# Understanding diffraction

Discussion in 'Accessories' started by timarmes, Sep 28, 2005.

1. ### timarmes

Hi,

I'm not sure where to post this question. We're missing
a "Technical" forum. Since this question regards lenses, which are
indeed camera equipment, this forum seems the best choice of a bad
bunch.

Can someone please explain to me why, exactly, diffraction is related
to the f/stop and not to the physical size of the aperture. I
understand that diffraction is due to light passing through a hole,
and will be more prominante for smaller holes. However the size of
the aperture at, for example, f/8 on a 200mm is entirely difference
to that of a 22mm yet the diffraction will be the same. Physically,
why is this?

Thanks,

Tim

2. ### peterblaise

.

I LOVE this technical stuff - it's so much easier to resolve, so to speak, than "real" photography! ;-)

Diffraction is NOT due to light passing through a hole, but rather to light passing over the sharp edges of a hole.

"Diffraction" itself is related to f/stop or aperture in that the GREATER the f/stop or aperture, the greater the edge surface area over which the light passes, and the GREATER the diffraction. The circumference is all that matters when calculating the potential diffraction.

But few people are concerned about diffraction on it's own. What is important is the amount of diffraction COMPARED to the amount of image forming light.

Diffraction is more prominent at smaller f/stops or apertures NOT because there is more diffraction -- because there isn't, there's LESS diffraction at smaller f/stops / apertures -- but due to the even smaller amount of image forming light getting through. What we observe at decreasing f/stops / apertures is actually an increase in the diffraction to "signal" ratio. That is, as we decrease our f/stop / aperture, even though the total diffraction goes down, at the same time the total amount of image-forming light is going down at an even greater rate at smaller f/stops / apertures. So the ratio between them changes in favor of the diffraction at smaller f/stops / apertures.

In ANY group of common activities, there tends to be abbreviations and resulting jargon -- which is intended to increase the speed and accuracy of communication between initiated members. However, this "coding" actually inhibits accurate communications with the uninitiated and thereby limits the growth and penultimate success of the group. What you have been hearing people say is probably, "... diffraction increases at smaller apertures ..." when what they should have been saying is, "... the total effect of diffraction increases at smaller apertures ..." or, "... diffraction-to-signal ratio increases at smaller apertures ..." That would have been more accurate if more time consuming.

Click,

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

3. ### winfried_buechsenschuetz|1

It's just the other way round. Diffraction is related to the PHYSICAL dimensions of the aperture (or rather the circumference of the aperture). Any aperture of same diameter (but different f-stop number depending on the focal length of the lens) will give the same amount of diffraction. The light rays passing through the lens do not know about the focal length (and the focal length to aperture diameter ratio, AKA known as f-stop number) but only know an amount of circumference where diffraction happens.

That's, BTW, one of the reason why people still use large format (4x5inch and above). They can use much higher f-stop numbers (like f/64) but still do not get more diffraction than with a 35mm format lens stopped down much less.

Most 35mm lenses perform best around f/11. At higher f-stop numbers, diffraction dominates although you cut a smaller center piece out of the lens diameter (and thus reduce most of the abberations which are related to the distance between light rays and optical axis). When using larger formats (and lenses with much longer focal length), you can stop down much more and still get an acceptable amount of diffraction.

4. ### charles_stobbs|3

Is it correct to say that diffraction is related to the perimeter (diameter, roughly) of the opening and the effect is spread out over the area of the image? Thus larger film sizes spread and dilute the effects.

5. ### roger_hicks|1

Strictly, it's numerical aperture (N.A.) not relative aperture. Relative aperture is merely a good-enough approximation. Google to find a definition of N.A. This may clarify matters for you. Or not.

Cheers,

Roger

6. ### oceanphysics

It's just the other way round. Diffraction is related to the PHYSICAL dimensions of the aperture
Wrong.
Diffraction effects at the film plane are a function of the focal ratio. This is because the amount of diffraction that occurs as the light passes through the aperture is only half the story. The deflected rays then spread out as they travel toward the film, and they spread out further the longer the focal length. It's easiest to think of this as a pinhole a certain distance (the focal length) from the film. But it works the same for lenses.
The reasons people get away with using very slow f-stops in large format photography is that the degree of magnification from the negative to the print is less, so the negative needs to be less sharp.

7. ### byronlawrence

we need more questions like this.

8. ### pvp

In fact diffraction is extremely dependent on the size of the aperture. However, it isn't the diameter of the aperture, but rather the angular size of the aperture as seen from the film plane that determines diffraction.

An f/8 aperture for a 200mm lens has a diameter of 25mm. Viewed from the film plane, a 25mm aperture 200mm away subtends 7.15 degrees.

In a 22mm lens, the f/8 aperture is 2.75mm, but because it is nine times closer to the film it subtends the same angle, 7.15 degrees, when viewed from the film plane.

(This geometry also explains, in a graphical way, why a given f/ number always produces the same illumination on the film regardless of the focal length of the lens.)

Since diffraction is a result of the light interacting with the edges of the physical aperture, and is (for practical purposes) not affected by the glass elements, then the total diffraction depends ONLY on the angle subtended by the aperture as viewed from the film. Since that angle is the same for a given f/stop in any lens, diffraction is always the same for a given f/stop.

9. ### richard_cochran

In any elementary physics textbook, you'll usually see a formula for angular diffraction expressed as a function of the hole diameter (or slit width; physics textbooks often study the problem in 2-d). The angular diffraction of a given wavelength is indeed inversely related to the hole diameter.
But photographers are rarely concerned directly about how wide an angle the point source spills out into. They are more likely to be concerned with how large the diffraction halo becomes at the film plane. In other words, how much it effects resolution as measured in lines per millimeter. To convert the angular spread to a size at the film plane, you must multiply by the distance between the diaphragm and the film plane, or in other words, the focal length.
Since diffraction at the film plane is inversely related to hole size, and directly related to focal length, it's easy to show it's directly related to (focal length)/(aperture diameter). And that is precisely the formula for the f-number that photographers use.

10. ### dan_fromm|2

Roger, numerical aperture and f/number are related by this equation:

f/number = 1/(2*NA)

Or, same thing written differently,

NA = 1/(2*f/number)

11. ### timarmes

Hi all,

When I draw out the situation you describe, with effectively two similar triangles, and it seems to me that the light that diffracts as it passes through the aperture at 200mm will have spread out further from the focus point than the light that deffracts at the 22mm mark, because it has further to travel before hitting the film. I would therefore expect more deffraction from the 200mm lens.

What am I missing?
Tim

12. ### roger_hicks|1

Dear Dan,

Good Lord! I am amazed that I had managed to stay ignorant of that for so long. I had always been told it was much more difficult and had therefore never investigated.

Thanks,

Roger

13. ### dan_fromm|2

Roger, I had no idea you were so easily intimidated.

Cheers,

Dan

14. ### alex_lofquist

Diffraction occurs whenever any wavefront passes an edge, be it in a circle, square, or slit. The wavefront need not be electromagnetic radiation but can be associated with a particle. It is always a direct function of the wavelength involved, and inversely related to the size of any aperture or slit in the system. Diffraction at an edge is a little more complicated, but there are many good treatises which discuss the subject and give examples. Diffraction isn't always something to but minimized, but can be put to good use in gratings and grizms.

15. ### roger_hicks|1

Dear Dan,

Well, I was told it a very long time ago and it had remained in the 'unexamined' file.

Cheers,

Roger

16. ### oceanphysics

Diffraction is NOT due to light passing through a hole, but rather to light passing over the sharp edges of a hole.
Actually it IS due to light passing through a hole.
Sharp edges certainly are not necessary. If they were, the optical performance of a lens could be improved by making the edges less sharp. Suppose we constructed an aperture and instead of a diaphragm we used a piece of glass, clear in the center and continuously, progressively darker toward the outside, until it was opaque. No edges. It isn't done because it would be a waste of time -- you'd still have diffraction.

17. ### pvp

...it seems to me that the light that diffracts as it passes through the aperture at 200mm will have spread out further from the focus point than the light that deffracts at the 22mm mark, because it has further to travel before hitting the film.
OK, I don't like trying to understand diffraction myself, because frankly it requires more math than I want to deal with in what is supposed to be a "fun" hobby! That may explain my tendency to try to reduce things to a qualitative understanding rather than insisting on numerical precision. Which of course makes things tougher to explain, even when I think I sort of understand what's going on...
We can probably agree on a gross description of what diffraction (for our purposes) is: when a beam of light (i.e., parallel rays from a point at infinity) passes through an aperture, the beam leaving the aperture is no longer quite parallel, but instead some of the light rays are spreading out. This is the phenomenon called diffraction.
It turns out that the ANGLE to which the light is diffracted is inversely dependent on the physical size of the aperture. Smaller aperture equals larger angle of diffraction. (So, the occasional photon that might [in theory] make it through an infinitely small aperture could go any direction at all, since the Airy disc for an infinitely small aperture will be infinitely large...)
So, yes, the smaller physical aperture in a 22mm lens will diffract the light to a greater angle than will the same f/number in a 200mm lens. However, since the 22mm lens is much closer to the film, the diffracted light rays get less time to spread out (as it were.) While the light from the 200mm lens does have more distance to travel, the diffracted rays are spreading at a narrower angle. In fact, the angular diffraction and the extra distance exactly cancel out, so the resulting Airy disc (the area that contains about 83.8% of the light that came through the aperture) is the SAME DIAMETER for either lens (at the same f/stop.)
When you toss it all into a bag and shake it up, what falls out is that the f/number alone is needed to calculate the diffraction effects that we need to worry about. Everything else cancels out.
Useful documents abound online. HERE'S ONE.

18. ### grahams

This refresher has been refreshing - it's 30 years since I was teaching this - now I need a teacher because my photographic memory has run out of film.

Thank's for raising the question, Tim.

Thank's for providing some answers, Alan.

19. ### mark_sirota|1

Many thanks, especially to Alan Davenport for his pithy answer. I suddely understand a lot more about this than I did a minute ago.

However, several other answers raise a question for me. Some of you said that it is related only to the circumference of the aperture (as seen from the film), but my understanding was that it's related to the ratio of the circumference to the area.

That is, the light passing an edge is diffracted, while the light passing through the middle is not. With a smaller aperture, the light passing edges make up a larger percentage of the total light because the circumference is proportional to the radius while the area is proportional to the square of the radius, so diffraction effects will be more noticable with smaller apertures.

Is there any truth to that, or am I blowing smoke?

20. ### peterblaise

.

First off, I'd like to say how much I really enjoy these discussions. I imagine that in moments, hours, at most, days, we are collecting an international group of people sharing insights and information that years ago took months if not years to happen. If Isaac Newton and Albert Einstein were alive today, they'd be happy and proud.

Secondly, I also enjoy our process. Some of us respond quickly with extemporaneous thoughts "off the top of our heads", and then the same or others respond with incredible research, experience and insights.

Thank you all!

I'm reminded of my childhood with a gang of boys huddled near the radio, listening to the game, discussing sports statistics and player's performances and skills. We haven't changed much, have we? But, where are the girls? Do any of us have any compatriots of the female persuasion who may pitch in on and expand these exchanges, or have we relegated ourselves to yet another "boys club"?

= = = = = = = = = =

Earlier on this thread: "... Useful documents abound online. HERE'S ONE...."

Peter Blaise responds: Since I at least read these threads in print, NOT on the web, the revealing of links is very helpful.

"HERE'S ONE" = http://www.mellesgriot.com/pdf/001.20-1.22.pdf from a greater volume from Melles Griot, this 3 page section entitled "Fundamental Optics, Diffraction Effects", covers circular aperture, slit aperture, energy distribution table, and gaussian beams, and is replete with formulae. Be aware that is is designed to support their business success, not define terms on their own, so they ONLY deal with diffraction in optical systems. Though this is appropriate for us, we may have to go a step further back to define our terms themselves and then apply them to our optical experiences here. Let's explore an example:

= = = = = = = = = =

Earlier on this thread: "...Actually [diffraction] IS due to light passing through a hole..."

Peter Blaise responds: Can anyone please tell me how a ray of light "knows" it's passing through a hole?

Presumably the "hole" is larger than at least a couple of rays of light otherwise it would not be much of a hole, right? Then, presumably, one "ray", say, one particular ray on the left side of the bunch ... well, how would it get information from a ray on the right side of the bunch, saying, "Hey, guys, this is a HOLE! Everyone on the periphery get ready to behave differently -- DIFFRACT"? What? Is there some sort of inter-light-ray/photon communication system that is sharing information about the media surrounding those light rays/photons? A communication system that moves information FASTER than the speed of light within the light ray/photon beam? If Isaac Newton and Alfred Einstein were alive to day they'd be spinning in their graves!

In order for the light rays/photons to take different action depending on passing through a hole or not, they would have to "see" in advance and then make their plan conditional in such information and then "spread the word" amongst themselves, "hole ahead, those on the periphery get ready to diffract". Otherwise, if they only assessed the hole once they were in it, then, at the speed of light, they'd already be at the image plane before they could respond.

No, it's the edge effect and the edge effect only. That fact that the edge is shaped like a hole is interesting to us as photographers with circular lenses, but it is not of much interest to the light rays/photos themselves, which behave as predicted by edge effect science, not hole science, if there were any!

= = = = = = = = = =

Earlier on this thread: "...Sharp edges certainly are not necessary..."

Peter Blaise responds: I put it to you that an edge is ALL any one particular light ray/photon "sees". See above for an exploration of the shared presumption * of a lack of a "super light speed inter light ray/photon communication system" that would inform one or more light rays/photons to behave otherwise due to remote environmental media information "observed" by one group of light rays/photons, information otherwise not immediately accessible to the first group of light rays/photons.

= = = = = = = = = =

Earlier on this thread: "...the optical performance of a lens could be improved by making the edges less sharp. Suppose we constructed an aperture and instead of a diaphragm we used a piece of glass, clear in the center and continuously, progressively darker toward the outside, until it was opaque. No edges. It isn't done because it would be a waste of time -- you'd still have diffraction..."

Peter Blaise responds: Isn't done? Oh, yes it HAS been done, and is commercially available today. The Minolta AF 135mm f/2.8 [T4.5] STF Smooth Trans Focus lens does take advantage of such a apodized "filter" within it's lens element system and aperture control system. It was the source of my first conversation here at photo.net with Philip Greenspun over how to calculate T value, "true" light transmission value. We don't have to go far - this lens is reviewed right here at photo.net:

http://www.photo.net/equipment/minolta/lenses

"...[Minolta] STF 135mm/f2.8 [T4.5] ... For those who think Minolta doesn't make exotic special-purpose lenses, here's a doozy. When it came time to fill the gap left by their long-discontinued 135mm/f2.8 ... the folks at Minolta decided to build a high-tech siege gun designed to wage war on the out-of-focus areas of your pictures, particularly portraits. I'd never heard of an apodization filter before, but it's there, along with a second ten-blade aperture designed to give precise control over aperture (the two features together are responsible for the T-stop designation). The goal is to produce a very smooth transition between in-focus and out-of-focus areas in your pictures, and reduce or eliminate distracting background effects ... this is the easiest-to-focus lens I've ever used, period... "

Trivia: it's also the ONLY electronic A-mount lens Minolta made that's MANUAL FOCUS only!

= = = = = = = = = =

* Perhaps we should define our terms more precisely at the beginning of our threads before we get off on a wild goose chase through our mis-meanings of them. Here, for our archives, from Google, the simple lookup: [define:diffraction] -- these aren't necessarily right or wrong, just already "out there" in the popular domain for our reference.

Definitions of diffraction on the Web:

* The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.
http://www.wrcc.dri.edu/ams/glossary.html

* A phenomenon exhibited by a light?s wave front when passing the edge of an opaque object (one that does not allow light to pass through it). The light becomes modulated, causing a redistribution of the light?s energy within the wave front. You will see it at the edges of the object?s shadow, in the form of minute dark and light bands. The edges of the shadow have a fuzzy appearance. Think of ripples meeting a rock in a pond. They go around the rock in a new series of ripples that can be seen on the sides
http://photographytips.com/page.cfm/1601

* Diffraction, the deviation of light from rectilinear propagation, is a characteristic of wave phenomena which occurs when a portion of a wave front is obstructed in some way. When various portions of a wave front propagate past some obstacle, and interfere at a later point past the obstacle, the pattern formed is called a diffraction pattern.

* Change in direction and intensity of light as it passes by an obstacle or through an aperture.
http://www.thebeerchair.com/html%20documents/astronomy%20dictionary.htm

* Energy redistribution due to an obstruction or change in the surface over which it is passing. Diffuse 1. To pour in different directions. 2. Spread out or dispersed, not concentrated.
http://www.yourwebassistant.net/glossary/d6.htm

* The process whereby RF [radio frequency] signals or sound waves are, in certain circumstances, deflected from their normal straight-line path by physical objects.
http://www.audiotechnica.com/glossary/

* A type of distortion due to multi-path resulting in the spreading out or ?smearing? of the received signal. It occurs when identical signals arrive via different paths and have different time delays.
http://www.bluewaveantenna.com/technical/glossary.html

* The spreading of light as it passes a sharp edge of an opaque object.
http://sohowww.nascom.nasa.gov/explore/glossary.html

* The bending of a wave front around an obstacle in the sound field. [3]
http://www.keithyates.com/glossary.htm

* The bending of light as it passes through a small slit or opening. When we study the diffraction of sunlight, we see a rainbow of colours.
http://www.ontariosciencecentre.ca/school/clc/visits/glossary.asp

* The tendency of waves to bend around corners. The diffraction of light establishes its nature as a wave.
http://astronomy.nju.edu.cn/astron/AT3/GLOSS_D.HTM

* scattering of X-rays (in this case) from a crystal. It depends on the "long- range" order in the crystal. More disorder means poorer diffraction especially at higher resolution.

* The modification of white light as it breaks up into the color spectrum.

* when light waves bend around an obstruction, i.e. [id est, "that is"] suspended particle, and move in a new direction.
http://www.serc.si.edu/labs/phytoplankton/primer/definitions.jsp

* A fundamental and inescapable physical phenomenon where, in all light beams, some energy is spread outside the region predicted by rectilinear propagation.
http://www.navitar.com/zoom/zoom_glossary.htm

* is the breaking up of white light causing spectral colours.
http://www.costellos.com.au/opals/glossary.html

* The process by which the direction of radiation is changed so that it spreads into the geometric shadow zone of an opaque or refractive object that lies in a radiation field. Diffraction is an optical ?edge effect,? differing only in degree from scattering. Diffraction becomes more evident when dealing with particles similar to, or larger than, the wavelength of the radiation. In meteorological optics, important diffraction phenomena include the aureole, Bishop's ring, corona, iridescent clouds, etc. The principle of diffraction may also be applied to the propagation of water surface waves, as into the sheltered region formed by a barrier
http://amsglossary.allenpress.com/glossary/browse

* The deviation in the path of a wave that encounters the edge of an obstacle.
http://www.fisicx.com/quickreference/science/glossary.html

* The change in the direction of a wave train at the edges of objects.
http://www.physchem.co.za/Common%20Files/Glossary.htm

* the scattering of light from a regular array of points or lines, producing constructive and destructive interference
http://www.learnchem.net/glossary/d.shtml

* An effect on wavefronts passing though an aperture. Wavefronts passing by the edges of the telescope pupil are bent and result in fringe patterns in the resulting image.
http://cfao.ucolick.org/EO/steinb/education_outreach/demoweb/home/glossary.html

* the spreading or bending of light that occurs when light passes around an edge.
http://www.icknowledge.com/glossary/d.html

* The spreading of a wave motion, such as light as it passes an obstacle and expands into the region that is behind the obstacle. Differentiation See magmatic differentiation, planetary differentiation, and sedimentary differentiation.
http://imnh.isu.edu/digitalatlas/glossary/letter.asp

* Deviation of a ray from a straight course when partially cut off by an obstacle, or when passing near the edges of an opening.
http://www.pqcorp.com/technicalservice/Glossary.asp

* bending of wave, i.e. light and sound, around obstacles in their path. Diffraction effects are common in microscope systems where apertures are used to help measure very small samples.
http://www.harricksci.com/infoserver/GLOSSARY/glossary.cfm

* when light passes sharp edges or goes through narrow slits the rays are deflected and produce fringes of light and dark bands
http://wordnet.princeton.edu/perl/webwn

* Diffraction is the apparent bending and spreading of waves when they meet an obstruction. It can occur with any type of wave, including sound waves, water waves, and electromagnetic waves such as light and radio waves. Diffraction also occurs when any group of waves of a finite size is propagating; for example, a narrow beam of light waves from a laser must, because of diffraction of the beam, eventually diverge into a wider beam at a sufficient distance from the laser. http://en.wikipedia.org/wiki/Diffraction

= = = = = = = = = =

Earlier in this thread: "... the light passing an edge is diffracted, while the light passing through the middle is not. With a smaller aperture, the light passing edges make up a larger percentage of the total light because the circumference is proportional to the radius while the area is proportional to the square of the radius, so diffraction effects will be more noticeable with smaller apertures..."

Peter Blaise responds, finally (oh, yeah, when did I EVER go "final" on any discussion?!?): THANK YOU! Now please re-read my FIRST reply at the top of this thread. Thanks for the paraphrase. At greater apertures, the total amount of accurate image forming light overwhelms the recording of diffraction effects at the image plane, even though there's more total diffraction due to there being more total edge surface to the aperture. At smaller apertures, the total diffraction is smaller due to the smaller total edge surface of the aperture. But even smaller still is the proportion of accurate image forming light remaining which can no longer overwhelm the diffraction effects from being recorded as a higher percentage of the light striking the image plane.

It's not "diffraction" per se that we're concerned about, it's "diffraction to signal ratio".

Apparently the effects of diffraction are made worse or lessened only by:

Relative aperture size:

- Wider apertures = greater diffraction, but lesser diffraction effects reaching the image plane as a portion of the total image forming light.

- Smaller apertures = lesser diffraction, but greater diffraction effects reaching the image plane as a portion of the image forming light.

Distance from the image plane to the aperture:

- Greater distance from image plane to aperture = lesser diffraction effects being "recorded" due to some diffracted light being bent away from the narrow angle to the image plane.

- Shorter distance = greater diffraction effects being recorded due to greater amount of diffracted light included in the wider angle to the image plane.

Any examples? After all, we can include pictures here at photo.net!

Click!

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

21. ### peterblaise

.

Add the word RECORDED above and below. Again, more or less diffraction is less important than the ratio of REDORDED diffraction to the RECORDED image forming information at the image plane:

It's not "diffraction" per se that we're concerned about, it's RECORDED "diffraction to signal ratio".

Apparently the RECORDED effects of diffraction are made worse or lessened only by two criteria:

One: Relative aperture size (that is, aperture edge "length" or circumference relative to aperture area):

- Wider apertures = more edge surface area over which light will defract = greater total diffraction, but lesser diffraction effects reaching the image plane as a portion of the total RECORDED image forming light.

- Smaller apertures = less edge surface area over which light will defract = lesser total diffraction, but greater diffraction effects reaching the image plane as a portion of the REDORDED image forming light.

Two: Distance from the image plane to the aperture:

- Greater distance from image plane to aperture = lesser diffraction effects being "recorded" due to some diffracted light being bent away from the narrow angle of approach to the image plane.

- Shorter distance from image plane to aperture = greater diffraction effects being "recorded" due to greater amount of diffracted light included in the wider angle of approach to the image plane.

Endless editing, eh?

Click!

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

22. ### kelly_flanigan|1

The confusion about diffraction seems to alot on photo.net . One wants a lens to be diffraction limited; this is perfection. A Diffraction limited design is the ULTIMATE in perfection. Optics speaks of diffraction by arc angle; OR a distance resolved on a plate; film or sensor; for a given focal length lens. For a telescope; aperture in inches/millimeters is what matters for resolving arc angle with double stars; not f stop. With an object that can be magnified; photo lenses have diffraction limits that varies with f-stop. A diffraction limited lens at F8 can resolve twice a diffection limited lens at f16. Most/many lenses increase in resolving power when stopped down a couple of stops; then drop some when further stopped down. Many photo books and sites water/dumb down diffraction; consult an optical book. Most folks have camera shake; miss focus; etc that really limit the resolving power of their systems.

23. ### alex_lofquist

It was suggested that diffraction could be reduced or eliminated by getting rid of the sharp edges in the aperture diaphragm (and also in the shutter blades in a between-the-lens shutter). This is true and practical in a few applications. This is known as Apodizing. It could be made from a graded filter with maximum transmission in the center with built-up density to a maximum at the periphery, according to a specific formula. It will sightly increase the width of the first maximum, but get rid of much of the fringes. Unfortunately, it would seldom seem practical when a variable aperture is required.

My experience in apodization was with optical spectrometers where scattered light was often a bane to good performance. Apodizing masks were applied to the diffraction gratings to minimize the secondary maxima.

24. ### kelly_flanigan|1

Apodizing masks are sometimes used with amateur reflectors; with mirrors somewhat not perfect in the hand done parabolic process; off of a spherical. Here a 6 inch might be tried with a 5.5; 5; or 4 inch mask; cut with a star cuttout. One can observe double stars with known arc separations; and trial and error make a custom mask.

25. ### pvp

Previously, on Understanding Diffraction:
Apparently the effects of diffraction are made worse or lessened only by:
Relative aperture size:
- Wider apertures = greater diffraction, but lesser diffraction effects reaching the image plane as a portion of the total image forming light.
- Smaller apertures = lesser diffraction, but greater diffraction effects reaching the image plane as a portion of the image forming light.
Distance from the image plane to the aperture:
- Greater distance from image plane to aperture = lesser diffraction effects being "recorded" due to some diffracted light being bent away from the narrow angle to the image plane.
- Shorter distance = greater diffraction effects being recorded due to greater amount of diffracted light included in the wider angle to the image plane.

=============================
This is simply incorrect -- exactly the opposite of what the math tells us about diffraction! Diffraction is greater for smaller apertures, less for large apertures. The circumference of an aperture is not mentioned in any of the formulas that define diffraction, only the DIAMETER of the aperture.
The AMOUNT of diffraction, if we need a quantity, is determined by the diameter of the spot formed (by diffraction) at the image plane. (Note that this is not referring to the "circle of confusion" that we use when discussing depth of field.) The diameter of the Airy disc for a particular wavelength of light can be given by the following formula:
y = 2.44 * lambda * f#
where lambda is the wavelength, f# is the numerical f/stop of the aperture, and y is the diameter of the Airy disc.
It's worth commenting at this point, that the resulting value will ALWAYS be larger than the physical diameter of the aperture: we can never escape diffraction.
Recall that the f# is, by definition, the focal length divided by the physical diameter of the aperture. Using "D" for the focal distance and "d" for the aperture diameter, our f/# becomes D/d and our formula for the Airy disc becomes:
y = (2.44 * lambda * D) / d
For purposes of this discussion, we don't care about a particular wavelength of light, nor for that matter the precise diameter of the Airy disc. So we can throw out the wavelength and the constant and come up with:
Diffraction [is proportional to] D / d
What this tells us is that a wider aperture ("d" is larger) the amount of diffraction is LESS, not more. And that for a longer focal length ("D" increases; the aperture is at a greater distance from the image plane) the diffraction will increase.
For any given scene, there is an optimum f/stop. "Optimum" being defined as the f/stop that will produce the best definition on the film over the required depth of field. If the aperture you choose is too large, defocus effects will predominate and you will lose definition. If the aperture is too small, diffraction will overwhelm the image and you lose definition.
I'll stop now.
For a much more concise discussion of the science than you'll get from me, see the hyperphysics page on diffraction, and pay particular attention to the link to Circular Aperture diffraction, which is of course what we're talking about.

26. ### mozart 2

Tim:

For current and future reference; here are some links:

http://www.gonda.ucla.edu/bri_core/na.htm

http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

http://www.kenrockwell.com/tech/focus.htm

Hope all of this is more than useful and not too confusing.

Bill

27. ### peterblaise

.

The original inquiry was "understanding diffraction ... in lens systems" not "understanding the RATIO OF diffraction TO IMAGE FORMING LIGHT ... in lens systems" but since the latter is all we care about, why not concentrate on that? AND, why not specify, completely and unambiguously that the latter is what we are talking about. So let us talk about all three elements in reference to each other:

- diffraction,

- apertures

- the image plane.

Let's not forget the definition of diffraction -- the bending of light as it passes over an edge. The more edge, as there is in a large aperture, the more diffraction. The less edge, as there is in smaller apertures, the less diffraction.

Now, let's also remind ourselves of another definition, aperture -- the hole that image forming light passes through on it's way to the image plane. The larger the aperture, the more image forming light will hit the image plane. The smaller the aperture, the less image forming light will hit the image plane.

Combine these -- the definition of diffraction and the definition of aperture -- and you get, AT the image plane:

- larger apertures = more diffraction but also w-a-y more image forming light AT the image plane, and so the RATIO of diffraction to image forming light is actually quite small AT the image plane. We call this "less diffraction" when we mean a "smaller diffraction/signal ratio".

- smaller apertures = less diffraction but also w-a-y less image forming light AT the image plane, and so the RATIO of diffraction to image forming light is actually quite large AT the image plane. We call this "more diffraction" when we mean a "larger diffraction/signal ratio".

So, earlier on this thread: "...Diffraction is greater for smaller apertures, less for large apertures..." Oops! Probably meant: "Diffraction/SIGNAL RATIO is greater for smaller apertures, less for large apertures." Or perhaps, "Diffraction EFFECTS RECORDED AT THE IMAGE PLANE are greater for smaller apertures, less for largeer apertures."

And, that's all we care about -- the actual ratio of diffraction to image forming signal AT the image plane.

The fact that there is less total diffraction at smaller apertures than at larger apertures is somewhat irrelevant, as, at smaller apertures, there is even w-a-y less image forming light, also!

Let me also edit an earlier post: "...The reason the circumference of an aperture is not mentioned in any of the formulas that define diffraction, only the DIAMETER of the aperture, is that diameter can be used to calculate the effect of the greater circumference and the resulting greater diffraction, but also the w-a-y greater image forming light."

As already mentioned, the diffraction goes down arithmetically with the circumference from larger to smaller apertures, but the image forming light goes down geometrically with the area from larger to smaller apertures.

Formulae mean nothing if they do not accurately represent what real-world facts we are trying to measure or predict. Formulae support, not prove!

Also earlier on this thread, let me identify and give credit where credit is due for some links and WHY there are relevant or not:

http://www.gonda.ucla.edu/bri_core/na.htm = Numerical Aperture and Resolution from Francon, M. 1961. Book "Progress in Microscopy". Hmm ... microscopy ... relevancy?

http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm = Sean McHugh offers support for lesser diffraction to signal ratio with longer lenses than with shorter lenses, presuming the aperture is farther from the image plane in a longer lens than the aperture to image plane distance is in a shorter lens.

http://photography.about.com/od/basics/a/bpaperture.htm = Peter Marshal about.com on Photography, generalized and simplified definitions of photo gear - aperture, auto manual, iris, boke[h], relative apertures, depth of field, diffraction limits, optimum apertures, including this gem which supports my point all along: "...when the lens is stopped down, a greater fraction of the light passing through it will pass close to the edge of the iris..." Nice paraphrase, Peter Marshal.

http://www.kenrockwell.com/tech/focus.htm = Ken Rockwell ... well. I'm not sure why Ken is EVER relevant to any discussion of photography -- he's a photographer, not a word-smith, as he freely admits! Here he chats about large format stuff unrelated to photographic subject content.

= = = = = = = = = =

Oh, and Roger Hicks, below is from your reference and for our archives of this thread -- Google searches for [define:numerical aperture] and [define:na] (non photo- optical- definitions removed):

Definitions of numerical aperture on the Web:

* The number that expresses the light gathering ability of a fiber. Related to acceptance angle. http://www.cetpak.com/Technical/glossary.htm

* The numerical aperture of a microscope objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance. All modern microscope objectives have the numerical aperture value inscribed on the lens barrel, which allows determination of the smallest specimen detail resolvable by the objective and an approximate indication of the depth of field. http://www.microscopyu.com/articles/formulas/formulasindex.html

* The sine of the acceptance angle of a fibre multiplied by the refractive index of the medium from which the light is entering (air = 1). http://www.fibreoptech.co.uk/glossary.asp

* A unitless measure of the ability of a lens to gather and focus light. NA = n sin q, where q is the angle of the light as it narrows to the focal point. A numerical aperture of 1 implies no change in parallel light beams. The higher the number, the greater the focusing power and the smaller the spot. http://www.dvdmadeeasy.com/glossary/n.html

* A unitless number referring to the light gathering ability of a fiber and is defined as the Sine of half acceptance angle. http://www.fontcanada.com/gloss.html

* In optical fiber, the sine of the maximum acceptance half-angle, q max , times the refractive index of the core (assuming an air-to-core interface). The larger the NA, the greater the amount of light that is accepted into the fiber for propagation to the distal end. http://www.polymicro.com/techsupport/techsupport_glossary.htm

* (NA): The light-gathering ability of a fiber; the maximum angle to the fiber axis at which light will be accepted and propagated through the fiber. The measure of the light-acceptance angle of an optical fiber. NA = sin a, where a is the acceptance angle. NA is also used to describe the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector. http://web1.mtnl.net.in/~powertel/glossery.htm

* The product of the angle formed by the cone of on-axis rays and the index of refraction of the medium in which the cone resides. With higher numerical aperture, more light will be collected. In a diffraction limited system, the numerical aperture is directly proportional to the resolution of the optical system. http://www.navitar.com/zoom/zoom_glossary.htm

* The characteristic of a fiber optic strand which defines its acceptance of impinging light. The degree of openness, light gathering ability, and angular acceptance are other terms describing this characteristic. http://www.nuhorizons.com/Glossary/Optoelectronics.html

* A ratio that describes the cone of light emitted by the condenser or accepted by the objective lens. Objectives with a larger NA have greater resolving power. (a 100x oil lens with a 1.25 NA can resolve smaller objects than a 100x lens with a 0.95 NA) http://www.bi-optic.com/vocab.html

* a measure of the acceptance angle of a lens. Higher numerical aperture means the lens gather more diffraction orders yielding higher resolution but at the expense of depth of focus. http://www.icknowledge.com/glossary/n.html

* (fiber optic) The light gathering ability of a fiber, defined as the sine of half the angle that contains 90% of the optical power that is captured by the fiber. http://connectors.tycoelectronics.com/glossary/glossary-n.stm

* This is a measurement of the light-gathering capacity of a fiber ? specifically, it measures how light spreads out after leaving the fiber. Numerical aperture also can be used to estimate the acceptance cone or angle when coupling to a fiber. http://www.corning.com/photonicmaterials/products__services/specialty_fiber/photosensitive/glossary.asp

* A figure that, in effect, indicates the capacity of an optical fibre to receive light. http://www.networkingsolutions.co.uk/acatalog/gl.html

* The numerical aperture (NA) of an optical fibre defines the characteristic of the fibre in terms of its acceptance of impinging light. It can be calculated and expressed as an index (a number). The higher the number the more light than can be accepted. http://www.aefos.com/html/glossary/n.htm

* The angle at which a fibre will gather light and propagate it down the core http://www.opticalfibresuk.com/glossery_of_terms.htm

* In microscopy, the numerical aperture, AN, of an objective is: where I is the index of refraction of the medium in which the lens is working (1.0 for air, up to 1.56 for oils), and a is the angular aperture of the lens. It is basically a measure of the diameter of the aperture compared to the focal length. In photography, the f-number expresses the same relationship. http://en.wikipedia.org/wiki/Numerical_aperture_(microscopy)

Definitions of na on the Web:

* The number that expresses the light gathering ability of a fiber. Related to acceptance angle. http://www.cetpak.com/Technical/glossary.htm

* Abbreviation of numerical aperture. The higher the value is the higher the resolution is. http://www.mo-forum.gr.jp/english/glossary/

* A unitless measure of the ability of a lens to gather and focus light. NA = n sin q, where q is the angle of the light as it narrows to the focal point. A numerical aperture of 1 implies no change in parallel light beams. The higher the number, the greater the focusing power and the smaller the spot. http://www.dvdmadeeasy.com/glossary/n.html

* Simply, a non-dimensional number that indicates the ability of a fibre or other device to receive light input. Specifically, the sine of the half angle of the acceptance or radiance cone of an optical fibre, multiplied by the refractive index of the material in contact with the fibre face. http://www.interconnect.co.za/links.html

* In optical fiber, the sine of the maximum acceptance half-angle, q max , times the refractive index of the core (assuming an air-to-core interface). The larger the NA, the greater the amount of light that is accepted into the fiber for propagation to the distal end. http://www.polymicro.com/techsupport/techsupport_glossary.htm

* The product of the angle formed by the cone of on-axis rays and the index of refraction of the medium in which the cone resides. With higher numerical aperture, more light will be collected. In a diffraction limited system, the numerical aperture is directly proportional to the resolution of the optical system. www.navitar.com/zoom/zoom_glossary.htm

* The characteristic of a fiber optic strand which defines its acceptance of impinging light. The degree of openness, light gathering ability, and angular acceptance are other terms describing this characteristic. http://www.nuhorizons.com/Glossary/Optoelectronics.html

* The sine of the vertex angle of the largest cone of meridional rays that can enter or leave an objective, multiplied by the refractive index of the medium in which the vertex is located. In air the NA must be less than 1. http://www.pmel.org/Surface-Glossary.htm

* A ratio that describes the cone of light emitted by the condenser or accepted by the objective lens. Objectives with a larger NA have greater resolving power. (a 100x oil lens with a 1.25 NA can resolve smaller objects than a 100x lens with a 0.95 NA) http://www.bi-optic.com/vocab.html

* A mathematical formula devised by Ernst Abbe for the direct comparison of dry and all types of immersion objectives for resolving power. Numerical Aperture is the sine of half the angular aperture of the objective multiplied by the refractive index of the medium between the front lens and the cover glass. NA ranges from 0.1 to 0.95 for dry objectives and up to 1.4 for oil immersion lenses. http://www.visioneng.com/technology/glossary.htm

Hmmm .. another source of heat more than light for me -- drat! I think I'll go out and shoot something! =8^o

Click!

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

28. ### neild

Very interesting discussion indeed! Thanks for the info...

29. ### dave_t

The more edge, as there is in a large aperture, the more diffraction. The less edge, as there is in smaller apertures, the less diffraction.
Eh? Where does this come from? Alan Davenport is right: for circular apertures (or any hole) the diameter is what's important.

30. ### peterblaise

.

Earlier in this thread:"...The more edge, as there is in a large aperture, the more diffraction. The less edge, as there is in smaller apertures, the less diffraction..."

Someone responded: "...Eh? Where does this come from?..."

Peter Blaise responds: ... from the DEFINITION of diffraction - see above in the miles and miles of postings to this thread. Diffraction is where light bends when going over an edge.

Someone responded further: "... Alan Davenport is right: for circular apertures (or any hole) the diameter is what's important..."

Peter Blaise responds: Important for what? Please be specific.

Diameter, as mentioned, is easily a part of any equation where the circumference of a hole (where the diffraction is) must be calculated AND the area of the hole (where the signal, or image forming light is) must also be calculated. The ratio between the circumference and the area is surprisingly identical to the ratio of diffraction to signal (image forming light) hitting the image plane! Go figure! =8^o

The dependence on "diameter" in a formula does not change the definition of diffraction.

As we use smaller apertures, the circumference (where the diffraction is) goes down arithmetically, and the area (where the "signal" or image forming light is) goes down geometrically. At some point, the ratio between them at the image plane favors the diffraction, hence the phrase "diffraction limited". I think the goal is to discover if there is actually ONE aperture for any particular lens, and what is that aperture where a lens's resolving power becomes diffraction limited. I think no one has yet offered specific examples of lenses and subjects and image enlargement where this can be illustrated.

Anyway, does ANYONE have ANY image where the ONLY thing wrong is "diffraction"? Otherwise, the image would be perfect? I certainly don't. I'd love to see some samples. Otherwise, this is an academic discussion. The only reason I'm hanging in here so long is to keep up the good fight against basing ANY of our photography decisions on fallacies, errors or misunderstanding.

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

31. ### dan_fromm|2

Peter Blaise, who writes at excessive length, remarked "I think the goal is to discover if there is actually ONE aperture for any particular lens, and what is that aperture where a lens's resolving power becomes diffraction limited. I think no one has yet offered specific examples of lenses and subjects and image enlargement where this can be illustrated."

40/4.5 Luminar. This lens is diffraction limited wide open. Image quality deteriorates markedly on stopping down at all. At the magnifications the lens is intended to be used at, stopping down reduces overall sharpness because the gain of DoF on stopping down is swamped by diffraction. Come to think of it, this is true of all of the Luminars.

55/2.8 AIS MicroNikkor. This lens is diffraction limited at f/4. Same thing. At normal distances, stopping down below f/4 hurts little, but when the lens is reversed and used above around 5:1 stopping down below f/4 is pure loss. Re hurts little, in an old test MP found that at normal distances the lens resolved best at f/5.6, went quite mushy at f/32.

You have to understand the tradeoff between sharpness in the plane of best focus and sharpness in depth. Most of us give up one to get the other. Copying flat subjects is a prominent exception.

Do a search in Usenet for Brian Caldwell's comments on diffraction limted optics. He posts as BC in several of the rec.photo.equipment.* newsgroups, also in sci.optics.*

32. ### dave_t

Diffraction is where light bends when going over an edge.
Your reasoning is right for some geometries (such as a linear obstruction) but not all (like an aperture). To see why geometry matters, it helps to think about why "light bends when going over an edge." There are a couple ways to do this, probably most people would use Huygen's wavelets , where destructive interference through a small aperture is less complete than through a larger one (and is only truly complete through an aperture of infinite size), generating greater spreading and stronger fringes -- what we would call more diffraction. Why the Huygen-Fresnel priciple works is an open question ("where do little wavelets come from, mommy?") but it does work, and work quantitatively.
Another way to look at it is to step back and ask what you're trying to do. A perfect imaging system would render a point in the object plane as a point in the image plane. To get that point (a spatial delta function) in the image plane you need a transverse field with a white wavenumber spectrum. Any obstruction (such as finite size) means that you cannot reconstruct a point in the image plane, and the smaller the aperture the worse the reconstruction -- we'd call that more diffraction. The actual pattern of the diffraction can be varied by changing the wavenumber spectrum (beyond the crude clipping of a window, that is, the "apodization" mentioned by Alex Lofquist) but you cannot eliminate the diffraction itself.

33. ### peterblaise

.

(a) Let's NOT comment on each other's posting styles.

(b) Thanks to a previous poster here for providing personal experience that your own 2 lenses (they are YOUR OWN lenses, right?) appear to have objectionable diffraction after certain apertures. I presume you decided for yourself after much experimentation and trying different apertures and subject distance, lighting and contrast ratios and film and subject matter and enlargement sizes? How about image samples? Can you assist us in how we might make a similar assessment of our own lenses regarding the threshold (if there be any) of being "diffraction limited"? Thanks!

(c) Maybe it's just me, but the reason I quote extensively is that I find it unreasonable to expect each participant now and in the future to independently go out and serendipitously find and read the exact same resource to which I am referring. I searched Google for [BC diffraction inurl:rec.photo.equipment] and [BC diffraction inurl:sci.optics] and found nothing. Please quote whatever you think is relevant for us to read rather than hope we will ever find the same thing that impressed you!

(d) No pictures?

Click!

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

34. ### peterblaise

.

Earlier in this thread: "... Your reasoning is right for some geometries (such as a linear obstruction) but not all (like an aperture)..."

Peter Blaise responds: Please tell me how a ray of light knows which of the different and supposedly competing or conflicting laws of geometry you want it to obey or disobey at any one time? I've asked this before but no one has yet to offer a theory of a super-light-speed inter-communications between waves/rays/photos of light.

I put it to you that each individual ray of light does not know nor care about anything except that it alone as hit an edge and diffracts accordingly.

Our observations are of the collective effect through a lens and (a roughly circular) aperture, and I am postulating that our observation criteria do not in the least concern any individual wave/ray/photon of light on it's trajectory, whose individual path is ignorant of the paths of other waves/rays/photons.

Or, does anyone have a lens with an aperture that is so small at to approach the size of a single wave/ray/photon such that the one ray can actually tell that it is tryign to s-q-u-e-e-z-e through a hole because that one wave/ray/photon is making contact with opposite sides of the hole at the same time? What is that, f/18,446,744,073,709,551,616 or something close? ;-)

Why are we spending so much time on the academic trivia on this? Does no one have personal experience and sample photos to share?

= = = = = = = =

Well, I DID go out and shoot something -- and I brought 'em back alive. That's the beauty of photography for me! I was out shooting wild flowers and butterflies and insects and stuff at the 40-acre Winkler Botanical Preserve here in Alexandria, Northern Virginia, US -- see satellite map below (lots of parks around where I call home) -- while my system at home was batch-scanning 20-year old shots of Arches, Capital Reef, Dead Horse Point, Moab, Utah, US and other travels over the years -- see http://data2.itc.nps.gov/parksearch/state.cfm?st=UT

Sorry, but neither experience turned up useful examples of diffraction to share.

Click!

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com

35. ### peterblaise

.

Ooops - here the elusive map to the Winkler Botanical Preserve:

and enter

[5400 Roanoke Avenue, Alexandria, Virginia, 22311 USA]

and I like the hybrid view with roads and sattelite.

Enjoy -- maybe I'll see you there?

Click!

Love and hugs,

Peter Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

PS - photo.net seems adverse to letting me upload an image ... oh well. So much for full participation. Maybe I need more 1,000 words to stand in for the pictures that are missing? ;-)

36. ### dave_t

Please tell me how a ray of light knows which of the different and supposedly competing or conflicting laws of geometry you want it to obey or disobey at any one time? I've asked this before but no one has yet to offer a theory of a super-light-speed inter-communications between waves/rays/photos of light.
You're making this more complicated than it really is. Some terminology first -- not to get into semantics, but I don't know what you mean by "ray". Photons and waves are both well defined, but you have to be careful about usage because there is a crucial difference, namely that waves are at field (as in "light is an oscillatory electromagnetic field") level and photons at energy (field^2) level, so that in the photon description phase information is lost - you can kludge it back in, but for interference effects such as diffraction working at field level is much more natural. You seem to be describing light as a stream of photons, which is fine if you keep the above in mind. Otherwise you'll have to work mighty hard to understand interference. Think about light also as a wave and not only does interference naturally follow, but so will the dependence on geometry (aka "boundary conditions")
I put it to you that each individual ray of light does not know nor care about anything except that it alone as hit an edge and diffracts accordingly.
Our observations are of the collective effect through a lens and (a roughly circular) aperture, and I am postulating that our observation criteria do not in the least concern any individual wave/ray/photon of light on it's trajectory, whose individual path is ignorant of the paths of other waves/rays/photons.

No offense but I don't think you believe that. Not unless you don't believe in interference. Not unless you don't believe in refraction. And you still haven't explained why it is that light diffracts at an edge.
Re: actual results from real lenses. I submit that for the vast majority of photographic work (high resolution 2-D imaging such as copy work and astrophotography excepted) such results are just as irrelevant as theory. How many times do we have to stop down for DOF, how many of us use heavy camera stands (or even beefy tripods), how many of us put a loupe on the ground glass for critical focusing, how many of us use the biggest format we can carry (since if we're talking ultimate sharpness we clearly don't care about DOF)? And ultimately, photography is about vision, not replication, and diffraction limited performance is important only to the extent that that helps express a vision. After all, some of the strongest visual images were done using a paintbrush and palette knife.

37. ### peterblaise

.

Earlier in this thread: "... you still haven't explained why it is that light diffracts at an edge..."

Peter Blaise responds: ... and I don't really care! All we know is that is does, and so that is what we are trying to incorporate into our photographic decisions. IF light diffracts at an edge, THEN as the edge becomes a significant portion of our lens setting, as it does at smaller apertures, we should be prepared to expect the resulting diffraction to become a greater part of our captured image.

----------

Earlier n this thread: "...Think about light...as a wave...and..."boundary conditions"...naturally follow..." [edited]

Peter Blaise responds: Ahh, so we do agree after all! I imagine that most of this thread is just describing the same general understanding differently and THINKING that we disagree when in actuality, we agree, after all, essentially!

Earlier in this thread: "...in the photon description [of light]...information is lost..."

Peter Blaise responds: By whom?

----------

Peter Blaise wrote earlier in this thread [clarified and distilled]: "...I put it to you that...light does not know nor care about anything except that it...[has] hit an edge and diffracts accordingly...[and light is not in the least concerned with] our observation criteria..."

A response: "...I don't think you believe that..."

Peter Blaise responds: Light doesn't care what I believe.

Earlier in this thread: "...you don't believe in interference...[?]"

Peter Blaise responds: Those are OUR observations. The light itself could care less that we see it as if it were "putting on a show" for us, so to speak. You're not suggesting atom smashing, are you? I believe that atom smashing in particle accelerators is all about small particles actually "interfering" with each other -- having a good head-on crack up! That's not what most of us are trying to do with our photography. If you think the light inside interference effects "knows" that those interference effects are observable by you and I, then I ask again, how does light "know" these things? Have we wandered into a discussion of the meaning of consciousness?

----------

Peter Blaise asked earlier: "Does no one have personal experience and sample photos to share [illustrating diffraction effects]?

Earlier in this thread, perhaps as a response: "...some of the strongest visual images were done using a paintbrush and palette knife..."

Peter Blaise responds: So I guess that's a "no" to illustrative photographic examples of diffraction effects, eh?

----------

Peter Blaise further responds: Reading through my library I'm struck with the following:

K. G. Birch wrote in "The Focal Encyclopedia of Photography", 1969, page 445: "...Diffraction: bending of light rays around opaque objects, specially apparent if the opaque object has a sharp edge... Diffraction: when a wave front meets a knife edge, it generates a new wave front at the edge itself. This spreads round the obstacle, leading to some light being apparently bent round the edge..."; and on page 845: "...it will be seen from the optical transfer function of a perfect lens that all spatial frequencies above zero lines per millimetre the lens performance is impaired, a necessary condition imposed by diffraction at the edge of the lens opening..."

Peter Blaise responds: Don't ya just love the references to "wave" and "ray" and "edge" when discussing and defining diffraction?!? ;-) I do not offer this reference as proof, but merely as one more reference, encouraging us all to keep researching and expanding our resources.

Also from "The Focal Encyclopedia of Photography", 1969, page 34: "...Airy disc. Central area of the image of a point source of light focused by a lens free from aberrations. The Airy disc is surrounded by concentric rings of light of feebler intensity which can in practice be ignores. The effect is an optical phenomenon connected with the wave nature of light and is called after its discoverer, G. B. Airy, Astronomer Royal in 1830..."

Peter Blaise responds: Optical phenomenon? In other words, we see it, but the light itself does not "know" it's putting on a show for us, eh?

-----------

I wish I could share a diffraction-limited image. Anyone? Anyone?

Click!

Love and hugs,

Pete Blaise peterblaise@yahoo.com http://www.peterblaisephotography.com/

38. ### dave_t

In the event that you're interested in and are willing to learn about the physical aspects of light (as opposed to photographic, which encompasses primarily artistic and documentary aspects) let me suggest someday sitting down with a good optics book. Hecht and Zajac's Optics is an excellent introduction and recommended for any photographer interested in the physics of light. If you're interested in something more advanced than H&Z just ask, I've got a whole stack.
Re: diffraction limited images: all it takes is a little looking around.

39. ### kelly_flanigan|1

diffraction is not just limited to light. Radio and microwaves diffract too.

40. ### pvp

Sound waves diffract. Waves in the ocean diffract. The fact that light diffracts, proves the wave behavior of light.