6x6 and FF equivalent look

[Ed_Ingold]

Regardless of that; I fail to see the relevance of changing the subject distance, perspective and angle-of-view when discussing comparative DoF between two different sized formats. When the whole point is to keep the end result as similar as possible.

The claim was made that DOF in an image stitched from multiple images would have less DOF than a single image made with the same lens. The exact opposite is true. The effective DOF depends on the end-to-end magnification. The merged image would need much less magnification for the same sized print, or even a much larger print, hence greater DOF. Secondly I was trying to clear up misconceptions on how DOF works.

 

Viewing matters! Estimates of DOF are based on viewing an 8"x10" print at a distance of 10", corresponding to a circle of confusion just visible to the unaided eye of about 0.2 mm.

Edited August 24, 2020 by Ed_Ingold

[Ed_Ingold]

The visual threshhold of sharpness would be 0.2 mm at 10", regardless of the print size. Of course, you would usually view a large print from further away in order to appreciate its scope.

[rodeo_joe1]

The claim was made that DOF in an image stitched from multiple images would have less DOF than a single image made with the same lens. The exact opposite is true.

I don't think that was the claim. What's claimed is that a larger aperture can be used than what's normally available for a given focal length and format. Because the format effectively becomes larger, while keeping the same aperture.

 

The linked example shows a stitched 'format expansion' taken with an 80mm f/1.8 lens. I know of no such lens available to cover, say, 10"x8", but if such a lens were available, it would give a DoF of about 1 metre with a subject distance of 3 metres. That's a horizontal angle of view of over 112 degrees (= a 12mm FL on 24x36mm).

 

Whereas any practical single-shot 80mm lens might need to be stopped down to an aperture of at least f/11 to cover 10"x8" - giving practically infinite depth-of-field, since 3m is just over the hyperfocal distance.

 

So I don't think there's any claim to break existing DoF rules. Simply to simulate an aperture and angle-of-view that's not possible with any obtainable real lens.

Edited August 26, 2020 by rodeo_joe|1

[Ed_Ingold]

DOF is a convenient fiction based on appearance, not rules set in concrete. The closer you look, the more the fictional aspect becomes obvious. For example, the 7x magnified view for focusing in a high resolution camera like a Sony A7Riii is enough to dispel any notions to the contrary. The tables, numbers and engraved marks are based on a particular way the results are viewed.

[glen_h]

DOF is a convenient fiction based on appearance, not rules set in concrete. The closer you look, the more the fictional aspect becomes obvious. For example, the 7x magnified view for focusing in a high resolution camera like a Sony A7Riii is enough to dispel any notions to the contrary. The tables, numbers and engraved marks are based on a particular way the results are viewed.

 

I suppose so.

 

But first, there is always diffraction. So no part if ever in perfect focus.

 

Next, no real lens is perfect, so it might have spherical and chromatic

aberration, so even less perfect focus.

 

So if depth of field adds a little to those, it will be pretty much not noticed.

[Ed_Ingold]

When you focus with the equivalent of a loupe, it's easy to see when the plane of sharpest focus is crossed. The procedure adopted by many is to focus through this point, then back by muscle memory. That doesn't mean perfect focus, but the best focus, For people of my age, manual focus was a way of life for decades, but never as precisely as available today. (A loupe on a ground glass mainly magnifies the texture of the glass.)

[rodeo_joe1]

DOF is a convenient fiction based on appearance, not rules set in concrete.

Indeed there's really no such thing as depth of field if you look close enough. There's one plane of focus only. (Extended by spherical aberration if you get picky!)

 

However, there is an area of acceptable focus surrounding that plane. Otherwise the concept of depth of field wouldn't have been invented. Sure, DoF tables make it look as if there's a sudden cutoff of focus at two fixed distances, but everyone knows they're just a guide. And they do work as a good comparator between format sizes, apertures and focal lengths.

 

Perhaps DoF tables or scales are less needed now that we have WYSIWYG viewing directly from the image sensor, but in the past even over-bright SLR screens didn't properly tell you what the final image was going to look like. And rangefinder or TLR viewfinders made DoF complete guesswork.

[tom_chow]

DOF is a convenient fiction based on appearance...

I think calling DoF fiction is a little extreme. It is a term used to describe the difference in aperture effects:

 

 

(Image sample borrowed from SLRLounge.com)

 

The image at the smaller aperture appears to have more depth. i.e. "Depth of Field". The term does not say perfect focus, and the original definition was "acceptable" focus, although it may mean different things to people these days.

 

What alternate term would you propose to describe the effect of aperture?

 

 

However, if you consider CoC, physical sensors (film or digital) have a thickness, and grain/pixel size, beyond which better focus does not change the image. Real lenses have diffraction and aberrations, over which there is no change in focus in a range of distance. There is a physical range that the image is as sharp as it'll be, something that is understood in photo micro lithography. The basis of this, and a definition of "acceptable focus" was used to define the tables for DoF (which I agree, needs to be majorly updated/redefined in the digital world).

[tomspielman]

Indeed there's really no such thing as depth of field if you look close enough. There's one plane of focus only. (Extended by spherical aberration if you get picky!)

 

However, there is an area of acceptable focus surrounding that plane. Otherwise the concept of depth of field wouldn't have been invented. Sure, DoF tables make it look as if there's a sudden cutoff of focus at two fixed distances, but everyone knows they're just a guide. And they do work as a good comparator between format sizes, apertures and focal lengths.

 

Perhaps DoF tables or scales are less needed now that we have WYSIWYG viewing directly from the image sensor, but in the past even over-bright SLR screens didn't properly tell you what the final image was going to look like. And rangefinder or TLR viewfinders made DoF complete guesswork.

 

My DSLR is getting pretty old and my other digital cameras have smallish screens that aren't always easy to see clearly in bright light. For those reasons, getting a good preview of DOF effects isn't always that easy for me. Maybe modern mirrorless cameras with good ELFs do a better job.

 

So I appreciate DOF scales. But lots of people never use them even if they are on the lenses.

[Ed_Ingold]

We're seeing diffraction thrown about carelessly. Diffraction in an image is estimated by the following equation

 

d = 1.22wN, where d = the circle of confusion, w = wavelength, and n = the relative aperture (f/stop)

 

If we use green light (630 millimicrons) as a representative wavelength, the circle of confusion at f/5.6 is 0.004 mm, well below 0.02 mm considered the DOF limit for 35 mm film. Whether a lens is considered "diffraction limited" or not depends on many factors. In the age of film, Leica lenses were touted as "diffraction limited" at f/11 or smaller. In the digital age, the same lenses are limited at f/8, or even f/5.6. The cell spacing on a 42 MP sensor is 0.047 mm, whereas ordinary film is comparable to 6 MP.

 

There is nothing unusual or wrong with a practical fiction. The concept of a "reasonable person" in law is known as a "legal fiction."

 

As illustrated above, I have learned that the best use of a DOF preview is to test the effect of aperture on OOF areas, not how well the subject is in focus.

Edited August 27, 2020 by Ed_Ingold

[q.g._de_bakker]

The 'phenomenon' DoF is not a fiction, but the idea that it can be calculated in an absolute way is. It depends on variables that are so variable that no calculation is able to tell us what to expect, except assuming one specific value for those variables.

There is another fictional thing concerrning DoF. DoF calculators (most of them anyway) use formulae meant to calculate hyperfocal distance. While that is a DoF related thing, it is not applicable to limited DoF (i.e. DoF that has a finite far limit as well as a near limit. But those formulae are used as if anyway).

Another thing is that, for some unknown reason, focal length appears as a factor too often in those calculations.

 

Diffraction limited is another often misunderstood concept. It means that the performance of a lens, in terms of resolving power, is no worse than can be expected if only diffraction would limit resolving power.

Diffraction increases with decreasing aperture size. It cannot be avoided, and does not depend on how good or how bad a lens design is.

It isn't estimated, but can be calculated rather precisely, though only for a given wave length.

 

If lenses would be perfect, their resolving power is best wide open, decreasing steadily and predictably when stopping down. (That's why astronomical telescopes need to be large in diameter. Not so they would be able to gather more light, but to increase their resolving power. Same would appky to camera lenses - better the larger their max. aperture - but only if expense would not be an issue.)

Lenses aren't perfect so at wide apertures, uncorrected lens faults limit resolving power more than diffraction would. Stopping down a bit would decrease both the limiting effect of uncorrected lens faults, and the maximum resolving power diffraction would allow (it approx. halves with every 2 stops a lens is stopped down). If the gain due to limiting the lens faults' limiting effect is larger than the reduction of resolving power due to diffraction, you would see higher resolving power at a smaller aperture.

 

At small apertures, all lenses are diffraction limited (Leica's boast as mentioned above is empty. It is rather difficult to find lenses that are not diffraction limited at f/11 or smaller). At larger openings, it depends on the lens design, i.e. whether lens faults are more limiting than diffraction.

So being diffraction limited says not much, if anything at all, about a lens when discussing resolving power at small apertures. It can be a distinctive mark of the degree in which lens aberrations are corrected when discussing performance at large apertures. It can be usefull when comparing lenses, to see at what f-stop a lens' resolving power is no longer limited by lens faults. The better lens is the one whose performance is diffraction limited at larger apertures.

 

Film or digital capture makes no difference at all. If a lens is diffraction limited at any given f-stop, it is diffraction limited at that f-stop. I.e. it will be able to resolve no more than diffraction allows.

Whether the medium used to record such a diffraction limited image is able to record the image's resolution in full says nothing about the lens.

Edited August 27, 2020 by q.g._de_bakker

[q.g._de_bakker]

I suppose so.

 

But first, there is always diffraction. So no part if ever in perfect focus.

 

Next, no real lens is perfect, so it might have spherical and chromatic

aberration, so even less perfect focus.

 

So if depth of field adds a little to those, it will be pretty much not noticed.

 

Perfect focus is a fiction. Unless you define it as the best you can get, given the circumstances. And i think we should.

There is a limit to resolving power (hence never perfect focus in the sense used above), which varies with a number of variables (diffraction and lens faults). So i'd rather say that (best achievable focus depends on ... etc. and this is or is not the best we can get given those circumstances), than that there is no perfect focus.

 

Depth of Field is about how noticeable it is that some part of an image is not in perfect focus, i.e. not as sharp as the part of that same image with the best (attainable and/or actually achieved) sharpness.

And that's one factor not included in DoF formulae: DoF is not about an (illusionary) absolute degree of fuzziness, but how noticeable it is that something is not as sharp as the sharper or sharpest part of an image. That varies with things like viewing distance (i.e the resolving power limit of the viewer's eyes). But it also means that DoF decreases when a lens is used that has better resolving power, or when film or digital sensor is used that is able to capture more of what a lens delivers.

Edited August 27, 2020 by q.g._de_bakker

[glen_h]

(snip)

 

If lenses would be perfect, their resolving power is best wide open, decreasing steadily and predictably when stopping down. (That's why astronomical telescopes need to be large in diameter. Not so they would be able to gather more light, but to increase their resolving power. Same would appky to camera lenses - better the larger their max. aperture - but only if expense would not be an issue.)

 

(snip)

 

Stars are close enough, at the distance we are away, to point sources. Larger telescopes collect more light.

 

Planets are not point sources, and also are brighter. Larger lenses resolve more detail.

 

In Galileo's day, diffraction wasn't yet known. Stars appeared not to be points, and so were

believe to be much closer than they actually are. Older astronomy had celestial spheres, with stars all at the

same distance on a sphere of stars. I suppose when you do that, they should all be the same size,

as diffraction makes them appear in a telescope.

[rodeo_joe1]

OK. Here's something to ponder:

Stars are effectively point sources, right?

An illuminated surface can be thought of as a collection of an infinite number of point sources; agreed?

Yet the brightness of stars through a lens is governed only by the absolute physical aperture size, while an illuminated surface (=n.point-sources) has a brightness governed by the relative aperture.

 

Anyone else see a paradox here?

[q.g._de_bakker]

Telescope size is all about resolving power. So important that they build large arrays of large telescopes.

The size sets the base line length for triangulation. The bigger, the greater the difference in angle at the extremes. And considering how far away the objects observed are, even a telescope the size of the earth would still leave much room for improvement.

 

Angular resolution = 1.22 • wavelength / diameter of lens or mirror.

 

That stars are small enough to appear to be point sources doesn't make resolving power any less important.

[glen_h]

There are two main uses for large telescopes. One is viewing the (relative) positions, needing resolving power.

 

Another is looking at the emission spectrum which doesn't.

 

In both cases, if you don' have enough photons, it doesn't matter if you have enough resolution.

 

Telescopes

[q.g._de_bakker]

There are two main uses for large telescopes. One is viewing the (relative) positions, needing resolving power.

 

Another is looking at the emission spectrum which doesn't.

 

In both cases, if you don' have enough photons, it doesn't matter if you have enough resolution.

 

Telescopes

When you want to examine the spectrum of a star that you cannot separate from other stars due to lack of resolving power, you'll find out that you need resolution for both purposes equally.

 

Photons come in streams.

[rodeo_joe1]

Photons come in streams.

Or waves!

 

Does coherent light come in Mexican waves?

[q.g._de_bakker]

No, photons do not come in waves.

[glen_h]

Yes photons come in waves. Funny thing, isn't it.

 

[glen_h]

Or this one:

 

[Ed_Ingold]

The larger the telescope, the greater the effect of atmospheric disturbances. The 200" telescope at Mt.Palomar has an effective resolution of 0.2 arcsec, which is about what you get with a high-quality 4" refractor. (As a guide, the "Trapezium" in Orion spans about 2 arcsec.) The best astronomical resolution is now obtained with large arrays of smaller telescopes, compensated for atmospheric effects using lasers. The best use of a large telescope is collecting photos from extremely dim objects, for which individual photons can be counted over minutes and hours of exposure. The CCDs are cooled with liquid nitrogen to reduce thermal noise.

[q.g._de_bakker]

The probability of finding a photon at a certain place at a certain moment comes in waves. Photons do not.

Funny thing, wave particle duality

Edited August 30, 2020 by q.g._de_bakker

[q.g._de_bakker]

[...] from extremely dim objects, for which individual photons can be counted over minutes and hours of exposure. The CCDs are cooled with liquid nitrogen to reduce thermal noise.

That's it. Or days, or weeks.