# Pixel density and lens quality

Discussion in 'Canon EOS' started by michael_young|3, Feb 6, 2010.

1. ### michael_young|3

A recent thread here asked about IQ issues on a 7D, and some of the discussion linked back to Merklinger's article on the Luminous Landscape (http://www.luminous-landscape.com/reviews/cameras/50d.shtml). To paraphrase, the pixel density of a 50D equates to 39 MP if extended to full frame. He concludes that image defects in his early tests were due entirely to the higher spatial resolution, and disappeared once the image was downsampled to more normal size.
The 7D's pixel pitch of 232 px/mm works out even higher, 46 MP if full frame.
Can this be right? P&S cameras with plastic gum drops (presumably, figuratively, and comparatively) for lenses have pixel densities ranging from mid-30 to 43 MP/cm^2. By comparison, the 5D2, 7D, and 50D have densities of 2.4, 5.4, and 4.5 MP/cm^2, respectively. If Merklinger's 24-105/4L is not sufficiently sharp for even the 50D's sparsely spaced pixels, how are the \$100 consumer cameras able to capture any image at all? Does it seem reasonable that a P&S houses the sharpest lenses made?
Also worthy of comment, HM goes on to say the 50D's pixel pitch makes the sensor diffraction limited at f/7.5, giving the relationship as N=1600/pixelpitch in millimeters. The 7D's 232 px/mm predicts diffraction softened images at smaller than f/6.9. Again in stark contrast, the P&S with 43 MP/cm^2 works out to 655 px/mm, making them essentially pinholes at smaller than f/2.4. (I guess that answers the first part!)

2. ### laur|1

Does it seem reasonable that a P&S houses the sharpest lenses made?​
What makes you think that is happening? At 100%, P&S 10MP images look close to a 5MP image seen at 200%. There's a point beyond which no more detail is being captured and the extra MP are just a marketing ploy. I think this ends up biting the industry in their rear-end as they first advertise cameras based on one single criteria and then they have trouble selling higher-end cameras to beginners that think their money should be buying them more MP. I think the fact that 7D comes up with 17MP instead of 10/12/15MP is sort of confirming this - camera companies still try to sell more MP.
I think that what you usually see clearly on high resolution DSLRs like 7D is that zoom lenses are inferior - their limitations become much more noticeable than before, because the sensor is good enough to capture that detail - on P&S it's just not capable of doing that, so if you can't see much detail, you won't see aberrations either.
I wish photosite density would be kept constant across cameras, sort of like how it was during the film era across various film formats - film was the same, except you got less of it on 135 than on 120. This would put an end to many useless discussions we have around noise performance, FF vs APC, etc - you would just pick the camera given your resolution needs (I am ignoring DOF issues, as I've always found those quite irrelevant - they're an advanced topic that most camera buyers won't ever know about - but noise is easily noticeable).

3. ### michael_young|3

Of course I don't think P&S lenses are sharper. Gumdrops, remember?
Maybe we can focus on the diffraction limited aperture for the sensors.
5D2: 155 px/mm, f/10.3 (also 1DsMk3)
7D: 232 px/mm, f/6.9
50D: 213 px/mm, f/7.5
300D: 136 px/mm, f/11.6
This roughly matches my experience with the 7D and 300D, and also with scrutiny of online ISO lens charts shot with the relevant bodies: diffraction limited at f/11 on FF; f/8 on 1.6x crop. It was explained to me (apparently in error) as a measure of quality of the lens that diffraction limit is reached at smaller f/ stop. I see now that this is not so, and is merely a function of pixel pitch. Maybe it's old news to you, but it certainly is a light bulb moment for me.

4. ### tim_klimowicz

Really cool info, Michael. In my own totally-unscientific tests with my 7D I was able to pretty clearly see the difference in sharpness between 5.6 and 8, where it got sharper and shaper as I went from 1.4 to 5.6, but then seemed to degrade as I went to 8 and beyond. It's interesting to see that the math backs it up with that figure of f/6.9.
Fortunately for me, I very rarely shoot beyond f/1.4 or f/2, so it's not really a concern for me 99.9% of the time.

5. ### michael_young|3

Further thoughts:
Color film (est. 9MP): 125px/mm, f/12.8
What are the consequences? For one, I can stop studying lens charts. The f/4 lenses I might have been considering are diffraction limited less than 2 stops down from wide open on the 7D. And for sure, TC 2x already pushes them past diffraction limit. I feel somewhat liberated.
Last.... I don't know that I agree completely with stopping the MP wars. We've only just very recently reached levels where the returns are diminishingly small. At any given moment in the past 5 years, somebody somewhere had already been saying that. So my very sharpest shots will be taken at wider than f/8. I'm weeping a little inside, but I can live with it. At worst, it's still better than "smaller" (but not lesser) bodies. At it's best, within this newly recognized limit, I already have shots that are pleasingly sharp, surpassing my very high expectations.

6. ### matthijs

Bob wrote smart things on this.

More pixels is not worse than less.

7. ### yoshio

Michael
my comment or thought is that everything is a swing, what you gain in one thing means you loose in another.
IF a lens for full frame has slightly less resolution than one for EF-S then it will be also benefited by the lower magnification applied to get the sensing area of 36x24 scaled up to a print of 8x12 than if taken on a 50D. The benefits of more pixels becomes a detraction with respect to noise related to the therefore smaller pixels. RN Clark has an excellent page on this on this.
If you really desire to enlarge more I believe strongly there is no substitute for larger capture format.It brings more with it than you think.
Lastly everyone forgets a key issue of lens testing, that is the process. You see when you test a lens you place it at a distance from the test chart which is related to the focal length . So when you use for instance Norman Korens test chart you place the lens at 51 times the focal length from the chart. For a 21mm lens this is 1.2 meters ... do you often photograph subjects at such distance?
Place the same chart at a more typical photographing distance and examine again what you get. Then compare this with a much lesser lens (say an EF24 f2.8) and tell me you can see any difference? I am willing to bet you also don't.
There are many many aspects to a good image, some of these technical issues may even be significant in that.

8. ### scott_ferris

Michael,
It is not helpful to isolate these figure,
"Maybe we can focus on the diffraction limited aperture for the sensors.
5D2: 155 px/mm, f/10.3 (also 1DsMk3)
7D: 232 px/mm, f/6.9
50D: 213 px/mm, f/7.5
300D: 136 px/mm, f/11.6"

They only mean something when referencing same sized enlargements. Diffraction is the same on all sensors at any given aperture. Think about it, the light doesn't know or care why it is being bent through a small opening, but it will react the same whenever it passes through a same sized opening regardless of the medium behind it. The reason the limiting figures are given differently is because the assumption of the refference is to arrive at the same sized COC at any given print size. You need to enlarge smaller sensors more to get to the same sized print. But if you have a tripod mounted 600mm f4 at f16 and take a picture of a bird with a 7D and then take the body off the lens and mount a 5D MkII and take a picture. Now enlarge those two images so the bird is the same size, not the print, just the bird, the images are identical, with identical diffraction and depth of field. Diffraction can only be referred to in the context of COC and that is only relevant when referring to reproduction size and viewing distance.
To question one area of the whole is impossible without a background understanding on all the other relevant aspects. So you need to understand COC, Airy discs, Nyquist limits, the concept of diffraction limited optics (good), optic resolution issues (imperfections, design limitations, manufacturing tolerances etc),sensor resolution, AA filter interference frequency etc .
The resolution limiting aspects are sensor density and how that is interfered with by the AA filter if present, diffraction and lens resolving power. If your lens is diffraction limited, ie it is not limited by its optics only by diffraction, then you are reduced to two factors, if your sensor is pixel dense enough to to resolve diffraction then you are limited to one, your image sharpness will be limited only by diffraction blur (your enlargement size and viewing distance). You normally end up with a trade off between the three.
If the optic is not limiting but the sensor is you are into Nyquist limit resolution until you stop down enough that the sensor can resolve the increase in COC. Most macro lenses will be in this example.
If the optic is limiting but the sensor isn't then you are done, once the diffraction gets worse than the optic limitations you are doubly done , most P&S's fall into this category.
Now with pixel dense sensors either the optics resolution or diffraction are normally the limits. Kit zooms will normally be optic limited initially but then move to diffraction blur. So stopping down past f10 or so the diffraction blur is worse than the lens blur.
Hope this helps, Scott.

9. ### g dan mitchell

A few thoughts:
• The density of the P&S cameras means little when it comes to determining what does and doesn't make sense. The very high photosite densities there are more about marketing than about photography.
• Watch out for that term "diffraction limited." It is easy to misconstrue what that is telling you. When a higher photosite density camera is said to have be diffraction-limited at a larger aperture, this does not in any way imply that a) that is the best aperture to use, or b) that the camera performs less well at smaller apertures when compared to cameras with smaller diffraction-limited apertures.
• Since it is impossible to imagine a photosite density that will provide resolution equal to that of lenses (for a range of reasons I won't describe), there are only two realistic choices: a) the sensor is capable of out-resolving lenses, or b) the lenses are capable of out-resolving sensors. I think most would prefer the former over the latter.
• Resolution is not the only issue. Even in situations in which the sensor out-resolves the lens, there is some thought that there can be advantages, for example you might obtain smoother gradients. If the cost of high photosite density sensors drops enough, cameras might do away with AA filters.
Finally, all of this is largely moot except in the most extreme situations: very careful and skillful photographer almost certainly using a tripod and making huge prints.
Dan

10. ### zml

Photography doesn't begin or end with DSLRs and P&S cameras... Digital MF is up to 60 mpixel (on a 40x53mm sensor) and I've been using a 39 mpixel back (on a 37x50 mm sensor) for a long while. Now, with most MF lenses being crap in terms of raw resolution compared to most pro-level 35 mm format optics, I should have ran into all kinds of "my back outresolves my lenses" issues but this is simply not the case. In fact, I can hardly think of a lens (both Mamiya MF and Canon EF L and even some non-L optics) that is not up to the PhaseOne 45+ back or an 1Ds3/1D3/1D4. And I'd welcome well documented, real-life - no charts and brick walls, samples of good quality optics not being up to the 7D or 50D sensor in terms of resolving power.
I can't say that I don't enjoy talking about how many photons will fit on a pinhead, but most of this stuff has absolutely no practical bearing on photography.
Edit:
Merklinger. Is that the guy who on the LL site proclaimed in 2003 (?) that the output from Canon D60 is on par with, or better than, MF film photography..?

11. ### mattb|1

IMO we are getting to the point where we the market will demand a revolution in lens design. Which I think is great news for us all. It will take photography where it could not go before. I can't say where it will go but I'm eager to see it unfold. Before that we will see a surge in applications that will try and correct the defects of lenses, somewhat like we have now but more effective and better work flow. Additionally, sensor design will also evolve at some point to make better use of the limits of lens design.
That said, I like reading luminous landscape and think they do a good job overall and I respect a lot of their opinions. However, they are not engineers and their speculation on this subject is more entertainment than fact. They may be some what correct in their estimation of current and past implementations of technology, they certainly can't say exactly what principles are in play or say precisely what is causing the IQ issues they are seeing. Could be that the 24-105 they used was a little off? I think it is good they are questioning the cameras on the market, it makes the manufactures try harder to provide better products.
IMO after about 15 MP technique and lens choice make major impacts to IQ when viewed at 100%. And, at 21 MP a lot of peoples technique can't keep up. Does that mean that we should settle for that level? Of course not, asking for more is going to push the engineering and technology.

12. ### scott_ferris

Matt,
I disagree, taking the stereo analogy, when CD's came out they were all sampled at huge rates. When people moved that music over to computers they realised there was little difference between native sampling rates at over 1,000bps and 192bps, indeed all but the keenest ear and on good equipment can tell the difference between full sized CD's and resampled 128bps audio. After the hype and when given a choice people are pragmatic and have common sense. Very few people print out images that are resolution limited at 21MP, and those that are have higher spec'ed equipment to buy anyway.
21-25MP is a practical overkill limit for most users, the lenses we have available, as Michael says, are generally more than up to the job and as you say, most people are not sufficiently proficient or interested in their photography to warrant or utelise even more MP. The next push will be ever higher iso, broader DR, better video etc, none of which will be limited by current lenses.
I agree with Michael 100% too. It is fun understanding the academic physics behind the process but until you actually run into image destroying lens/resolution/diffraction problems (which I have never seen in normal use, apart from a few very crappy lenses) then it is just a thought process and we shouldn't worry about it much.
Take care, Scott.

13. ### michael_young|3

Allow me to further refine the thought. These are all good and worthy thoughts, but the point I was making is quite a bit more simple. Given no other reason to select one aperture in preference to another, we select one that fits the lighting while giving us good sharpness. It's a very common thought (isn't it?). Until yesterday evening, because I had been reading lens charts as though they held an answer, I would have set f/8, and maybe stop it down a third or two further for just a bit more DoF. Two months ago, I would have set it even smaller, f/11 to f/16. A lifetime of shooting sharp pictures says that's a good target if the light allows. The 7D is my first to fall far from this range. A very casual browse through my LR catalog bears it out. f/5.6 and f/8 good; smaller not so good. If we define "good" as eye pleasingly sharp at 1:1.
Who should care? I care. I was unknowingly making an ill-informed choice, based on assumptions that don't apply to the current situation. Readers outraged or perplexed by Darwin Wiggett's blog should care. His proof of the 7D's inconsequence is based on f/16 shots. You might care, if you've ever set it on a tripod in bright daylight and locked the mirror to shoot the sharpest image possible. If you carry more than one lens in your bag, you probably care. For sure, if you can distinguish the quality of a prime versus a zoom, you care deeply. I think that covers everyone not carrying a P&S for its acceptable IQ.
So, is this a universal relationship, diffraction limited N=1600/pixelpitch? I believe it is, insofar as the common definition of DoF is universal and acceptable. The 1600 comes from the same CoC that defines DoF. Nobody to my knowledge argues its validity strenuously.
This is news. The effect is easily observable in casually shot images. Diffraction degrades images shot on the 7D at apertures smaller than the relatively large f/6.9. This differs considerably from prior experience, where f/11 was a reasonable and safe estimate. Whether or not it shows up in 20x30 prints is a separate question. It's readily evident at 1:1, a very important magnification because that's how I like to view some images. Surely, if mediocre was good enough, I wouldn't have bothered to have this thought.
But all that aside, it still doesn't explain the phenomenon that started all this. I have a Tokina 80-200/2.8 ATX lens that displays ruinous CA everywhere in the frame on the 7D, but was not objectionable or even noticeable on a 300D. None of the above seems to explain this.

14. ### scott_ferris

Michael,
It seems to me you are missing your own point.
When you enlarge you 7D image to 1:1, or pixel for pixel, you are enlarging it a lot more than you were your 50D. If you print out your 7D and 50D the same size the diffraction is exactly the same, it is just as blurred in both images. The diffraction is no more or less with the two cameras, but now, due to the increased pixel density, your diffracted blur is better resolved by the 7D. The effects of diffraction on the COC are entirely dependent on the reproduction size and viewing distance. You can't enlarge your 7D image more and say it shows more diffraction than your 50D, it doesn't, they are identical.

15. ### scott_ferris

Or put another way, compare your 300D (at 6mp) at 300% to your 7D (at18mp) at 100%. You will see the same CA and diffraction. You are looking at the detail far far closer on the 7D.

16. ### zml

IMO we are getting to the point where we the market will demand a revolution in lens design.​
Got any practical (i.e. based on know physics and not involving liquid nitrogen cooling...) ideas how to vastly improve transmission of visible light through a complex optical system..?

17. ### michael_young|3

That's all true, Scott. Nonetheless, two shots taken side by side at different apertures will show differences in the detail captured in the parts at critical focus. If I have no other reason to choose one over the other, meaning DoF and other considerations aside, I want the aperture capturing the most detail.
The difference isn't theoretical. It's quite pronounced in the tools I most often use to view pictures. Looking at the 2 shots side by side, one at f/7.1 and one at f/11, I won't say, "Oh, they're the same. It was certainly good enough when I had only 6 MP. Watch what happens when I downsample it and gain back all that wasted disk space. You'll see. They're the same. They have to be: the lens doesn't know or care. All that extra detail? You didn't need it. How big were you going to print anyway? It isn't all that good to begin with."
That doesn't sound like a conversation I would have. It actually went like this: "What the CR*P? I'd better go reshoot that."

18. ### scott_ferris

Michael,
You need a reality check on your viewing technique then. The diffraction and blur and DOF and CA are exactly the same on your 300D and your 7D, you are disappointed that your 7D doesn't look better than your 300D at 3x the magnification, that is crazy, it will look a damn sight better than the 300D at the same magnification!
It's analogous to saying, my old car went 60mph and did 30mpg, my new car is rubbish because although I go everywhere at 180mph it doesn't still do 30mpg. If you went 60 in your new one it would do more than 30mpg.
The 7D is way way better than the 300D, it suffers diffraction exactly the same as the older camera, you can't look at it 3x closer and not expect to see more blurring, you will see three times more, but it will give you more detail at the same size. 1:1 for the two cameras is not the same size, you have to understand that.

19. ### zml

The difference isn't theoretical. It's quite pronounced in the tools I most often use to view pictures.​
Photography has always been about the intended output so if you shoot for people who only pixel-peep @100% (on a 72 DPI monitor...a joke in itself if you get my drift...) then all I feel is pity. The truth is that on real life output - regardless of its size - on something different than a 72 DPI monitor neither you nor the the most acutely sensitive person would be able to tell apart a well-executed picture taken - say at f/8 and f/11, even though (say, for 1Ds3) f/10 is "diffraction limit." Now, of course we (photographers) should be aware of the technical aspects of the craft (after all photography is a technical endavour) but we also should keep the whole technical aspect in perspective (i.e. does it matter for the intended application...) And the size of the intended output is not always the most important consideration: say, if you shoot for glossies (esp. higher quality mags) you'll soon realize that you really need to put an extra "oomph" in your pictures because, despite their smallish size, they are intended for "nose surfing" - close-up viewing from the "average" reading distance. And so forth... It's good to be aware of all this stuff but I draw a line at demonizing it..

20. ### michael_young|3

Re. 300%: 173%, actually, or equivalently 58% going the other way. It's a squared relationship.
The analogy is a pretty good fit. You might not notice if you didn't have a reason to care. If you didn't understand the relationship or misremembered it in your haste, the disparity with your expectations will drive you nuts.

21. ### michael_young|3

This is crazy. If it means something to you that f/6.9 on the 7D is as sharp as you'll get, I'm glad I was able to help. It's contrary to long previous experience, not intuitively obvious, and explains a lot. Marginalizing the observation as "shooting only for peepers", or somehow turning it into a comparison with the 300D is just nuts.
I like these grapes; they're sweet and juicy.

22. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

23. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

24. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

25. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

26. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

27. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

28. ### kelly_flanigan|1

Here with my old 4x5 digital scan backs; the lens requirements are only modest. The 35 and 50 megapixel backs have a 7x10cm scan area. The small guy shoots a 5000x7000 pixel image; the big guy a 6000x8400 pixel image. 10cm is 100,000 microns; thus the smalll guy has a pitch of 14 microns; the big guy 12 microns. If the lens is *good enough* for the sensors pitch; one can get a sharp image.
The poor assumption you all have led your self into a mental rut is assuming that a 100 buck P&S digital has a bad lens; since your assumption is gravely in error; you conclusions are faulty and in error. Optically it is easier to make a good lens for a smaller format camera that a larger one. P&S digitals are made by the millions too; thus the tooling costs per unit sold are way less than a low volume lens.

29. ### g dan mitchell

Here is a simple thing that every photographer who wants to understand sharpness, aperture, and how the two interact in their gear.
On a slow day, take your camera and tripod and lenses out somewhere and shoot a series of test photographs. With each lens carefully focus on a subject that provides a decent target - don't use a test target, just a scene of the sort you usually photograph, but pick one that doesn't provide any real DOF challenges. With each lens:
• Camera on tripod.
• Focus by whatever means you prefer.
• Turn off AF.
• Shoot RAW.
• Starting at your largest aperture make an exposure at each whole f stop in aperture priority mode. Use mirror lockup (or live view) and a remote release.
• If you use a zoom repeat the process at several representative focal lengths across the range of your lens.
• Try with each of your lenses.
• Take the resulting images through your normal basic workflow including sharpening.
Take a close look at what you end up with. Compare center sharpness and corner sharpness at a variety of apertures. Note the amount of vignetting. Note the effects of barrel/pincushion distortion.
The object is not to see if you have a "good copy." It is not to determine what aperture you should always shoot at. The goal is to quickly understand the capabilities of your lenses so that you can intelligently apply choices about lens, aperture, etc to your photography. You'd learn much of this eventually by trial and error, but you can significantly jump-start the process by this means.
This I discovered about my lenses include stuff like:
• One lens has significant barrel distortion and vignetting at the wide end at f/4 but is still very sharp.
• Another provides surprisingly good bokeh for a f/4 lens.
• Another turns out to produce quite decent IQ at its longest FL but is even better at a slightly shorter FL.
• Resolution of one zoom is so good that it usually isn't worthwhile using a prime in its FL range.
• On a full-frame camera, diffraction blur is inconsequential at f/16 even in quite large prints.
All the theorizing in the world becomes quite irrelevant in the light of what you find out about your own gear by using it.
Dan

30. ### yoshio

Dan
noone wants to know for themselves, they may not believe their eyes. It requires them to interpret the evidence and have faith in themselves. Most will prefer to be told by experts. It reminds me of a "monty python" episode ...
"Is this the right room for an argument"​
but your test is a good idea

31. ### mike_hitchen

Michael Young: Have you taken the same picture with a 300D and 7D and printed them to the same size? Surely that is the only test that matters? I would tend to agree with Dan onthis one, viewing each image at 100% is irrelevant if your end point is to print so surely that should be the ultimate test.

32. ### michael_young|3

Mike, I first looked at slides and negatives through a microscope in 1972. Why don't we pick up the validity or value of understanding the image micro-structure some other time? The fact of the matter is, photographers will continue to examine their images up close, whether they plan to print that size or not. Well, maybe not photographers per se. Real Photographers (tm) such as yourself care only about the message conveyed by the image, the art of the image, rather than the image itself or the technology behind it.
Alas, photographers are also camera owners. Male camera owners in particular, being male and owners of newly acquired high tech equipment, will examine in excruciating detail the performance of their newly acquired, expensive equipment. They will have questions about what they perceive to be softness and inferior performance. Will you have an answer?
The newest, very fine pitch sensors are now able to resolve diffraction effects at surprisingly wide apertures. Ultimately, your answer of "Don't do that. It's stupid and meaningless." is itself stupid and meaningless. If instead you can put a number on it that they can understand -- f/6.9 on the 7D -- they'll move on and start taking some Real Pictures (tm) and stop shooting lens tests, sooner rather than later.

33. ### mike_hitchen

Michael - my point was not whether your comaprisons are stupid and meaningless (your words not mine) but was to pick up on Dan's comment about the validity of comparing 100% views from sensors of different resolution. By printing at the same size you are forced to compare apples with apples. Do you think a picture with f8 on a 7D (past its diffraction limit) will be sharper or less sharp than a picture taken with the same lens on a 300D (within its diffraction limit)
They will have questions about what they perceive to be softness and inferior performance​
Yep. Sure they will, and to do this it is not the data you collect, but (a) the question you are asking and (b) how you analyse the data and I feel you are not analysing the data in the correct way.
If you had the 7D and the 300D, and you wanted to shoot a landscape using f10, you would put aside the 300D and pick up the 300D because the 7D is limited at f6.9?
Male camera owners in particular, being male and owners of newly acquired high tech equipment, will examine in excruciating detail the performance of their newly acquired, expensive equipment.​
I agree completely . Been there, done that with hifi as well as cameras. And I still do sometimes

34. ### michael_young|3

would put aside the 300D and pick up the 300D because the 7D is limited at f6.9?​
Of course not, Mike. Is that how this conversation shapes up?
I hear the 550D will have similar MP as the 7D. Get ready for the deluge of even less sophisticated but completely expectable Y-chromosome driven questions about its crappy resolution.

35. ### g dan mitchell

Mike observed:
By printing at the same size you are forced to compare apples with apples.
Absolutely true! I believe that comparing at 100% on the screen actually tells us very liitle when it comes to comparing cameras with different pixel dimensions and sensor sizes. However, making photographs than then taking them through a typical workflow to a typical final medium at a given size - be that a print (my preferred form) or a uniform size on-screen display - is a much more meaningful real world test.
The classic misunderstandings are represented by a couple of types of posts that we frequently see on forum boards:
• "My 5D2 isn't as sharp as my 5D when I compare 100% crops from the two cameras." (Of course it isn't - you are looking closer at a smaller portion of the image!)
• "My 5D2 is diffraction limited at an earlier point as I stop down, so a 5D is better for shooting at smaller apertures." (The two cameras will produce precisely the same amount of diffraction blur in a print made from respective photographs at the same apertures.)
These and similar misconceptions arise all the time when people obsess over 100% magnification crop comparisons on the screen... but they disappear in actual output.
Dan

36. ### michael_young|3

Dan, knowing it won't stop them from clicking just once on the image in LR and getting the 1:1 view. Wouldn't it be nice to have the magnification default to a preset for "8x10 at 240 dpi, corrected for Epson 3800 dithering" instead? We don't have that today. It just snaps to 1:1. On what else would they base their questions?
The irony is we're all saying the exact same thing. Me personally, I would much rather have someone say to me, "Nah. Put on your sharpest lens and shoot it again at f/(whatever). It'll be tack sharp or something else is wrong." That curmudgeon stick of "Well, don't look at it if it bothers you" is so completely meaningless I haven't the words to begin to rebut.

37. ### g dan mitchell

Michael, it still wouldn't resolve (pardon the pun) the issue, though there is a "as printed" sizing option in Photoshop and I presume in Lightroom. Here you would still dealing with different and less than optimum interpolation.
Really, the only meaningful result is one that is based on the final photograph in whatever form you intend to present it. If you always (or mostly) go to jpg at size X by Y, then it is very important to understand how different lens/body systems will or will not differ. If you print, seeing the result in a print is the most significant thing.
Dan

38. ### scott_ferris

Michael,
Your assumptions are that less educated owners are as naive as you are obtuse. You are trying to make a point about others not understanding something that you can see. They can see it too, but don't think they don't have the common sense to realise they can't do 30mpg at 180mph. Whilst your point is valid, anybody with an ounce of common sense can see through your "issue".
Take care and time for another bottle of wine, Scott.

39. ### michael_young|3

Let me see if I understand you right. Downsampling the larger image to the smaller image's downsampled dimensions gives an equivalent image that's valid for comparison.
Thanks a bunch. Why not point out that water is wet? Just how much new understanding did that inject into the conversation?

40. ### michael_young|3

Not at all, Scott. I shot something at f/7.1 and it looks sharp. I stopped down further with the expectation that it would be sharper still, and it wasn't. Same body, same conditions, same lens, same viewing software, moments apart. Shouldn't I want to understand why? Now that I can put a number on it, I do wonder why water being wet is such an insight while this is so shruggable.

41. ### scott_ferris

That is now a second issue, are you trying to stir up a "my 7D is not as good as my 300D" comment or are you ignoring the answers to "my 7D is not as good at f8 as it is at f7.1"?
You know the answers to both. It takes no real understanding.
Again, your 7D is magnifying everything, CA/diffraction/lens limitations everything, more than your 300D, enlarge them the same and the 7D will look better. Your 7D images look sharper at f7.1 than at f8 because the engineers have done such a good job you can now resolve diffraction blur accurately at f8, your 300D wasn't good enough to do that. So again your 7D is better than your 300D.
Your new 7D does not break the laws of physics, diffraction is more at f8 than f7.1. Zooming in to 100% on the 7D allows you to see that, on the 300D it did not. You have bought a better magnifier, why are you complaining because you can see smaller things?
Like I said, you raise a point, a small and inconsequential one, but a point. Accept that anybody that noticed this, seeming anomaly, and did not have your understanding of it would have been lead to the answer long before this.
Back to your bottle of wine, Scott.

42. ### michael_young|3

I can hardly claim credit for discovering that images suffer from diffraction. I merely learned to determine with precision where that limit is. You dense brick. I'll be by tomorrow to install the breathalyzer on your keyboard.

44. ### michael_young|3

Yup. All of that. Followups to usenet, please.

45. ### mike_hitchen

Micheal. I ams till trying to udnerstand what pont you haev been getting at in this thread.
OK, the 7D is diffration-limited at 6.9, the 300D is diffraction-limited at 11.6. Now that is fair enough and the only conclusion you can draw is that the 7D is able to demonstrate the diffractino limitations before the 300D is.
But after that you seemed (to me at least) be then using these figures to try and define one characteristic of 'the better camera'. Given that diffraction is a characteristic of the lens not the camera, if you used the same lens on the 7D and 300D, I would hope to God the engineers at Canon have done a good enough job that despite its diffraction limit being 6.9, the 7D would still be better than the 300D at 11.6 when using the same lens on both cameras.
And this being the case, who cares how the diffraction limits compare - the diffraction limit is only relevant when using a specific camera under specific conditions and you want to know its limitations. Inter-camera comparison is almost meaningless.

46. ### g dan mitchell

OK, the 7D is diffration-limited at 6.9, the 300D is diffraction-limited at 11.6. Now that is fair enough and the only conclusion you can draw is that the 7D is able to demonstrate the diffractino limitations before the 300D is.
But after that you seemed (to me at least) be then using these figures to try and define one characteristic of 'the better camera'. Given that diffraction is a characteristic of the lens not the camera, if you used the same lens on the 7D and 300D, I would hope to God the engineers at Canon have done a good enough job that despite its diffraction limit being 6.9, the 7D would still be better than the 300D at 11.6 when using the same lens on both cameras.
And this being the case, who cares how the diffraction limits compare - the diffraction limit is only relevant when using a specific camera under specific conditions and you want to know its limitations. Inter-camera comparison is almost meaningless.
At the risk of getting sucked into something that I may regret... an observation that might help people understand what "diffraction limited" doesn 't mean.
I don't know the figures, but let me grant for the sake of the example that a 7D might be "diffraction limited" at f/6.9 and the 300D might be "diffraction limited" at f/11.6. If you aren't careful, you might misinterpret this to mean that you should shoot the 7D at f/6.9 and the 300D at f/11.6, or you might imagine that you would get a worse photograph from the 7D at f/11.6 than you would from the 300D. Both assumptions are wrong and betray a serious misunderstanding of what so-called diffraction limiting means and how it affects photographs.
Imagine the following experiment. You have a 300D and a 7D. You have a really excellent lens that will work on both cameras. You first set up the 300D and carefully focus and make a photograph at some aperture, let's say f/11. Then you remove the 300D from the tripod, mount the 7D in its place and attach the same lens you used for the first image made with the 300D. You carefully focus and using the same aperture you make the same photograph.
Take both photographs through optimized workflow and make prints at the same size. The amount of diffraction blur in each will be identical . Repeat the experiment at any aperture and the result will be the same - the amount of diffraction blur in your print will be the same.
Dan

47. ### scott_ferris

Mike and G Dan,
This is a very good link , it explains, and demonstrates the difference between diffraction limited aperture and the diffraction cutoff frequency, via another thread in which I also posted that link I seem to have placated Michael somewhat.
But shhhhh, we don't want to wake him

48. ### michael_young|3

Sheesh, Guys! No angst here, or at least not any more than usual. Why don't we start over?

I first stumbled across DLA mentioned in passing in Merklinger's article on the Luminous Landscape site. I later read Scott's first words about it written in Oct, 2008. TDP reviews first reported DLA figures in their equipment reviews with their review of the 50D. Me personally, I had been living under a rock until buying the 7D in November. Up till then, I had been happily shooting the 300D, in all its glorious 6 MP, since it was first released in, when was that, September, 2002? 2003? If there's any contradictions in the thoughts I expressed earlier, it's because I've been trying to make sense of the concept. Stay tuned for even for more refinements, revisions, and reversals. DCF used to mean discounted cash flow. Now, it'll be a mantra until I work out a better figure of merit for choosing an aperture when shooting smaller than DLA. After that, I'll likely fall back asleep under that rock until the 7D's replacement obsoletes physics.

Merklinger gave the simple relationship of: DLA = 1600/pixelPitch in millimeters. Equivalently, that's DLA=1.6 * pixel spacing in um. How convenient. I multiply by 1.6 easily in my head, having apparently hardwired some neurons to do so for crop sensor related thoughts. The long and short is, I soon had two numbers in hand: f/6.9 for the 7D, and whatever it was for the 300D. They match my experience exactly. Where f/11 had been a perfectly good best sharpness aperture for film and the 300D, it produced mush on the 7D. Mush only at 1:1 to be sure, but mush nonetheless.

Mush relative to what?

Sure, the print is the final measure of image quality. No question about it. I don't exhibit in galleries, and I don't hawk them at craft shows. The ones that are good enough to print, I've tended to print as large as I can. I am therefore looking for the best image quality for the circumstance, and that includes sharpness of detail. I care very much about mush and its converse. With all due respect to anyone else's measure of goodness, I buy L series lenses for reasons other than their color scheme. Surely Canon doesn't make them just for me, in case I might want to buy one. I'm not alone in caring about sharpness.

I recognize that peeper-baiting is common sport here and perhaps elsewhere. In an equipment forum, though, it can't be anything but noise and flaming. I'm not here to discuss pinhole cameras and alternate processes, or even fine art and exhibitions. I'm here talking with you because the quality of the image means something to me.

And so that brings us back to DLA, and DCF. Knowing where it is on your gear is a good first step, but not enough. I want to know what I'm trading off when I stop down further, as is often the case, for other considerations. I believe the trade off can be expressed meaningfully as "no larger than X by Y at full-frame at this aperture."

As to why focus only on aperture, ... We can solve global warming and Bayer noise some other day. Aperture isn't the only choice or decision to be made, but it's the one that changed the most for me when I bought the 7D.

49. ### g dan mitchell

Sure, the print is the final measure of image quality. No question about it. I don't exhibit in galleries, and I don't hawk them at craft shows. The ones that are good enough to print, I've tended to print as large as I can. I am therefore looking for the best image quality for the circumstance, and that includes sharpness of detail. I care very much about mush and its converse. With all due respect to anyone else's measure of goodness, I buy L series lenses for reasons other than their color scheme. Surely Canon doesn't make them just for me, in case I might want to buy one. I'm not alone in caring about sharpness.
You won't get any less detail at f/11.6 on the 7D than on the 300D in your "large as I can print" images.
If you prefer to test with something other than a print, consider an onscreen jpg at some realistic size and do the same comparison - even though you won't be able to see the whole image on a laptop try 1000 x 1000 pixels.
The "diffraction limited" business never favors either camera over the other. It is absolutely neutral and is a lens property that is independent of whether you use a \$50 used kit lens or a \$2,000 prime - you get the same diffraction either way.
The camera that becomes diffraction limited at a larger aperture actually has an advantage if you intend to make big prints. If you compare the two cameras at a small aperture equal to the diffraction-limited aperture of the camera with "more megapixels" the effect on sharpness will be equal and might be the limiting factor to some extent assuming you use good lenses. If you use very good lenses and continue to open the aperture the "more megapixel" camera can continue to improve the resolution of the image - but the "fewer megapixel" camera will not get any better.
Dan

50. ### michael_young|3

Dan, the only comparison I'm making is between what I'm now holding in my hand and what I'm now holding in my hand. I'm interested in capturing the most detail I can, if that's what I'm doing, consistent with the shot in front of me that moment. If it mattered all that much, I would have driven the pickup truck and unlimbered the view camera trunk instead. But I didn't and likely won't ever again. What matters that moment, when considering which aperture to shoot, if I decided that's what I care about, is whether f/7.1 or f/whatever else is more optimal. It's much simpler this way. I don't even carry the 300D.

51. ### michael_young|3

Ahh, fuhget about it. DLA/DCF crashes headlong into output-referred criterion, primarily DoF, and can't be so readily reconciled. It'll take a larger brain than mine that has spent more time thinking about this to make sense of it.

It is still worth noting, though, that the effect and concerns are real, and a relatively new phenomenon with the very fine pitch, small sensors. Previously, DoF conflicted minimally with DLA. The important apertures of f/11 and f/16 had relatively little loss of detail with diffraction. To that end, the following tables list the equivalent megapixels of detail for various apertures smaller than DLA, for each of several bodies for comparison. I included the 300D, since that is my most recent frame of reference.

The pixels/mm values come from the same equivalence of DLA = N = 1.6 * pixel_pitch, but reversed to solve for pixels/mm. If this is valid, the equivalent MP values are valid. The short summary version is this: for apertures smaller than DLA, effective resolution depends only on sensor size, not pixel spacing or density. Bigger, of course and as always, is better.
7D
f/N Pixels/mm Equiv. MP
6.88​
232.6​
17.9​
DLA
8​
200.0​
13.3
11​
145.5​
7.0
16​
100.0​
3.3
22​
72.7​
1.8​
1DMk4
9.12​
175.4​
16.0​
DLA
11​
145.5​
11.0​
16​
100.0​
5.2​
22​
72.7​
2.7​
5D2
10.24​
156.3​
21.1​
DLA
11​
145.5​
18.3
16​
100.0​
8.6
22​
72.7​
4.6​
50D
7.52​
212.8​
15.0​
DLA
8​
200.0​
13.3
11​
145.5​
7.0
16​
100.0​
3.3
22​
72.7​
1.8​
300D
11.84​
135.1​
6.3​
DLA
16​
100.0​
3.4
22​
72.7​
1.8

52. ### g dan mitchell

Michael, am I misreading you, or are you suggesting that I'd get less detail at f/11 on a 18 MP cropped sensor camera than I would at f/11 on a 8MP cropped sensor camera if the same lens is used in both cases?

53. ### alan_bryant|1

If this pissing contest continues much longer, I'm going to start building an ark

54. ### michael_young|3

Dan, they should be the same. Are they not? Oh. The 300D's sensor is a few square mm larger, 22.3mm for the 7D, 22.7mm for the 300D. In other words, if they are diffraction limited, the resolution depends on area alone.

55. ### michael_young|3

An 8 MP APS-C (22.3mm wide) sensor is diffraction limited at f/10.3. Its performance at f/11 is degraded by diffraction to the same effective resolution as other diffraction limited APS-C sensors at that aperture: 7 MP .

56. ### g dan mitchell

An 8 MP APS-C (22.3mm wide) sensor is diffraction limited at f/10.3. Its performance at f/11 is degraded by diffraction to the same effective resolution as other diffraction limited APS-C sensors at that aperture: 7 MP .
OK, I think we're on the same page. Neither sensor image is more or less degraded by diffraction at any aperture.
Dan

57. ### michael_young|3

Yup. It also doesn't change with sensor size. The figures above maybe muddied things by including Equiv. MP. The important number is pixels/mm, which is really lines/mm, a more common definition of sharpness. It also translates more readily to print size.
Notice that the 5D2 sensor appears to have been engineered for about 150 lines/mm, or f/11. Maybe a different way to look at the 7D is as the center crop of a (non-existent) 46 MP full frame sensor. That's relevant for birders shooting for very fine plumage details at wide apertures with their super-lens primes. No real need to end the MP wars just yet, and conversely hasn't been compelling need to chase escalating MP's for some time. It just depends on what you need, and if you know how to wring it out of the equipment you own.

58. ### g dan mitchell

Well, it does change with sensor size. A diffraction blur of a given measured size on the sensor is smaller as a fraction of the overall frame width as the format size increases. This is why you can shoot at smaller apertures on larger formats without giving up image sharpness.
The fact that sensor/film size makes a difference to potential resolution and selection of aperture was a given with film and continues to be with digital capture. (Whether the difference does or does not make a difference to a given photographer is a different question.)

59. ### michael_young|3

Call it what you want, Dan. The lens is still diffraction limited to 150 lines/mm at f/11, no matter what you stick behind it. It would help me, but I've already given up hope of it, if you'll separate output side references from strictly input side discussions. Doing so might even help you keep from sounding like such a muddled mess. The only thing _I'm_ discussing here is whether the sensor can resolve and capture those 150 lines/mm. If it can, the sensor is diffraction limited at that aperture. If it can't, it isn't.

60. ### g dan mitchell

"separate output side references from strictly input side discussion?" Pardon me?
And thanks for the advice on how I can avoid "sounding like a muddled mess." Much appreciated.
Have fun... Bye now.

61. ### michael_young|3

If you're going to pout, we're not gonna play. I had mixed feelings about including the equivalent MP. They do drive home the point that a 1Ds can be reduced to the same pixel dimensions as my cellphone. Mayhaps you're right. MP numbers reach a gut-level understanding that the more abstract lines/mm figures don't.
As for not losing sharpness at small aperture... What have we been talking about if not losing sharpness?

62. ### yog_sothoth

Keep in mind that a Bayer sensor (or any real sensor) may need more MP than the theoretical minimum to resolve those line pairs.