Myth busted: smaller pixels have more noise, less dynamic range, worse diffraction, etc.

[Colin]

So if I ditch my 40D and pick up a 5DII FF Camera my pictures will be sexier with my L glass?
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Depends on who you're photographing.

Great point. Good models (hot or not) are a great advantage. Appreciate them

[cian3307]

I see the new G11 has a 10MP sensor, reduced from the 14.7MP of the G10. Is this the beginning of the end for the MP race? Canon are marketing the G11 as having better IQ due to the lower MP's!

[Daniel Browning]

I see the new G11 has a 10MP sensor, reduced from the 14.7MP of the G10. Is this the beginning of the end for the MP race?

It's possible.

Canon are marketing the G11 as having better IQ due to the lower MP's!

Many photographers have been clamoring for lower MP's for years, begging manufacturers to cut resolution. Now that Canon has finally given them what they asked for, I would expect them to take full advantage of the situation and say that reducing the number of pixels had tremendous benefits on image performance. Even if it's a total fabrication, I expect Marketing to say whatever users want to hear if it will sell more cameras.


What's weird is that when I read the press release, it says the noise improvement is only due to an improved sensor and software. Nothing about the benefits of lower resolution. I expect Canon will remedy that situation quickly. They wont let a marketing opportunity go untapped.


Where do you read that Canon is saying the IQ is "due to the lower MPs" and not due to an improved sensor/DIGIC?

[Jon Ruyle]

Many photographers have been clamoring for lower MP's for years, begging manufacturers to cut resolution.

I wonder how many of these same photographers use 1.4x and 2x extenders, which one only needs to do if one doesn't have enough resolution. (Or if one feels the need to see something cropped to a particular size in the viewfinder, I suppose... I've never found that useful but I guess some people do).

[clemmb]

Daniel Browning,
I am slowly coming around. I am an electrical engineer by day and a self taught professional photographer by night and weekend. I have a reasonable understanding of these things but have not studied them as in depth as you obviously have. I agree now that, (smaller pixels have more noise, less dynamic range) is a myth that has been busted but, your heading implies that worse diffraction is also a myth. I am not on board with this yet so lets discuss.

In Bryans Canon EOS 50D Digital SLR Camera Review he states that he generally regretted going much past f/8. I have run tests with my cameras to see what I could find. On my 5D I can see this affect very slight at f16 and a little more at f22 but still very exceptable. Realistically in a print of almost any size viewed from a normal distance this could not be noticed at all. Same goes for my XT. With my XTi it backs up a stop. I believe it is at the unacceptable or near unacceptable point at f22 for the XTi. It is extremely rare that I ever shoot at f22 but do sometimes shoot at f16 and f11.

Now my rough calculation of the 7D is that it&rsquo;s DLA is 6.9. Would I be like Bryan and regretted going much past f/8 with the 7D? I am very curious to see some test of this camera at high f-stops/small apertures. I downloaded some full file images from a 7D from the canon website. I must say, I am impressed.
I also downloaded some from the 5DmkII and 1DsMkIII. These images are really amazing. I am sure my next camera will probably be a 5DmkII.

Give me your thoughts on DLA and/or educate us on how this is a myth.

Mark

[Chuck Lee]

Would I be like Bryan and regretted going much past f/8 with the 7D?

Yes.


Not to butt in, but Mark you can go to Bryans ISO charts for almost any lens and see the diffraction effect on APS-C vs FF. It is pretty obvious and is why he makes the claims he does.

[clemmb]

you can go to Bryans ISO charts
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I have seen these. What you see in these charts may not be noticable in an inlargement viewed from a normal viewing distance.


Mark

[Daniel Browning]

Thank you very much for the response, Mark!

...your heading implies that worse diffraction is also a myth. I am not on board with this yet so lets discuss.

I meant to discuss diffraction, but I completely forgot about it. It's one of my favorite topics, so I'm glad you brought it up!

In Bryans Canon EOS 50D Digital SLR Camera Review he states that he generally regretted going much past f/8. I have run tests with my cameras to see what I could find. On my 5D I can see this affect very slight at f16 and a little more at f22 but still very exceptable. Realistically in a print of almost any size viewed from a normal distance this could not be noticed at all. Same goes for my XT. With my XTi it backs up a stop. I believe it is at the unacceptable or near unacceptable point at f22 for the XTi. It is extremely rare that I ever shoot at f22 but do sometimes shoot at f16 and f11.

To clarify for the reader, I would point out that comparing the 5D and XTi is mixing two effects: sensor size and pixel size. One must factor out the effect of sensor size in order to draw conclusions about pixel size. (And you may have done that; I'm just sayin').

Now my rough calculation of the 7D is that it&rsquo;s DLA is 6.9. Would I be like Bryan and regretted going much past f/8 with the 7D?

If you are happy with *some* improvement, then you will not regret it. Diffraction will never cause the 7D to have *worse* resolution. But in extreme circumstances (e.g. f/22+) it will only be the same, not better. At f/11, the returns will be diminished so that the 7D is only somewhat better. (If you use the special software below, you can get those returns back.) In order to enjoy the full benefit of the additional resolution, one must avoid going past the DLA.

Let's compare the XT and 7D. The maximum theoretical improvement in linear resolution that would be possible going from 8 MP to 18 MP is 50% (sqrt(18/8) or 5184/3456). That means if the XT can resolve 57 lp/mm, then the 7D could resolve 86.4 lp/mm (50% higher). But that would only be true when you stay under the DLA. At f/5.6, you should be able to get the full 86.4 lp/mm. But at f/11, you will get something in the middle (say, 70 lp/mm). At f/18 you're back down to 57 lp/mm again. (For green light. Blue has less diffraction and red has more.)

Give me your thoughts on DLA and/or educate us on how this is a myth.

There are many things that can affect the resolution of an image, including diffraction, aberrations, motion blur (from camera shake or subject movement), and mechanical issues such as collimation, back focus, tilt, and unachieved manufacturing tolerances.

There have been some claims that these issues can cause small pixels to actually be worse than large pixels. The reality is that all of these factors may cause diminishing returns, but never cause returns to diminish below 0%.

The most frequently misunderstood factor in diminishing returns is diffraction. As pixel size decreases, there are two points of interest: one at which diffraction is just barely beginning to noticeably diminish returns (from 100% of the expected improvement, to, say, 90%); and another where the resolution improvement is immeasurably small (0%). One common mistake is to think both occur at the same time, but in reality they are very far apart.

Someone who shoots the 40D at f/5.6 will get the full benefit of upgrading to the 7D. The returns will be 100% of the theoretical maximum improvement. Someone who shoots the 40D at f/11 will *not* get the full improvement. The returns will be diminished to, say, 50%. Someone who shoots the 40D at f/64 (for DOF) will not get any increased resolution at all from the 7D. The returns have diminished to 0%.

Under no circumstances will the smaller pixel ever be worse, and usually it is at least somewhat better, but sometimes is only the same. When the returns diminish to 0%, it means that the sampling rate is higher than the diffraction cutoff frequency (DCF). This is different from the Diffraction Limited Aperture (DLA).

Diffraction is always there. It's always the same, no matter what the pixel size. When the f-number is wider than the DLA, it means that the image is blurred so much by large pixels, that it's impossible to see the diffraction blur. Smaller pixels simply allow you to see the diffraction blur that was always there.

The DLA is the point at which diffraction *starts* to visibly affect the image. It is not the point at which further improvement is impossible (the DCF). For example, the diffraction cutoff frequency for f/18 (in green light) is 4.3 micron pixels (the 7D). So if you use f/18, then you can upgrade to the 7D and still see a benefit. For example, if you compare the 50D and 7D and f/11, you'll see an improvement in resolution, even though the 50D DLA is f/7.6.

Another important factor is that diffraction can be deconvolved in software! Normal sharpening helps, but specialized algorithms such as Richardson-Lucy are really impressive, and there are several free raw converters that include that option. There are two important limitations: it doesn't work well in the presence of high noise power (at the sampling frequency), and we don't have the phase information of the light waves. The practical result of these two factors is that RL deconvolution works great at ISO 100 for increasing contrast of frequencies below the diffraction cutoff frequency, but it cannot construct detail higher than the cutoff. (I haven't seen it, anyway.)

Lens aberrations can be an issue too. Usually even the cheapest lenses will have pretty good performance in the center, stopped down. But their corners wide open will sometimes not benefit very much from smaller pixels, so the returns in those mushy corners may be 0-5% due to aberrations. Stopped down, though, many cheap lenses are surprisingly good.

And there's the mechanical issues. If the collimation is not perfect, but it's good enough for large pixels, then it will have to be better to get the full return of even smaller pixels. This relates to manufacturing tolerances of everything in the image chain: the higher the resolution, the more difficult it is to get full return from that additional resolution. Even things like tripods have to be more steady to prevent diminishing returns.

OK, as a reward for those of you who read through this long-winded post (novella?), here are some pretty pictures. First, a warning. I'm about to do something morally wrong and illegal by manipulating some of Bryan's copyrighted photos and redistributing them on his own forum. Kids, don't try this at home. (And Bryan, sorry in advance.)

This comparison is the 5D (12 MP) with the 1Ds Mark 3 (21 MP) using the EF 200mm f/2.8. The 5D has a much weaker AA filter, relative to the pixel size, than the 1Ds3, so that will skew the results in favor of larger pixels looking better. Furthermore, although the same raw conversion software (DPP) and settings were used for each camera, Canon might be using a different de-Bayer algorithm behind the scenes for different camera models (I don't know).

I have simulated the same print size by re-sizing the center crops with a good algorithm. Do not examine the thumbnails below: you must click on the thumbnail to see the full sized image. (The thumbnails themselves are not intended for analysis.)

Set "f/5.6" is below: The 5D and 1Ds Mark III at f/5.6. There is no visible effect at all from diffraction in either camera. The aliasing/debayer artifacts (green and color patterns) are a natural result of the weakness of the anti-alias filter. As expected, the 1Ds Mark III, with over 50% more pixels, has higher resolution. This set establishes a baseline of how much improvement is possible when there is no diffraction at all. (Some people have a hard time seeing the difference between 12.8 MP and 21 MP, so look carefully.)





Set "f/8" is below: The 5D and 1Ds Mark III at f/8.0. Diffraction is beginning to have a very slight effect here, which is noticeable on the 1Ds, but not the 5D. It is softening the very highest frequency of detail. The 5D's 8.2 micron pixels add too much of their own blur for the diffraction to be visible.





Set "f/11" below: The 5D and 1Ds Mark III at f/11.0. Now diffraction is very obvious, even in the 5D. But it's plain that the 6.4 micron pixels still resolve more detail.





Set "f/16" below: The 5D and 1Ds Mark III at f/16.0. This focal ratio results in a *lot* of diffraction, as you can see. However, you can still see that the 21 MP provides more detail than the 12 MP. The difference isn't as large as f/5.6, above, but it's there. Returns have diminished, but not to 0%.





Furthermore, note that in all the cases above, the higher megapixel camera provided more contrast (MTF) in addition to the increased resolution. Yet this is with very little sharpening ("1" in DPP) applied. RL deconvolution would greatly increase the contrast in the diffraction limited images.

To summarize: the diminishing returns depend on the circumstances, but the higher the resolution, the more often the returns will be diminished. So there will be many times where smaller pixels provide higher resolution, and some times where they only have the same resolution, but never worse.

[clemmb]

I meant to discuss diffraction, but I completely forgot about it.
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Easy to understand how you forgot. With as long a disertation as this was, it is easy to forget where you started.


As usual great information. Your explanation helps quite a bit.


It will be a while before I upgrade a body(camera of course, no hope for me) but I am wanting the 5DmkII. I love my 5D and only keep my XTi for back up. Gave my XT to my son. My next big purchase will be glass. Probably 70-200 f4 IS.


Thanks for the response


Mark

[cian3307]

Hi Daniel, I've been reading through your DLA theory and think I have grasped the gist of it. AmI right in saying that DLA is always present but the higher the density of a sensor, the sooner it becomes visible. The less dense the sensor, the less able it is to resolve the DLA?