Diffraction 101: continued

Started Feb 19, 2010 | Discussions thread
Daniel Browning
Senior MemberPosts: 1,058
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Re: Diffraction 101: continued
In reply to eNo, Feb 20, 2010

post some samples that show diffraction one format at different pixel counts

Here are some sample images that do just that. They are based on the following ISO 12233 shots from the-digital-picture.com:

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=245&Camera=453&Sample=0&FLI=0&API=6&LensComp=245&CameraComp=356&SampleComp=0&FLIComp=0&APIComp=6

This comparison is the 5D (12 MP) with the 1Ds Mark 3 (21 MP) using the EF 200mm f/2.8. There are several imperfections in the comparison:

  • The two cameras likely have different OLPF MTF curves relative to their respective Nyquists (5D looks weaker to me).

  • Although the same raw conversion software (DPP) was used for each camera, it's possible that the versions differed.

  • The same DPP settings were used for each camera, but Canon might be using slightly different raw conversion behind the scenes for each camera model.

  • This is with very little sharpening ("1" in DPP) applied. Additional sharpening or RL deconvolution (in images that aren't to noisy to do that) would greatly increase the contrast in the diffraction limited images and make them even more suitable.

I have simulated the same print size by re-sizing the center crops with ImageMagick.

Set "f/5.6" is below: The 5D and 1Ds Mark III at f/5.6. Aliasing/demosaic artifacts (green and color patterns) are clearly seen. As expected, the 1Ds Mark III, with over 50% more pixels and the same sensor size, has higher resolution. This set establishes a baseline of how much improvement is possible when the effect of diffraction is minor. (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 relative to f/5.6 above, 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 from smaller pixels have diminished, but not to 0%.

Note that in all the cases above, the higher megapixel camera provided more contrast (MTF) in addition to the increased resolution -- this is largely due to the beneficial effect that smaller pixels have on the OLPF.

I think this shows that as you increase resolution in the face of diffraction, there is a long ways between the point of diminishing returns and the point where returns diminish to nothing.

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