Diffraction discussion continued

Started Jul 27, 2013 | Discussions thread
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quadrox
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Diffraction discussion continued
Jul 27, 2013

I wish to continue some of the discussion that I feel remained unresolved from the old thread (http://www.dpreview.com/forums/post/51847507). So if you don't care to participate, just close this thread and hope it dies a quick death.

I want begin by summarizing what I think most of us can agree on. If any of this seems like a contradiction to what I have said previously, then please consider that as me having lost track of what I originally meant to argue in the previous discussion (especially things about a certain website).

  1. Diffraction is not a reason to pick one camera/lens system over another, as equivalent photos will have the same amount of diffraction. Lens quality/selection and sensor quality/design are the only reason to go with a specific format when concerned with image quality).
  2. Diffraction has an effect on all images regardless of pixel size (but sometimes it is so small it is not noticeable).
  3. Diffraction will never make images taken with smaller pixel size sensors worse compared to images taken with sensors having larger pixels. Smaller pixels will always yield more detail (at least theoretically).

If I have contradicted any of the above previously, it was unintentional and just a matter of choosing my words poorly. Nevertheless, there is one issue where I feel there will still be disagreement between different posters, and I would like to try and resolve this in this thread.

Fun starts here

I realize than bayer patterns and AA filters complicate this discussion further, but for the sake of the discussion I would like to assume a FOVEN style sensor with no AA filter and no gaps between pixels.

While I absolutely hold points 2 and 3 above to be true, I also believe the following:

When considering viewing an image at 100%, there will be some pixel size for a given sensor format where any further decrease in pixel size (increase in sensor resolution) will not yield any practical increase in detail due to diffraction. Because the effect of diffraction per pixel will increase or decrease smoothly with pixel size, it is difficult to specify a precise threshold for when diffraction makes the picture appear blurry (when viewed at 100%).

Nevertheless one could argue for two specific thresholds - one where the image will definitely appear blurry, and one where it definitely won't (always considering 100% view).

The 'definitely blurry' threshold is probably around the point where the airy disc has the same size as the pixel size, probably even quite a bit earlier. Since the airy discs won't be aligned with sensor pixels, having them be the same size will virtually guarantee that some of them will be projected onto adjacent pixels, which would result in a slightly blurry image.

The 'definitely not blurry' threshold probably requires the airy disc to be quite a bit smaller than a sensor pixel - maybe one half to one quarter of the pixel size. If I understood the Nyquist stuff better, I could probably give a more precise figure. But somewhere around this number should guarantee that most pixels sample more than one airy disc, which will thus look reasonably sharp (I cannot explain it better - I would appreciate it if someone could rephrase this to make sense).

Does anyone disagree with the above? If you do, please make sure you have understood what I am trying to say, and try to assume I am saying something that is correct and see if you can make my words give meaning, instead of automatically assuming I must be trying to say something that is obviously wrong.

Thank you for your attention!

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