Why does machine-vision circle of confusion use pixel pitch or Airy disk, while photographic DoF CoC usually does not?
Asked 10/2/2023
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I’m confused by two different uses of circle of confusion (CoC). In a machine-vision paper, the permissible CoC is defined from the larger of the sensor pixel pitch or the Airy disk diameter. But in photographic depth-of-field discussions, people often say pixel size usually does not determine CoC, and that viewing conditions matter more.
These seem contradictory. If pixel pitch and diffraction limit resolution at the sensor, why aren’t they normally used as the CoC for photographic depth-of-field calculations? What is the missing distinction between these two contexts?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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Pixel pitch does not determine the standard CoC limit, and it is not a factor considered in the DoF calculations.
That's because the accepted CoC limit for normal viewing by an average individual is far less demanding. The CoC standard requires less than 2MP of image resolution for viewing any size displayed image... every camera far exceeds that requirement. I.e. the limit is human perception (~20/40 vision viewing an image from a distance equal to the image diagonal (~45˚ HFOV)). Note that most images viewed on your computer are at ≤2MP.
But the airy disk most certainly relates to pixel pitch in terms of recorded resolution. If the airy disk is much smaller than a pixel, it cannot be resolved. As the airy disk becomes larger than the pixel, contrast is reduced (which is a large portion of our perception of sharpness/resolution). When it becomes ~ 2x the size of the pixel, recorded resolution drops (depending on contrast requirement). And when it becomes larger than the CoC limit, visible resolution decreases in the output image (when viewed according to the standard).
The scenario where the airy disk exactly matches the pixel pitch is less than ideal as well; because it results in aliasing errors when the two do not line up (which is more probable than not).
And for viewing conditions other than standard the standard CoC is not applicable. I.e. for a large format print to be viewed at short distance in a gallery, you might want to use a much smaller CoC limit (or for machine vision).
You might like to read this paper: Do Sensors Outresolve Lenses' Capabilities?
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
2y ago
0
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They’re using the same term for different goals.
In traditional photography, CoC for depth of field is usually based on what looks acceptably sharp in the final image to a human viewer. That depends mainly on viewing conditions: print/display size, viewing distance, and human visual acuity. With modern cameras, pixel size is usually much smaller than this perceptual limit, so it normally isn’t the factor used in standard DoF calculations.
In machine vision, the system evaluates the image at the sensor level. It is effectively always “pixel peeping,” so sensor sampling and diffraction matter directly. In that context, it makes sense to use a CoC tied to pixel pitch and/or the Airy disk, because those limit what the system can actually resolve.
So there isn’t really a contradiction:
- photographic CoC = acceptable blur in the final viewed image
- machine-vision CoC = acceptable blur relative to sensor sampling and optical resolution
The Airy disk and pixel pitch are very relevant to recorded resolution, but not usually to the conventional photographic DoF standard, which is based on human perception rather than sensor-level analysis.
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