Thread: An optics question about diffraction and image circles

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  1. #1 An optics question about diffraction and image circles 
    Senior Member Karim D. Ghantous's Avatar
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    I wonder - is the diffraction limit related to image circle, or just to focal length, or is it a combination of multiple factors? For example, in Micro 4/3, f/8 is probably the most you want to stop down before diffraction properly sets in. For VistaVision, it's f/16, or maybe f/11 to be conservative. But, I am not clear on whether this has to do with image circle or something else.

    The implication is that if diffraction is related to image circle, then a medium format lens could be used on a Micro 4/3 camera, and you could stop down much more than you could on a dedicated Micro 4/3 lens of the same focal length.
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  2. #2  
    Michael Tiemann, Chapel Hill NC

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  3. #3  
    Senior Member Karim D. Ghantous's Avatar
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    Michael, thank you for the reference. This is the conclusion:

    The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important
    So that answers my question, I think. If I disagree, I can perform an experiment and see for myself.
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  4. #4  
    You forgot that pixel size is also a part of the equation. If you have a 4k medium format sensor, pixels are huge and it would take a ginormous airy disk to become diffraction-limited. If you have a 4k m43 sensor, the pixels are very much smaller, and a much smaller airy disk can creep across more than 3 pixels, creating a diffraction-limited system. Fuji GX100 33mm height vs m43 13mm height is > 2.5x difference in pixel-based airy disk limit.

    The comment about f-stop related only to the lens's theoretical contribution to the problem. The pixel pitch also matters. And if there are other lens aberrations, they may completely obscure diffraction limited as coma, astigmatism, spherical aberation, chromatic aberation, and other gremlins emerge.
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  5. #5  
    Senior Member Karim D. Ghantous's Avatar
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    Thank you Michael. I understand it a bit better now.
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