Diffraction Limit

Started Aug 28, 2013 | Discussions thread
Erik Magnuson
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Re: Diffraction Limit
In reply to Krich13, Aug 28, 2013

Krich13 wrote:

Obviously, if the diffraction limited spot is smaller than a pixel, diffraction limit of resolution can not be reached. Megapixels still matter.

This is where you go off track: diffraction spots do not line up perfectly with pixel centers. Imagine two images, one with the spots lined up with pixel center and then one with the same spots offset by the pixel radius so each "spot" overlaps two pixels. The second image will have lower edge contrast than the first -- and we measure resolution via contrast retained, e.g. MTF50.

Diffraction always limits resolution. You just don't typically notice because other lens aberrations are more visible than diffraction at wider f-stops. The amount of resolution lost may be very low, difficult to measure, and visually insignificant, but it's there.

Megapixel's don't matter except that they can resolve everything better: detail, diffraction, and other lens aberrations. Megapixels mean you make larger prints or greater crops which magnifies both defects and details.

Obviously, the same diffraction-limited spot would occupy larger portion of the smaller sensor than of a larger sensor.

Which is important if comparing the same size output (print or screen) using the entire sensor area. And why f/64 or f/128 is not usually much of an issue for 8x10 film cameras.

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Another way to look at resolution is angular resolution (this is tha one relevant for photography. Minimum resolvable angle depends only on the wavelength and the lens diameter (or absolute aperture).

The minimum angle resolvable (in radians, multply by 57 for degrees)

is 1.22/1.02/0.98 D/lambda (Rayleigh/Abbe/Sparrow) where D is the diameter of the lens/aperture.

This agrees with what I say above: diffraction always limits resolution. Stop thinking about pixels: diffraction, DOF and lens aberrations limit the resolution before point sampling occurs.

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Erik

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