At what aperture does diffraction start degrading IQ on a Foveon?

Started Mar 26, 2013 | Questions thread
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 Re: At what aperture does diffraction start degrading IQ on a Foveon? In reply to Petroglyph, Mar 27, 2013

There's something here that has not been cleared up. We're talking here about relative apertures. So the actual diameter of an aperture labelled f/8 will depend on the focal length of the lens that it's fitted to, given that this diameter is, by definition, one-eighth of the focal lenth of the lens.

And the diffraction of light, which is always present at the edges of the aperture, will increase proportionally as this diameter is decreased, and the diffraction-free area in the centre gets smaller.

If we take the three members of the DPM-family as an example, and for the sake of simplicity, take the focal length of the DP1M lens to be 20mm, then the diameter of the aperture at f/4 will be 20 divided by 4, so 5mm. Put that 5mm aperture into the 30mm lens of the DP2M, and you get f/6. Put it into the DP3M and it becomes f/10. And with a 320mm lens on a 10x8 camera, a 5mm aperture will give you f/64.

Set all three DPM cameras at f/8 and you get three different aperture-diameters, and therefore, three different levels of diffraction with the same f/number.

So to write, as Petroglyph does, "With APS/C sized Bayer sensors I can start to see diffraction reducing IQ at apertures smaller than f/8". Has little meaning, unless all other conditions are given as well - such as, for example, the focal length of the lens, and the degree of enlargement at which the image is viewed.

David

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