Diffraction's impact on pictures

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Lee Jay
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Diffraction's impact on pictures
9 months ago

It's recently become apparent that many people are confused about what diffraction does to photographs.  People talk about diffraction limits without defining what they are and about pixel-count equivalents of diffraction limits without stating what criteria are used to calculate those pixel counts.

So, let's look at this another way - MTF.  MTF can be thought of as contrast at a particular resolution or, more simply as "sharpness".  Higher means more contrast/sharpness.

So what do the numbers mean?  Well, you can't have an MTF of 1, as it requires infinite aperture and other ridiculous things.  However, it can be quite high, in some cases.  Many people would judge any MTF larger than 0.5 to be "sharp".  However, that doesn't mean MTFs below 0.5 are useless - far from it.  As for the diffraction limit, well, it's zero technically, which is the blue/red boundary on the plot below.  However, contrasts of just above zero are pretty close to useless.  Some people use 0.05 for extinction.  Lord Rayleigh used a criteria that now bears his name that equates to about 0.09.  I've done my own testing and I can't tell a lot of difference between 0 and 0.1 and so I think you can pick anything in there and it won't make a lot of difference.

That said, below is the plot that I calculated.  This plot includes one important assumption, that being that you need three (3) pixels to represent one line pair, not two.  This is a simplified way of accounting for things like the Bayer mask, the AA filter and the large area of each pixel.  In my testing, I don't find this assumption to be far from the mark visually.  Assuming 2 pixels for this is certainly wrong as it requires perfection, and the real number is almost certainly between 2 and 3, so 3 is a bit conservative.

Look up your pixel size (i.e. if you have a full-frame sensor which is 36mm wide with 6,000 horizontal pixels, pixel size = 36mm/6000 = 0.006mm = 6 microns) and look at your f-stop to get the MTF that you can get with a perfect lens having no aberrations.  Aberrations will reduce MTF further but by a totally lens-specific amount, so it's impossible to generalize about them.

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Lee Jay

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