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Old July 4th, 2011, 10:11 AM
Jerome Marot Jerome Marot is online now
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Join Date: Jan 2011
Location: Munich, Germany.
Posts: 3,833

Actually, on small formats, the effects of diffraction are limiting the photographer's choices quite a lot nowadays. I'll give some examples:

-on 35mm film, or on a 12 mpix full frame DSLR (which is about the same resolution), the effects of diffraction are measurable from f/8 and start to be visible over about f/11. Considering that typical 35mm primes were open around f/2, f/2.8, and because aberrations tend to diminish with aperture, we have that idea that lenses have a sweet spot between f/5.6 and f/11...

-on a 12-16 mpix crop format DSLR (or on a 24 Mpix full frame DSLR), because we have higher linear resolution, diffraction starts to be measurable from f/5.6 and starts to be visible over about f/8. In addition, the typical zoom lens usually has a maximum aperture of f/4 or f/5.6 so, basically, all what we can choose is full aperture or closed one stop to improve corners a bit. Anything above that and we will start to degrade center sharpness.

-on the typical digital P&S, diffraction usually start to be a problem as soon as you close the diaphragm. Luckily, these cameras usually have no diaphragm... ;) Some use a neutral density filter instead. This is also known from professional video cameras (which also have a small sensor, but lower resolution -HD is 2 Mpix- and usually faster lenses): don't close the iris (the video name for diaphragm) above f/5.6, use a ND filter instead. Many of these cameras have a set of ND filters built-in for that reason.

Of course, there is more to a picture than absolute sharpness. Sometimes we need more depth of field and need to compromise resolution. Sometimes we need less depth of field and don't care about optical aberrations in the corners. But for a reasonably flat subject (landscape far away), these are the constraints we have. In other words: don't go above f/11 (f/16 for film) unless you must.
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