gdanmitchell
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HS-LD wrote:
Yes, very good things to keep in mind.
But just like those things, if you want to produce images with the highest IQ your system is capable of you need to know what apertures that system works best at, which is why knowing when diffraction kicks in is important. Also it allows you to know when you are sacrificing that highest image quality for some other parameter. It's all part of knowing your tools.
It eventually occurred to me that there are basically three modes for selecting aperture.
1. If you need to minimize DOF or are willing to accept minimal DOF in order to keep your shutter speed high enough, you open up as much as you can, often going to the maximum aperture of the lens.
2. If aperture isn't relevant to the "interpretation" of the image you typically will shoot at or very close to the aperture that best balances things like average sharpness across the frame, minimal diffraction blur, best sharpness on your central subject. The rule of thumb on FF has been f/8, but f/5.6 or so is probably a good bet here on APS-C.
3. If you need to maximize DOF you likely stop down "a bit more" up to the point at which you feel that diffraction blur will be too much of a liability to offset the extra DOF. On APS-C this is probably in the f/11 range, though in some cases you might push to f/16.
"Sharpness" is a complex thing, and it is about more than just measuring stuff. (Not that measuring doesn't have value, too.) I like a concept I call subjective sharpness." For example, let's say you are photographing a person. Let's say that your "sharpness" aperture is f/5.6. However, if you open up to, let's say, f/1.4 and blur the background the person will stand out better from that background, increasing the subjective sense of sharpness. This can be amplified in other ways. Perhaps you select a background with a more uniform texture, colors that don't stand out, perhaps will less light so that it is a bit darker. (or more light to make it brighter than your primary subject.) How you light the subject matter, too.
The point is that while it is worthwhile knowing what the "sharpest aperture" is, the difference between that aperture and on a stop away (or even more in some cases) may be insignificant if it gets in the way of other elements that contribute to the sense of overall sharpness in an image.
Anyway, good general rule of thumb — if you haven't tested each lens — is that f/5.6 is probably pretty good on most APS-C lenses.
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