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Anonymous
>>84745 Because when dealing with real-world lenses, the optical limit set by the angular resolution is often much higher than the film or sensor can resolve, so you don't see any difference as you stop down until you hit the diffraction limit.
Meanwhile there are a number of other factors that reduce the lens's resolving power well within the detectable limits of the film/sensor. Chromatic aberration, spherical aberration, internal flare, and manufacturing variability all have a much more pronounced effect on the real-world resolution of a lens, and all of them are reduced by stopping the lens down.
It's an easy experiment to do at home. Photograph a test pattern under controlled conditions at different apertures, and with most lenses you will see the lowest sharpness and contrast wide open with a marked improvement as you stop down, peaking 2-3 stops from wide open, and falling off due to diffraction 1-2 stops from the lens's minimum aperture.
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