Originally posted by jatrax Diffraction is not something that just suddenly happens at a particular f-stop. It is always present it just gradually becomes more noticeable as you stop down.
exactly, It is also important to note that Diffraction is as much a sensor dependant as it is lens dependant - there are no 35mm lenses that perform well at f/22 or higher. With a takumar 50mm f/1.4 lens the image quality starts a bit on the soft side, but still usable at f/2.8 we see improvement, at f/4 this trend continues until we reach the optimum aperture of f/5.6, at f/8 the resolution is slightly lower (but still extremely good) at f/11 it lowers a bit more, at f/16 there is a observable loss of acutance, at a hypothetical f/22( Takumars can't stop down this far) a considerable amount acutance in the image will be lost.
Lenses follow a shallow bell curve*, there is a peak performance and then there are the F/stops on either side of that optimal point.
*Pancake lenses tend to have a bit of a linear characteristic in terms of resolution, with a well designed pancake lens (The DA70mm f/2.4 springs to mind)the image quality at f/2.4 is already extremely good, the only reason to stop that lens down is to increase DOF.