There are many reasons that a lens doesn't make a perfect image. One group of reasons is called abberations. For example, the surfaces of your lens are spheres, and is causes light rays to focus at different distances, which softens the image. This is called spherical abberation. This problem improves as you stop down the lens, from small f-numbers to large.
Light is composed of waves, which causes it to bend as it passes through an opening. This is called diffraction. This effect also softens the image. Unlike the classical abberations, this effect gets worse as you stop down, because the bending is larger for smaller openings (larger f-numbers).
As you stop down abberations improve and diffraction gets worst. There is a broad middle range of apertures in which your lens will be near its best. If you use your lens near wide open the image won't be sharp because of abberations. If you stop your lens all the way down (e.g., near the f64 )
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