Lenses are almost always sharpest at intermediate apertures, not the smallest. This runs counter to intuition: Since small apertures provide greater depth of field, you might think they’d provide maximum sharpness. Not so. Without getting into the science, the smallest apertures tend to degrade images. There may be more depth in the image, but if you blow up the area you focused on, you’ll almost always find it somewhat softer at small apertures than at larger ones.
Photographers who want maximum sharpness in images will avoid smaller apertures for this reason. An old rule of thumb is that lenses tend to be sharpest at an aperture 2 to 3 stops down from maximum. On an f/2.8 lens, this would be f/5.6 to f/8.
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