Aperture question, re: sharpness

Started Feb 26, 2014 | Questions thread
Conrad69 Junior Member • Posts: 48
Re: Aperture question, re: sharpness

Slightly simplifying, two physical phenomena affect lens sharpness: aberrations and diffraction. Aberrations are a result of lens design/construction, diffraction is a fundamental physical effect.

Diffraction limits the resolution of a lens in a way that resolution decreases when the f-number increases. If a lens would be aberration free, the highest resolution would be obtained wide open. In fact, the world's highest resolution lenses are around f/0.19, but they're not photographic lenses and cannot be used as such.

A general trait in most photographic lenses is that aberrations are the highest wide open, and reduce when stopped down. This is the reason why sharpness is not optimal wide open, then improves when stopping dow, but decreases again at smaller apertures due to the abovementioned diffraction efffects. This is no absolute truth, however. There are some lenses that perform best wide open, and there are lenses that have a significantly worse performance at smaller apertures than the diffraction limit.

Hope this helped.

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