What sharpness really is
We all talk about sharpness, but what does the word actually mean?
Sharpness is not a scientific term, it’s all about impression – if we think part of an image is sharp, then it is. Sharpness is influenced by two lens qualities – resolution and acutance – but also by subject, composition and our own expectations. Resolution is the ability of a lens to separate fine detail, such as narrowly-spaced lines, while ‘acutance’ refers to how abrupt edges are between dark and light tones – local contrast, in other words. Improving both of these qualities, particularly resolution, is what lens designers and manufacturers strive for, and lens quality is judged on this more than anything else.
However, beware of thinking of sharpness as a gold standard. You would nearly always expect, or want, some parts of an image to be sharp, but images can be powerful without this quality. Maximum depth of field to give sharpness throughout the frame may be good for some images, but it’s also predictable, and prevents the creative use of blur. To quote Henri CartierBresson: “Sharpness is a bourgeois concept.”