What sharp­ness re­ally is

We all talk about sharp­ness, but what does the word ac­tu­ally mean?

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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.”

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