I was just informed by someone that if you want the absolute sharpest picture possible you should open up the aperture as wide as possible and that although depth of field is increased as you close down the aperture, the actual sharpness of the picture decreases. This goes against everything I ever hear about lenses. Have I been wrong all these years or is this guy just an idiot?

-- shadowman (mshadow818@msn.com), January 22, 2002

Answers

He is an idiot or thinks he's playing practical joke.

Sharpness worsens on any lens as you open the aperture (smaller f stop number). Sharpness increases as you stop down (larger f stop number), but if you stop down too far then diffraction enters the picture and sharpness decreases again, though only slightly. Each lens has a point, typically around f/8 or f/11 somewhere, where it's at its sharpest.

http://www.photo.net/bboard/q-and-a- fetch-msg?msg_id=000f7T

-- NK Guy (tela@tela.bc.ca), January 22, 2002.

There's a tiny grain of truth hidden in what he said, but as NK pointed out, most of it is garbage. When you focus at a given distance, only objects at precisely that distance are truly in focus - i.e. a point on the subject resolves to a point on the film. Anything that's not at that distance but is within the depth of field is not technically in focus; a point within the DOF will resolve to a small area of light on the film. However, due to the design of the human eye and due to the design of film, a sufficient small area of light is indistinguishable from a point of light, so it appears to be in focus even though it isn't truly in focus. Depth of field also varies according to enlargement; that which appears sharp in a 4x6" print might appear slightly out of focus in a 16x24" print becuase you've enlarged the small area of light to the point where it's no longer indistinguishable from a point.

But that's really being pedantic; for almost all pictures you might take, anything within the depth of field will appear to be sharply focused. If your priority is to have a certain range in focus and the rest out of focus, choose an aperture that gives you the depth of field you want. If your priority is to have the sharpest results possible for the subject, use your lens' sharpest aperture - usually 2-3 stops down from wide open. If you don't know what that aperture is (and it's not usually information you'll find published), f/8 is usually a good choice; "f/8 and be there" as they say.

-- Steve Dunn (steved@ussinc.com), January 22, 2002.

With a perfectly designed and built lens, it will be at maximum sharpness wide open. Stoping down increases diffraction that reduces sharpness. Also there are several very good prime lenses that are as sharp as they ever get while wide open, and stopping them down does not help a bit. But I agree, the vast majority of lenses made today increase sharpness as you stop down, until diffraction makes it worse again.

-- Jim Strutz (j.strutz@gci.net), January 22, 2002.