Is there something special about f8?

[johnd]

I've been told that lenses are tuned to perform best at f8. Is this a commonly held point of view? I imagine that every lens would perform best at some particular aperture, but is there something special about f8?

I understand the depth of field issues with different apertures, but what I'm particularly asking about is sharpness, or is the answer that the greater the depth of field the greater the sharpness by definition? In which case why f8?

And what about zooms, is there a rule of thumb about their performance at say minimum or maximum focal length, or somewhere in between.

I would much appreciate your comments.

Cheers
John

[MCWB]

John,

Obviously it depends on the actual lens in question, but usually lenses perform best a few stops down from wide open. f/8 is three stops from f/2.8, two from f/4 and one from f/5.6. f/11 adds another stop, so f/8 to f/11 is usually a pretty safe bet for most lenses. Massive generalisation there of course though!

[Matt. K]

Well answered MCWB
There is another part to the equation that also effects the performance apart from the F/stop and that is the distance focussed upon. Most good quality lenses have a "magic" sweet spot and it's usually, but not always, around f5.6 or f8 or f11 and a certain focal point. When you discover where this sweet spot is for your particular lens then try and exploit that setting as much as possible. Get your lens welded!

Finding the sweet spot for zoom lenses is more a matter of chance because of the high number of variables.

[johnd]

Matt and MCWB, thanks for your responses.

So, as a generalisation, f8 isn't a bad place to start looking for a sweet spot.

Cheers
John

[Matt. K]

johnd
Try f/5.6 and f/8 focussed out at around 3 metres/4 metres etc. See what happens.