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I was on another photography board, one I don't frequent, and I saw someone say the following.

A f4 lens will never be as sharp as a 2.8 lens.

Firstly it is very very general, but secondly I don't its all that valid, but I'd be happy to be proved wrong.

A 2.8 lens gives you more light to focus with, but is there something in the design of every 2.8 lens that makes it inheritly more sharp then a f4 lens ?

I'll acknowledge that most 2.8 lens are semi-pro to pro lenses, so are expected to peform better, but to outright say a 2.8 will always be faster ? Seems silly logic to me..


am I missing some obviously law of physics here ?

photozone.de is a good reference site for lens comparisons - they review lots of lenses and do a fine job of it.

Excerpt from their 70-200 f/2.8 L USM review:

In the lab the lens showed that it is far from being qualified for retirement. In fact the performance figures beat both its IS sister lens as well as the EF 70-200mm f/4L - not by much (about a half "school mark") but clearly measurable. At all focal length it scratches the resolution limits of the 8MP APS-C sensor. At f/2.8 the results are already very-good at f/4-5.6 they're excellent. Even when combined with the EF 1.4x II there's only a marginal performance penalty.

So I'd suggest the statement is possibly true of some lenses, but not as a generic "rule of thumb". I can assure you that my f/4 lens (70-200mm f/4L) is a hell of a lot sharper than my EF 50mm f/1.8 II (nifty fifty) - better glass for a start!

Cheers

Leigh

A f4 lens will never be as sharp as a 2.8 lens.

Bollocks!
To take it to the absurd...
Get an f2.8 lens. any one. set the aperture to f4. Glue the setting in place. It is now an f4 lens to all intents and purposes. Are they telling me it has magically lost sharpness? I don't think so. If you don't like this test, take another f2.8 lens (you've already ruined the first one ) Scratch the objective with wet & dry. It is still an f2.8 lens, but I bet just about any f4 lens in the world will be sharper.

My point is f is a measure of the focal length relative to its diameter (I forget the exact relationship). It has nothing at all to do with a lens's sharpness or otherwise.

That said, a faster lens is generally better built as it is aiming for a more discerning market, so it is likely it will be sharper at the same f stop than its slower companion. There is no guarantee of this however.

Craig, I think what they might have meant or failed to either properly express or understand is generally a lens won't generally give the sharpest results wide open.

As you probably know, lenses typically start to give their sharpest results a couple of stops down from wide open and then diffraction limitations start to come into play around f/16 on your average DSLR. You've probably seen this too - the 'G' (70-300) is a particularly good example - it will deliver good results, just keep it between f8-11.

So a 'faster' lens will usually start to deliver sharper results quicker (aka lower in the aperture range) than a 'slower' lens.

But for a fair comparison, you really would need to compare similar designs and build for an accurate test. for example, say I take a 18-70 DX and a 28-70/2.8 and set both of them to to 35mm @ f/4.5. The 28-70 is already a stop and a bit down, the 18-70 nigh on wide open. The results are a given here - you've got Nikon's better consumer kit lens up against their top-of-the-line mid-coverage pro zoom. Not exactly fair. That's not to say that the 18-70DX is rubbish though!

So you come back to similar designs at similar build quality - like what Hart posted - the Canon twins or your standard 50mm f/1.4 vs f/1.8 debate.

And on those two comparisons, there's a few articles about already.

Good links:
Diffraction: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
Acutance and Resolution = Sharpness: http://www.cambridgeincolour.com/tutorials/sharpness.htm

I don’t believe there is correlation between the size of aperture and sharpness of the lens.
I think it’s to do with the quality of the glass used and the construction of the elements in a lens rather than the size of the aperture. Most PRO zoom lenses are happened to bef/2.8 at wide open. We can find older primes f/2 which are not as sharp as modern zoom f/2.8 Once again, in my opinion, sharpness is correlated with the quality of glass and the design of the lens.

For example Nikkor 17-55mm f/2.8 DX is sharper than Sigma 30mm F/1.4 HSM on the borders from f/2.8 onwards.

http://photozone.de/8Reviews/lenses/nik ... /index.htm
http://photozone.de/8Reviews/lenses/sig ... /index.htm