I understand that all lenses seem to perform worst at their extremes - at either focal length or aperture. My question is, at least for some, why? For example, I am considering buying a 28mm for my 40D to use in low light situations. The Canon 28s are apparently lousy wide open - which is where I would expect them to be most used.

Is it that hard to optimize a lens for wide open use?

Please accept my apologies if this is a stupid question.

Lens aberrations are greater as you increase maximum aperture size. It's much harder to design a sharp f/1 lens than a sharp f/8 lens. The difficulty of correcting aberrations and achieving the necessary manufacturing tolerances increases exponentially with the physical diameter of the lens. A typical webcam lens will resolve a point light source to a much smaller Airy disk than any 35mm-format lens, but it will only cover a very small image circle, and would be a ridiculously tiny aperture on a 1Ds.

The problem is even more difficult for wide angle lenses.

No.

It's even harder. Especially for wide-angle lenses.

The Canon EF lens you're contemplating isn't "lousy." It is, at full aperture, just not as good as it is when stopped down. It will be still good enough for most available-light applications. Other wide-angle lenses sporting the same or similar speed (e. g. Sigma 28 mm 1:1.8 EX DG) are no better. So if your subjects and intents require a super-fast wide-angle lens then go ahead and buy one. But don't expect it to be as good at full aperture as at, say, f/2.8 or f/4. It will still be better than a lens that's too slow for your application, or no lens at all.

If you're planning to use the lens on an APS-C-format camera only then you may consider the Sigma 30 mm 1:1.4 EX DC which is no wide-angle lens but a standard lens (i. e. for APS-C format only). It does have its weaknesses, too, but still may be better wide open than a super-fast wide-angle lens which you don't need (on the EOS 40D, that is).

-- Olaf

I use the Canon 28/1.8 fairly often, and it's a decent lens. I only get it out when I need something faster than f/2.8 -- so I am almost always using it at f/2. It's actually quite sharp, at least compared to a zoom at f/2.8.

The more the light rays have to be bent, the bigger the problems; for example rays of different wavelengths refract differently.

The light rays have to be bent more, if

- the focal length is shorter,

- the entrance pupil diameter is larger,

- the curvature of the lens is stronger.

Accordingly, it's easier to design a lens with long focal length, large diameter lens (so that the curvature is less), using only the center of the lens (small aperture). Of course, if the lens is large, a larger aperture will be utilized, but it will be sharper close to the center.

Turn these around: short focal length, large aperture.

Faster lenses mean glass honed at larger diameters. Think of it this way. Using a hand saw, can you cut as straight a line for 36 inches as you could for three inches?

Even the best of machines are capable of inaccuracies and in wide angle lenses small inaccuracies add up to unacceptable distortion.