Quote Originally Posted by RalphLambrecht View Post
is a fast lens, stopped down, just as good as a slow lens at the same aperture?
It depends on the lenses and the situation.

For example…

At f/16, I see a significant difference in image quality between my fast normal lens on my small format camera and my slow normal lens on my medium format camera. The medium format image is so much better.

At f/16, I see very little difference in image quality between my high-priced 50mm f/1.4 lens and my low-priced 50mm f/1.8 lens.

At f/16, the close-up images produced by my high-quality 55mm f/3.5 macro lens are so much better than those produced by my high-quality 50mm f/1.4 lens.

At f/5.6, the close-up images produced by my 105mm f/2.8 macro lens have more image detail than those produced by my 105mm f/2.5 lens. However, the portrait images produced by my 105mm f/2.5 lens are better than the portrait images produced by my 105mm f/2.8 macro lens.

http://www.flickr.com/photos/11336821@N00/6012452194/