[EMAIL PROTECTED] wrote: > I have another question which is not fully Nikon specific. How is it > possible that the speed of a zoom lens does not change when the focal > length is changed? (I have an old Sigma 70-150/3.5 lens, but I have seen > that with some Nikkor lenses as well.) There are two types of zooms: constant aperture, and the other sort. Constant aperture zooms are usually more expensive than the others, as they have to have mechanics inside to adjust the aperture as you zoom. They also need to have a wider aperture to start with, as the apterture in the one you're talking about needs to be f3.5 at the long end, 150mm. Because *effective* aperture, eg. f8, is the result of a sum which includes the focal length of the lens, the *actual* aperture, (which could be 3mm, 5mm, and so on) will have to change as the lens gets longer/shorter. A zoom which has a constant aperture and is f3.5 at 150mm will actually be capable, mechanically, of a wider aperture at 70mm, but they choose not to build in the facility. Hope this helps... Best Wishes, Paul http://www.paulbrad.demon.co.uk
