Kevin asks: > I have a question concerning fast lenses as to why they are so expensive. I under stand that a fast lens allow for more light by providing a larger diaphragm opening. I am curious as to why this results in such a large price increase between, for example,a f3.5 and a f1.4 lens. Anyone know the answer? well... in addition to what others have said diameter of the aperture = focal length / aperture (f number) so if we are considering an 85mm f3.5 lens we are talking of an aperture of 24.3mm so if we are considering an 85mm f1.4 lens we are talking of an aperture of 61mm all the elements (of course) have to be large enough to allow the light to pass unobstructed through that aperture with the correct angle of view so the front element in particular becomes quite large - you can't just have a tiny front element and a big aperture inside, so we're talking about big bits of glass in fast lenses. Imagine having to grind a precision lens that is more than twice the size of it's smaller, slower counterpart and still maintain good optical characteristics! karl
