Originally posted by gokenin I was on the way into work this morning and a thought occured to me ( rare at 7 am) why is it that on a zoom lens f2.8 across the board is considered a fast lens but on a prime if you have anything over F2 its thought of as a slow lens? or am I just wrong? or should i not try to think before coffee
.
Because it's much easier and less costly to construct a quality optic with a fixed focal length with a large maximum aperture than a zoom - also, there are very few telephoto primes over 85mm that have greater than f/2.8, so focal length plays a part, too. 28mm --> 85mm seems to be the sweet spot for large-apertures in lens design - those lenses can have f/2 and greater max apertures without being enormous or too cost prohibitive.
Olympus makes some f/2 zooms for their 4/3 systems, but they're huge and costly.