Originally posted by L33tGreg The f numbers are written like f/2.8 where 2.8 is in the denominator, and a larger denominator makes a smaller number (and thus a smaller aperture).

Quite. We must remember that an f-stop is a

*fraction*, the ratio of the opening to the focal length. So on a 100mm lens, a 50mm opening is 50/100 or 1/2 (f/2), and a 25mm opening is 25/100 or 1/4 (f/4). Just as 1/2 is a bigger number than 1/4, so f/2 is a larger aperture opening than f/4. And apertures jump in the odd scale of 1--1.4--2--2.8--4--5.6--8--etc because those related to the square root of two, about 1.414. Iris openings are (or should be!) fairly circular; opening the iris 1.4x lets in 2x the light.

A fixed-maximum(widest)-aperture zoom adjusts the iris to keep the aperture constant; a variable-max-aperture zoom doesn't make that adjustment, because the mechanism for doing so costs money. Thus variable zooms cost less -- they're made more cheaply. If I have two otherwise-comparable zooms from the same maker even, I'll can bet that a constant f/3.5 lens will perform better than a variable f/2.8-4 zoom.