Whenever you see a lens' focal ratio listed as a "range" (e.g. f/3.5-5.6) instead of a single value (e.g. f/4) it means the lowest possible focal ratio really depends on the focal length (how far have you "zoomed" the lens.)
That means that at the 18mm end it can go as low as f/3.5. But at the 135mm end it can only go as low as f/5.6.
But it's not quite that even. It turns out variable zooms QUICKLY increase their focal ratio as you zoom in from the lowest focal length and roughly by the time you're half-way through the zoom range they are already at the high-end of the focal ratio range. In other words, you can use f/3.5 at 18mm... but probably by the time you make it to 60mm you'll already be at f/5.6 as your lowest possible focal ratio.
High end zooms can provide a constant focal ratio at any focal length. For example there's the EF-S 17-55mm f/2.8 IS USM which can provide an f/2.8 focal ratio at any focal length in the zoom range. There are lots of lenses that can do this, but they tend to be more expensive lenses. Less expensive zoom lenses are usually always variable focal ratio lenses.
Tim Campbell
5D III, 5D IV, 60Da
