Aperture measurements

Lenses almost always have their maximum aperture setting engraved or stamped on one end of the barrel. On a zoom lens you'll see two measurements, often stated as f/3.5-f/5.9 or similar.

Rather than being opposite ends of a single scale these describe the maximum aperture at the wide angle and telephoto (maximum zoom) lens positions respectively. Always buy a lens with the smallest number you can afford in each position.