SLR Lens Focal Length
Focal length is a more accurate way of looking at zoom in a telephoto lens, or how much magnification there is in a prime lens on an SLR. Common focal lengths are 18-55, 24-70, 50mm, 60mm, 70-200 and 70-300. These numbers will be printed on the lens by the glass.
Adjusting Focal Length
Rotating the lens barrel adjusts focal length on an SLR zoom lens; these numbers are printed on the lens barrel. For instance, a 24-70 will often list 24, 35, 50 and 70 on the barrel of the lens.
Aperture numbers, expressed as an F number, are also shown.
Variable Aperture numbers
If you see two F numbers, it is a variable aperture lens such as the Nikon 18-200 lens, which is an F3.5-5.6.
Point and Shoot
Point-and-shoot lenses will display the aperture for the lens, which is always variable. They will also usually list the optical zoom (3x, 4x) and focal length.