A lens is essentially a tube with optical elements inside through which light from a scene, is focused onto the recording medium. The maximum amount of light that can pass through a lens is denoted by its aperture or f number. Whilst the maximum aperture of a given lens cannot increase, it can be reduced by a diaphragm that makes the aperture smaller. A lens with a low f number is described as a fast lens, whilst higher numbers denote a slower one.

The f number sequence is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 64, with each increase or decrease in f number denoting a halving or doubling in the amount of light transmitted through the lens. It is also possible to have intermediate apertures, so f/3.5 is midway between f/2.8 and f/4 for example. This might seem confusing, but in reality it does not matter. Our eyes adjust automatically to prevailing light levels and do not perceive differences in the same way such as with length or volume of a liquid, for example. The camera controls or the aperture ring on a lens set the aperture and the photographer need only be aware of whether a wide or small aperture is in use, remembering the lower the f number, the wider the aperture.

Lenses are also described by their focal lengths, which are expressed in millimetres. A low number such as 20 indicates that a lens has a wide angle of view, while higher numbers such as 200 show that the lens has a narrower angle of view and magnifies objects by making them appear closer. These are often referred to as telephoto or long lenses.

In order to create image magnification, the optical elements inside a telephoto lens are placed further apart than for a wide-angle one. Since this lengthens the path through which the light travels, telephoto lenses tend to have smaller maximum apertures and are physically longer compared to shorter focal lengths.

There are fast telephoto lenses available, which are made possible by increasing the diameter of the elements, but this has a detrimental effect on both weight and cost. The larger an element, the more difficult it is to eliminate defects in the manufacturing process and being made of glass, the lenses are heavier. Increasing the maximum aperture of a telephoto lens can easily treble the cost, typically becoming specialist equipment mainly used by professionals.

The introduction of crop sensor DSLRs has meant that for any given focal length, there will be a telephoto effect compared to conventional 35 mm equivalents. This has been a boon for those who shoot subjects such as nature or sport, but the downside is wide-angle lenses have become more moderate. Manufacturers have addressed this by introducing lenses with short focal lengths specifically designed for crop sensors. By their very nature, they are incompatible with film cameras or full frame DSLRs as the image created by the lens will not fill the entire frame.

A lens with a fixed focal length is known as a “prime”, while the majority of lenses available today are zooms that have variable focal lengths. Since a fast lens requires a wider tube and hence larger internal elements, they are very expensive to produce and weight increases dramatically. Primes have a simpler optical structure and at the shorter focal lengths especially can be very fast. Zooms are more complex and typically have smaller maximum apertures. Depending on the design, the maximum aperture can vary across the zoom range, with aperture reducing at the longer end.