Lenses and Focal Length

The focal length of a camera is the distance from its lens to the focal point, which is where the film or sensor is.

Short focal lengths give a wider field of view, while long ones magnify but give a narrow field of view.

Zoom lenses let you change focal length (“zoom in or out”), while fixed or prime lenses have only one set length and therefore can’t zoom. But prime lenses are better some ways : they usually allow much wider apertures and can be built cheaper, so you get better quality and more light for a lower price.

Lenses are divided into a few basic categories based on their focal length in mm:

Length	Name/Type
< 20 mm	Super wide-angle or Fish-eye
20 – 35 mm	Wide angle
50 mm	Normal – no distortion, same as the human eye
80 – 300 mm	Telephoto

The lengths above are for 35 mm film cameras. Digital cameras have sensors smaller than 35 mm, so to get the same effect, multiply the length by about 1.4 e.g. a “normal” film camera lens is 50 mm, but on a digital, it would act like an 80 mm lens, which means it will start to magnify like a telephoto.

Here’s a link to another explanation: http://www.dpreview.com/learn/?/key=focal+length

Fisheye Lenses have a very short focal length, giving almost 180 degrees of view. This is good for showing a very wide scene, like most of a room. But the edges are bent – check the walls of the room below. What part of the picture is most and least distorted?
Telephoto Lenses magnify the view and narrow the field of view. One effect is Foreshortening, where things that are distant appear to be close together, like the cars on the 401 below – they were many metres apart, but look bumper-to-bumper.
Zooming In – a zoom lens can go from wide angle to telephoto, like these shots of Owen Sound taken from a jet.
Telephoto lenses also let you get closer to things that are far away. This is particularly good for nature shots, where it might be imposiible or dangerous to get close. But the more magnification, the steadier you have to hold your camera!