focal length & angle of view

'The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.'-Wikipedia.

Contrary to common belief, focal length isn’t a measure of how long or short a lens is physically, but the distance in millimetres from the optical centre of a lens to the imaging sensor when the lens is focused at infinity.

Rather than being fazed by the physics, it’s easier to think of the way in which focal length affects image size.

For a camera with a full-frame sensor, for example, a standard lens (one that gives a similar perspective to the human eye) is 50mm.

Lenses with focal lengths less than 50mm are referred to as wide-angles – simply because they have a wider angle of view. Lenses with focal lengths greater than 50mm are known as telephotos, and these offer greater magnification thanks to their much narrower angle of view.

Angle of view? What’s that?


Essentially, the angle of view is the amount of a scene that a lens can take in, measured in degrees. For instance, a fisheye lens may offer an extremely wide 180° angle of view, meaning that it can capture everything in front of it (and to each side).
A 200mm lens, on the other hand, will offer a much narrower angle of view of 12.3°. This allows you to fill the frame with a considerably smaller amount of the scene that you’re trying to photograph.