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The A to Z of Photography: Focal length

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Focal length is the optical distance from the front element of a lens to the point where light rays entering the lens converge to form a sharp image on the sensor, and is expressed in millimetres.

It also suggests the characteristics of a lens – its angle of view, i.e. how wide or how narrow a view of a scene will be captured, and the magnification, or how large or small an object will appear in the viewfinder. 

Shorter (lower number) focal lengths produce a wider angle of view, with objects appearing smaller in the frame, while longer (higher number) focal lengths produce a narrower angle of view, with objects appearing larger in the frame.

For instance, an 8mm fisheye lens may be able to take in a huge 180-degree view – and there’s a risk that your feet will inadvertently end up in the frame if you’re not careful. Super-telephoto lenses, on the other hand, may offer an angle of view of just three or four degrees.

Focal lengths compared

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Lenses of different focal lengths can produce the same angle of view on different camera bodies depending on sensor size. For instance, on an APS-C camera, 35mm is roughly the same as 50mm on a full-frame camera on account of the 1.5x or 1.6x crop factor, as is 80mm on a medium format body. Although the focal length is different with each camera format, the effective focal length – the angle of view produced by the lens – is the same. 

Prime vs zoom

A lens can either have a fixed focal length ('prime' lenses) or a variable focal length (zoom lenses), with options available in both types covering the full range of focal lengths, from wide-angle through standard to telephoto. So why would you select one type of lens over another?

A zoom lens offers flexibility in that you have a range of focal lengths within a single lens, which is great if space and weight are limited. Generally speaking, though, the maximum aperture of a zoom lens isn’t as wide as that of a prime lens, so if you want additional light-gathering capability for shooting in low light, and the potential for a shallower depth of field, a prime lens may be the best option.

James Abbott is a professional portrait and landscape photographer, and a freelance photography journalist producing words and pictures for the best photography magazines in the UK. James is also a qualified college lecturer and has taught photography and Photoshop to a wide range of age groups and situations including one-to-one, group and distance learning.