THIS MONTH: FOCAL LENGTH

Everything you need to know about choosing lenses – in a nutshell

FOCAL length isn’t actually a measurement of how long or how short a lens is. It’s actually the distance in millimetres from the optical centre of a lens to your camera’s sensor when the lens is focused at infinity. Too much detail? Don’t worry, all you need to think about is the way in which focal length affects image size.

For a camera with a full-frame sensor, the ‘standard’ focal length is about 50mm. This is the focal length that provides a similar perspective to the human eye. Lenses that have focal lengths shorter than 50mm are known as wide-angles, as they offer a wider angle of view, while lenses with focal lengths greater than 50mm are known as telephotos; these have a much narrower angle of view that can make an object look much bigger in the picture. (We explain angle of view over the page.)

Lenses for digital cameras have the focal length – or the focal length range in the case of zoom lenses – marked on them, but the type of camera you’re using can makes a difference to how much of the scene or subject you capture through the lens. Full-frame sensors get their name because, at 36 x 24mm, they share similar dimensions to a frame of 35mm film. This means that they are exposed to the full view captured by a lens that’s designed for a film or full-frame camera.

Most digital cameras, however, have sensors that are smaller than full-frame. As a result, they’re exposed to a smaller area of the image in the centre of the lens. It’s for this reason that they’re sometimes called ‘cropped’ sensors. This can be good news if you’re taking pictures of distant subjects, as you effectively get a bit more ‘reach’; but it’s not so good for landscapes and interior shots, where you won’t be able to squeeze as much into the shot as you could with the same lens attached to a full-frame camera.

You’ll often see the ‘35mm equivalent’ (really the ‘full-frame equivalent’) focal length listed alongside a lens’s actual focal length. This provides a standard way to compare different lens and camera combinations. It’s calculated by taking a lens’s focal length and multiplying it by the ‘crop factor’ or ‘focal length multiplier’ of the sensor. An APS-C sensor has a crop factor of 1.5x or 1.6x, depending on the camera, so this means a 100mm lens on an APS-C camera has an 35mm equivalent focal length of 150mm/160mm. In other words, you’d need to use a focal length of 150mm/160mm on a full-frame camera to get the same view as a focal length of 100mm on an APS-C camera.

To get around this problem, manufacturers also make a range of dedicated ‘digital only’ lenses with shorter focal lengths to allow for the crop.

50mm provides a similar perspective to the human eye