Figuring out focal length

Get your head around the fundamental aspect of choosing and using lenses

The lens you select to shoot a subject with has a huge impact on your pictures. It’s the choice of focal length that’s the key decision, as this affects what you can include in a shot and has an impact on both your viewpoint and composition.

The focal length is the distance from the optical centre of a lens, to the camera’s imaging sensor when the lens is focused at infinity. The ‘standard’ focal length for a full-frame camera is 50mm, with wide-angle lenses having shorter focal lengths and telephoto lenses having long focal lengths.

When we talk about focal length, it’s actually the angle of view we’re talking about. The angle of view is a way of measuring how much of a subject or a scene the lens can ‘see’. Lenses with long focal lengths have a narrow angle of view, while lenses with shorter focal lengths offer a wider angle of view.

Wide-angle lenses are able to pull a great deal into a single picture. They’re a good choice when you can’t move farther from a subject but you want to squeeze lots in. They let you to capture a subject and its surroundings in a single picture, which is why they’re a go-to lens for landscape photography.

Lenses with long focal lengths, such as 100mm to 400mm and beyond offer progressively narrower angles of view that make distant subjects appear bigger in the picture. This is why they’re essential for wildlife and sports photography, where you may be unable to get physically close to the action. The drawback to such narrow angles is that it’s easy to lose track of a moving subject, as a small shift in the position of the camera can have a huge effect on which area is picked by the lens to capture.

Angle of view

A lens’s angle of view is measured in degrees, either horizontally, vertically or diagonally across an image, and is listed in the lens’s specifications. Prime lenses have a fixed angle of view, whereas the angle changes on a zoom lens as it’s zoomed through the focal length range. The field of view – the physical distance captured across an image – changes depending on how near or far a camera is to the scene being photographed, but the angle of view is determined by the focal length of the lens that you’re using.