Focal length, angle of view and crop factor

What focal length is

Focal length is roughly the distance from the optical centre of the lens to the sensor when the lens is focused at infinity. It is printed on every lens in millimetres: 24 mm, 50 mm, 70-200 mm.

It is a property of the lens itself. A 50 mm lens is 50 mm on any camera; what changes from camera to camera is how much of its image the sensor actually records.

Angle of view

Angle of view is how wide a slice of the world ends up in the frame. A shorter focal length gives a wider angle; doubling the focal length roughly halves it.

Approximate diagonal angles on full frame (36 x 24 mm): 14 mm about 114°, 24 mm about 84°, 35 mm about 63°, 50 mm about 47°, 85 mm about 29°, 200 mm about 12°, 400 mm about 6°. The diagram on this page plots this curve.

Ultra-wide: under about 20 mm. Wide: about 20-35 mm. Normal: about 40-60 mm, close to how a scene feels to the eye. Short tele: about 70-135 mm. Long tele: 200 mm and up.

To try any combination of lens and sensor, use the angle-of-view calculator on this page.

Crop factor and equivalent focal length

Crop factor is how much smaller a sensor is than full frame, measured across the diagonal. Typical values: APS-C about 1.5 (about 1.6 on some cameras), Micro Four Thirds 2, 1-inch compacts about 2.7.

A smaller sensor records only the centre of the lens's image, so the view looks tighter. Multiply the focal length by the crop factor to find the full-frame lens with the same angle of view: 35 mm on APS-C (x1.5) frames like about 52 mm; 25 mm on Micro Four Thirds frames like 50 mm.

This "equivalent focal length" is a way of comparing framing, not a new focal length. The lens itself has not changed.

Phones use the same idea. Their real focal lengths are tiny (often 4-9 mm) because their sensors are tiny, so makers quote an equivalent: a main camera labelled "24 mm" or "26 mm" frames like that lens on full frame. A "3x" or "5x" camera is roughly 70-120 mm equivalent.

Perspective is about distance

A common belief is that long lenses "compress" a scene and wide lenses "stretch" it. In fact, the relative size of near and far things depends only on where the camera stands.

Try it: shoot a person with mountains behind from 30 ft at 200 mm, then from the same spot at 50 mm. Crop the 50 mm picture to the same framing and the perspective is identical; only detail is lost.

What really happens is that a long lens tempts you to step back to fit your subject in, and the greater distance makes the background look larger and closer. A wide lens tempts you to step in, so nearby things look big and the background shrinks. That is why a face shot from arm's length with a wide lens looks distorted, and the same face from about 6-10 ft looks natural.

Prime or zoom

A prime lens has one fixed focal length. It is usually smaller, lighter and brighter (f/1.4 to f/2 is common), and you frame by moving your feet.

A zoom covers a range, such as 24-70 mm or 70-200 mm, so you can reframe without moving. It is convenient for travel and events, but tends to be larger and less bright for its price.

Many people start with a zoom, notice which focal lengths they use most, then add a prime there.

Typical uses (full-frame equivalent)

Landscapes, interiors, architecture: about 14-35 mm.

Street, travel, everyday: about 28-50 mm.

Portraits: about 85-135 mm, from a flattering distance.

Sport and wildlife: 200-600 mm, because you cannot get close.

These are approximate habits, not rules; any focal length can work if you think about where you stand.