Equivalent focal length: what 24 mm, 70 mm and 120 mm mean

Equivalent focal length (35 mm equivalent) is a conventional number in millimetres that shows a smartphone camera's field of view compared with a classic camera. 13-16 mm is ultra-wide, 24 mm a typical main camera and 70-120 mm a telephoto.

Different focal lengths give different fields of view

In plain words

Having the same "ruler" for all cameras is handy, like having the same shoe sizes in every country. Smartphone sensors differ, so their actual focal lengths tell you nothing. The equivalent translates them into one "language": see 24 mm and you know how wide it is; see 120 mm and you know it is a strong zoom.

How it works

A smartphone lens's real focal length is just a few millimetres.
Field of view depends on both focal length and sensor size.
To compare, it is converted to a 35 mm film frame (36×24 mm).
The smaller the number, the wider the view: 13 mm fits a whole room.
Divide the telephoto's focal length by the main camera's to get the zoom factor: 120 / 24 = 5x.

The equivalent is a shared "ruler" for all cameras

Where you'll see it

In flagship camera specs and in the camera interface: "24 mm", "35 mm", "70 mm", "120 mm". Some phones let you switch between "focal lengths" like a professional camera.

Does it matter when buying

It is a handy way to understand what a camera can do. For portraits, photographers like 50-85 mm - faces look natural. For landscapes and cramped rooms, ultra-wide 13-16 mm. If you see "3x optical zoom" and a 70 mm focal length, it all adds up.

Myths

Myth: Focal length is about quality. Reality: It is about field of view, not sharpness.
Myth: 5x optical zoom means 5 mm. Reality: Zoom is a ratio of focal lengths, not a length itself.

FAQ

Which focal length is best for selfies?

About 24 mm for groups and 30-35 mm for one face - less distortion.

Why do people look stretched at the edges of an ultra-wide shot?

A wide angle creates perspective distortion.

How do I work out the zoom factor?

Divide the telephoto's equivalent focal length by the main camera's.