Beecher's Handouts /

Focal Length / 17.1 - MMs

Lenses have two types of MMs: diameter and focal length.

Let's look at each one.

Diameter

The front of a lens has a diameter, such as 67mm.

You need to know the diameter of the lens if you're buying a filter.

The easiest way to determine the diameter is to look at the inside surface of the lens cap.

The diameter is usually printed there.

Focal Length

Technically, focal length is the optical distance from the front of the lens to the sensor.

Practically, focal length is how much you can see of a scene.

A wide-angle lens "sees" a lot, from side-to-side.

A telephoto lens magnifies a distant part of a scene, and brings it closer.

Some lenses have only a single focal length, a fixed focal length, such as a 500mm mirror lens.

Zoom lenses have many focal lengths.

For example, a 28mm to 200mm zoom lens has all of the focal lengths between 28mm to 200mm.

Examples

18mm

50mm

70mm

450mm