What is the speed of a lens?

The speed of a lens depends upon the maximum amount of light it is capable of transmitting through to the film. This speed rating is determined by the diameter of the lens and the distance the light travels from the lens to the focal plane or film, and is measured in f stops. Sometimes this is also talked about as how fast a lens is.

It relates to the widest or most open aperture that the lens has. Because the exposure is a combination of both shutter speed and the lens aperture, and having a faster lens or wider aperture allows more light through the lens, a photographer can work hand held with out camera shake in dimmer conditions.

Although we don't usually use the widest aperture for most shooting, with 35 mm SLR cameras it is useful to have a bright viewing screen so faster lenses are of benift when we view the image before we push the shutter, - this extra light provides better illumination to focus the image.

However - faster lenses mean larger pieces of optical glass and while they have the potential to allow more light through the lens, because of the extra glass, they cost more $$. So a photographer needs to assess if they need to spend the $$ on the extra speed.

A range of similar lenses of differing speeds might be manufactured as follows:

50 mm f1.2
50 mm f1.4
50 mm f2
50 mm f2.8

The speed of the lens is usually imprinted around the face of the lens, and in the image above it is imprinted in the lower right corner as 1:2

This means that when a subject is photographed at size on the negative smaller than a ratio of 1 to 1 f 2 is the fastest aperture of this lens, if however the ratio is larger than 1 to 1 the aperture setting ill be untrue and will have to be calculated. Hence the embossing 1:2

Want to learn more? - do a workshop or one on one with Lloyd Godman