by ʇolɐǝz ǝɥʇ qoq

Submit your Photo
Hall of Fame

Please participate in Meta
and help us grow.

Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

It seems very counter intuitive that the amount of light let in by a lens is referred to as the lens speed. This term confused me for a while because I couldn't logically see how a speed would be associated with a particular lens, being either fast or slow.

Is there any reason why this is referred to as the speed of the lens?

share|improve this question
I suspect that long long ago, as you sat rigid backed and unmoving as the minutes ticked away and your portrait was taken, and the "faster" the lens the sooner your ordeal was over, that "speed" was a very meaningful term indeed :-). – Russell McMahon Apr 14 '12 at 21:25
up vote 13 down vote accepted

Indeed, Speed is way too overloaded in Photography, we have lens speed, shutter-speed and ISO speed!

These terms indirectly refer the possible shutter-speed. A fast lens has a bigger maximum aperture (represented by smaller numbers) which lets more light in. As a consequence, a fast lens lets you use fast shutter-speeds.

The same is true of ISO. A high ISO is called fast because you can use a faster shutter-speed with it.

share|improve this answer

Consider the exposure triangle - aperture, shutter speed and ISO. When you adjust one, you can adjust the others accordingly. Ignore ISO for now, as that is a function of the camera, and consider the aperture and shutter speed variables.

The wider the aperture, the more light can enter the camera. This means that for any given exposure, the shutter speed can be shorter - in other words, the shutter speed is fast. Faster shutter speeds are generally desirable as they freeze motion and counteract camera shake.

So a fast lens is generally a wide aperture lens, like a 1.4 or 2.8, because compared to a slow lens like a 4 or 5.6, you can get many more shutter speed stops with them.

share|improve this answer
Hmm exposure triangle mentioned....awaits @mattdm to swoop down. – rfusca Apr 14 '12 at 19:48

Consider that you are taking a photo with a specific amount of light, say a dark-cloudy day. You get an exposure setting from the ASA rating of the film or digital sensor, the f-stop and the shutter speed.

If you put a F5.6 lens on your camera body, you will have to have a slow shutter speed.

Put an F1.4 lens on it, and your shutter speed will be must faster.

As @ital says, we tend to overload the term speed in many areas within photography.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.