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?
Answer
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.
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