Fast lenses ...

[Osi Osgood]

So, I have heard this expression occasionally, but what makes one a "fast lenses"?

[Paul Adsett]

A fast lens is one with a very large aperture, typically less than about f1.3. For the most light on the screen you need at least an f1,1 lens , preferably an f1.0 lens. Even better is an f1.0 fixed focal length lens ( non-zoom lens) which will have better light transmission and contrast than a complex multi-element zoom lens.
The light transmission is inversely proportional to the square of the aperture number. So an f1.0 lens will transmit 1.7 times as much light as an f1.3 lens and 2. 6 times as much light as an f1.6 lens

[Brian Fretwell]

I imagine this expression came from still cameras where a large aperture lens would allow a fast (high speed) shutter to be used.

[Mark Mander]

Designed for low light situations ,Mark