What do you mean when you say a lens is fast?

[LAW2]

This comes up when talking about prime lens that have a larger aperature (low Fstop 1.8. Are we talking about shutter speed because the aperature is so large?

Sorry wrong forum. I don't know how to move it.

 

[Big Mike]

A fast lens or a fast aperture would be about F2.8 or bigger (F2.0, F1.8, F1.4 etc). On the surface, this does not refer to shutter speed...a fast lens is a fast lens...on or off the camera.

Why do we call it fast? Probably because a wider aperture allows us to use a faster shutter speed.

 

[JEazy]

Because they let more light in faster than slower lenses?

 

[Rob]

Often with camera lenses there are two options of the same focal length, for example a 70-200 f4.5-5.6 and a 70-200 f2.8. The one with the smaller f-number allows a wider aperture and costs a great deal more. They are often built better. We call them "fast" lenses, simply because they allow more light to be let through to the film/sensor and therefore the exposure can be faster (usually a desirable thing).

Prime lenses, which do not zoom, often have fewer elements and a wider aperture, making them "faster" than an equivilent focal length zoom.

Rob

 

[Soocom1]

Many lenses as described by Rob are also of the large element range for yet another but closely related reason:

Faster lenses also allow the image to be much brighter, and thus makes the image more color saturated.

 

[THORHAMMER]

also in low light you can focus a lot faster and more accurate with them and a good body in conjunction.... they dont hunt for focus very often unless your in total ddarkness....