When you’re reading about sports photography, you’ll probably come across the term fast glass or a fast lens. What does this mean?

A fast lens is a lens with a large maximum aperture – in other words, a really low f/value. The term is somewhat relative, and anything from a Canon 70-200mm f/2.8L to a Canon 50mm f/1.2L would be considered fast… although that f/1.2 is probably about as fast as you’ll get.

This often means really expensive, too, although you can find some more economical lenses under $1,000.

So Why Are These Lenses “Fast”?

Two related settings affect an image’s exposure – aperture and shutter speed. As the aperture gets larger, more light comes in. To compensate for this and keep the exposure the same, you can increase the shutter speed. A “faster” lens has a larger maximum opening, allowing you to use a quicker maximum shutter speed.

This is especially important with regards to indoor sporting events. You need a high shutter speed to freeze the action, but you’re often constrained by crappy lighting conditions. Professional venues will have decent lights, but high school gyms are going to make you pull your hair out. With a not-so-fast lens that maxes out at f/5.6, like the run of the mill Canon 55-250mm f/4.0-5.6 zoom lens I originally got with my first dSLR, you’re either going to have to jack up the ISO or settle for blurry photos.

A zoom lens with a maximum aperture of f/2.8 is much better suited for indoor or night sports photography. Simply put, the glass is “faster.” And this is, unfortunately, one area where the quality and expense of your gear makes a big difference.