These are colloquial expressions describing the maximum aperture value or values of which the lens is capable. Slow lenses have a very small maximum aperture, which means less light enters the lens and so longer time periods are required to expose the film or image sensor. Fast lenses have a very wide maximum aperture and so shorter time periods are required to expose the film or image sensor.

The larger the maximum aperture of a lens the more light it lets in. And so faster lenses are generally more desirable than slower lenses. First, fast lenses let you take photos in lower light levels using available light rather than blasting the scene with ugly light from a flash unit. Second, you can see through the viewfinder better since fast lenses let in more light and so the view through the finder will be brighter with a fast lens.

As explained above, lens f stops are ratios, and so smaller numbers indicate larger apertures. A lens with a maximum f stop value of 1.4 is, therefore, fast. And a lens with a maximum f stop value of 5.6 is slow by comparison. However, f stop numbers are the ratio between the focal length of the lens and the aperture, which means that it’s very easy to design a fast 50mm lens (1.8 is a typical maximum aperture value) but very hard to design a long 200mm telephoto lens with a maximum aperture so large.

In fact, designing fast lenses in general is more complex and expensive than designing a lens with a small maximum aperture, so fast lenses tend to cost more than slow ones. It’s also harder to design and build a fast zoom lens than it is a fast prime (fixed focal length) lens. Faster lenses are also usually physically larger than slower lenses of equivalent focal lengths. The reasons for all this are tied into the complex mathematics of optics.

Note that autofocus lenses for EOS cameras have the focus motor built into the lens, not the camera. And some lenses focus more rapidly than others. So sometimes you hear people talking about a lens having a fast or slow focus motor speed, which is a separate matter altogether from the optical properties of the lens.