Photography has a lot of confusing terminology, and often there are several ways to describe the same idea. One of the more annoying ones is when lenses are called “fast”. How can a lens have a speed? If you drop two lenses, they’ll both hit the ground (and break) at the same time. It’s an old term, so let’s find out.
If you recall, the two basic things you have control over when you’re taking a photo are the shutter speed your camera uses and the aperture of the lens. The shutter speed is measured in seconds or fractions of a second while aperture is measured in f-stops.
The wider the aperture of the lens is open, the faster the shutter speed needs to be to get a good exposure. If you took a photo with the aperture set to f/8 and the shutter speed set to 1/30th of a second, to get the same exposure with an aperture of f/2.8, you’d need to set the shutter speed to 1/250th of a second. This is where the idea of lens speed comes from.
Back in the early days of photography, some lenses had fixed apertures. A fast lens, then, was one that required you to use a fast shutter speed, while with a slow lens, you had to use a slower shutter speed. Now, there are very few fixed aperture lenses on the market outside of smartphone cameras, but the terms are still used interchangeably with wide aperture.
If you see someone recommend you use a fast lens for portraiture, or say that a certain lens is too slow for astrophotography, all they’re talking about is the aperture. They could just have said use a lens with a wide aperture or that the lens’ aperture isn’t wide enough for good star photos, but photographers like to be awkward.
Image Credit: LeonRW.