You may have heard the term “Fast Lens” when referring to a good quality lens. I find the term “Fast” a bit misleading, although technically accurate.
A fast lens is a lens with a large aperture. So a 50mm lens with the largest aperture of f5 may not be considered a fast lens, a 50mm lens with the largest aperture of 1.4 is very fast. Remember the aperture size is the size of the opening or diaphragm of the lens. A smaller number means a bigger opening.
The reason it is a “fast” lens is that, if you recall from my previous post on aperture, a larger aperture lets in more light, and thus the lens aperture does not need to be left open for as long as a lens with a smaller aperture.
This is significant! Imagine shooting an indoor wedding with low light. Let’s say the church doesn’t allow flash, and doesn’t allow you to use your tripod. If you shoot with a slow lens, you will need to use a slower shutter speed to get a well lit shot of the bride and groom. Since we humans tend to shake, a longer exposure will reveal that shaking, and will be exhibited as blur. A faster, better lens will be able to take the same shot in a much shorter exposure, thus leading to less camera shake and resulting in a sharper image.