Many people are told they should use a fast lens for certain types of photography, such as sports and concerts, but aren’t really sure what they are. Basically when somebody refers to the speed of a lens they are referring to its maximum aperture.
For instance, the bigger the aperture is, the faster the lens is. However, remember the smaller number on the aperture setting actually represents a larger aperture as it lets in more light.
For example the f/1.8 aperture is larger than the f/5.6. The other part of the lens is its focal length, for example a 300mm lens.
A fast lens will have a low aperture number, meaning it can let in a lot of light, which will allow you to crank up the camera’s shutter speed.
This is ideal for low-light photography and for instances where you need to use a fast shutter speed to capture quick moving objects, such as indoor sports and wildlife photography. They’re also ideal for taking photos in places where you can’t use a flash.
Typically, a fast lens is considered to be one that has a maximum aperture setting of f/4 or higher, such as f/2.8, f/1.8, and f/1.4 etc., Remember that a larger aperture will produce a shallower depth of field. This is a good way of making your subject stand out against the background.