What is an F-Stop?
Before we get started, let's review exactly what an f-stop is (if you're brand new to photography, you might want to start by reading one of our free digital photography tutorials).
Aperture refers to the opening at the front of your lens that allows light into the camera. The larger the aperture (opening), the more light is able to enter, and the less time you need to capture a properly exposed photograph.
Aperture is expressed in terms of f-stops. Here's where it gets slightly technical: the f-stop is the ratio of the focal length to the aperture, where focal length is simply the distance from the camera's sensor to the focusing element in the lens. For example, suppose you're using the nifty fifty, which is the nickname for a 50mm f/1.4 lens. The 1.4 means that if the lens is wide open (at its greatest aperture), the focal distance is 1.4 times as much as the opening in the lens. If you stepped down to f/32, however, the opening is now only 1/32 of the focal distance, so it lets in a lot less light.
Lenses with larger apertures (particularly zoom lenses that keep the same maximum aperture throughout their zoom range) are more expensive than lenses with smaller apertures and variable apertures, which is why entry-level lenses will often have maximum apertures like 3.5-5.6, which means that it can open as wide as f/3.5 at one end, but only to f/5.6 at the other end. A pro lens, by contrast, will often have a maximum aperture of f/2.8 for the entire zoom range.