All about Apertures and F-Stops
The second factor governing lens choice is aperture, which is the adjustable opening that regulates how much light passes through a lens. You will usually be most concerned with the largest aperture (smallest number). The lower the number, the better that lens is for low-light or high-speed use. The range of apertures varies from lens to lens, affecting exactly how much light they're capable of allowing in.
As discussed in Chapter 6, apertures are expressed in f-stops, such as f/1.8. This measure is a ratio that reflects the focal length of the lens (f) divided by the actual diameter of the aperture. The smaller the number, the more light the lens is capable of letting in. Lenses with extremely low f-stops, such as f/1.2, let in a great deal of light and can handle fast shutter speeds.
Lenses with low f-stop numbers are often referred to as fast lenses. Slow lenses are those that have higher f-stop numbers, generally in the f/5.6 range and above.
Fast lenses are wonderful things to own, but they do come at a higher price. Not only do they cost more than comparable (but slower) lenses, they're also bigger — they have to be to allow in more light. They also weigh more and take up a great deal more room in your camera bag. While weight and bulk may not seem like important factors initially, they will definitely come into play if you're lugging your equipment on an all-day hike. That wonderfully fast lens may end up being your worst nightmare after you've hauled it for a few hours.
Normal lenses aren't the only prime lenses. Any lens with a fixed focal length is considered a prime lens, so wide-angle lenses and telephoto lenses also fall into this category. Compared to zoom lenses, they generally weigh less and are faster. They can be sharper than zoom lenses as well.
