Unfortunately, there is a bit more to it than just the amount of light that gets through in a given period of time because the aperture affects something else called the “depth of field.” Depth of field is simply the distance between the nearest and furthest subjects in an image that can be rendered acceptably sharp at a given setting for the lens. Depth of field is also affected by the focal length of your lens, so before we move on to a discussion of aperture, we need to come to terms with focal length and depth of field.
The focal length (what our friend, the “f” in “f-number,” stands for) is a measure of the power of a lens to bend light. If you focus a lens on an object on the horizon (effectively at infinity), the distance from the lens to the film or sensor on which the image appears is the focal length; it is given in millimetres (mm). The greater a lens’ ability to bend light, the closer the image will be to the lens when it is in focus, and the shorter the focal length will be.
When you hear people talk about the length of a lens, it is the focal length to which they are referring. For small format (DSLR or 35mm) cameras, short focal length lenses are wide-angle lenses, and they range in focal length from roughly 10mm to 30mm. These lenses tend to have very long depth of field. In this format, a focal length of 50mm is considered “normal” (i.e., capturing a 45-degree angle of view, similar to the human eye). Generally speaking, lenses with focal lengths longer than 50mm are considered telephoto lenses in this format. These lenses have a relatively short, or “narrow,” depth of field.
Back in the days of film, the best lenses were “prime” lenses, which meant they had a fixed focal length. Today, “zoom” lenses, which allow you to change focal lengths without changing lenses, are of much better quality than they were before, so much so that it’s relatively uncommon for people to use prime lenses anymore (with the possible exception of the 50mm lens). In fact, some zoom lenses, including my very own 28-300mm, span the range from wide-angle to telephoto. These “one-size-fits-all” lenses are not quite as sharp as their more specialised cousins, but because of their wide focal length range and lightweight (plastic) casings, these lenses are great for travel and are often packaged with DSLRs as “kits.” You may have a similar lens yourself.