The aperture in a camera’s lens is simply the opening through which light passes to strike the sensor or film. Instead of quantifying aperture size directly, for example, speaking of an 8mm opening, we usually speak of the “f-number” or “f-stop”. The f-stop is the relative aperture in comparison to the focal length of the lens; that is, the f-stop is the focal length of the lens divided by the diameter of the aperture. For example, if the focal length is 100mm and the aperture is 25mm diameter then the f-stop is 100 / 25 = 4. Since most people use aperture and f-stop interchangeably, we’d probably say that the aperture was “f4” in that example. For more details, check Wikipedia.
What about f-stops for zoom lenses? If the physical opening remains constant but the focal length changes, then the f-stop changes with the zoom setting. This is the situation with the Canon G9 and most digital cameras having built-in zoom lenses. By tinkering with my G9, I found the zoom points for which the maximum aperture (f-stop) changes and constructed the table below.
The table shows, for example, that the maximum aperture for the wide angle (7.4mm focal length) is f2.8 but near mid-zoom, say 25mm focal length, the maximum aperture is f4. At maximum zoom (44.4mm focal length) the maximum aperture is f4.8.
Notice that, contrary to the simple definition, the maximum f-stop for the G9 is not exactly proportional to the focal length. This apparent contradiction is an indication that the available physical aperture is also changing. If this were not the case, then the f-stop at 44.4mm focal length would be f16.8 instead of f4.8 !
Try this little experiment. Put the G9 (probably any camera) at maximum wide angle, Aperture priority (Av mode) at f8. Oh – you might want to turn off the focus assist light! Point the camera towards yourself and look deep into the lens. Push the shutter button halfway (you don’t have to actually take a picture) and watch carefully for a little bit of movement deep in the assembly. You’ll see a small circular opening become even smaller. Now change the f-stop to f2.8 and try again. You’ll see that the small opening doesn’t change. Now zoom to about mid-telephoto and repeat the tests. Notice that the initial size of the circular opening (the physical aperture) is somewhat larger at mid-telephoto. Zoom all the way in and repeat. Although it becomes more difficult to see deep into the lens, the initial opening becomes even larger at maximum telephoto settings and it changes from a large opening to a smaller one at f8.
Finally, remember that, counter-intuitively, a larger f-number (f-stop) actually means a physically smaller opening (aperture) that lets in less light.