Telescopes are usually described in terms of their aperture. A 70mm scope refers to the size of the aperture. But things are different for camera lenses. A 300mm lens means the focal length is 300mm.

Camera lenses almost never mention aperture. This is because their apertures are variable. With a camera lens, the focal length stays the same, but when you change the f/stop, you stop change the aperture.

The aperture on a camera lens is defined by the size of the opening in the diaphragm. It is usually located internally in the lens, either between lens elements or behind them. A diaphragm is an iris-like device that can open and close to let more or less light in.

A wide-angle lens is seen with the aperture wide open (left) and stopped down to f/5.6 (right). This shows how the aperture gets smaller and lets in less light as the lens is stopped down. It also shows that the front element of a lens is not the same thing as the aperture. The aperture is governed by the size of the stop at the diaphragm.

The apparent aperture of a camera lens can be deceiving. You may have a telephoto lens that looks like it has an opening of 2 inches in the front, but when it is stopped down to a smaller f/stop, the effective size of the opening may only be 1 inch or even much less.

Wide-angle camera lenses can be even more deceiving. A fisheye lens may have an enormous piece of glass for its front element, but this is so it can have a wide field of view. It's effective aperture may be tiny.

You can easily calculate the size of the aperture of your lens by simply dividing the focal length of the lens by the focal ratio you are using. For example, a lens with 50mm of focal length, used at f/2 has an effective aperture of 25mm. This same lens, stopped down two stops to f/4 would have an effective aperture of 12.5mm.

On the Canon 1000D (Digital Rebel XS), you change the aperture of a lens by holding down the AV (Aperture Value) button on the back of the camera (arrowed) and turning the main control dial next to the shutter button. The F number (arrowed) will change on the LCD display to indicate the working aperture. Here the aperture is set to f/3.5.

The size of the aperture determines how faint the stars are that you can record. Since a larger aperture lets in more light, it can record fainter stars. A lens should be used at the widest aperture at which it performs acceptably. For astrophotography, collecting more light is more important than getting the absolute sharpest image possible. Your lens may be really sharp at f/11, but your exposures may be so long that you can't possibly track accurately that long to get an image that is not ruined by trailing.

Most fast lenses perform well enough for astrophotography when stopped down 2 stops from maximum aperture. Most slow lenses don't need to be stopped down that much and can even be used wide open in some cases if some aberrations are tolerated.

The best way to evaluate your own particular lens is to simply go out and shoot some tests on a star field. Try the lens wide open. Stop it down one stop and double the exposure. Stop it down another stop and double the exposure again. Then enlarge the images to 100 percent and examine the stars in the center of the frame and at the far corners of the frame.

Aperture - The Bottom Line The aperture of a camera lens is the size of the opening that lets light in. A camera lens' aperture is controlled by the diaphragm or iris inside the lens.