Discovering Lens Apertures

The lens aperture is an adjustable control that determines the width of the opening that admits light to the sensor. The wider the aperture, the more light that can reach the sensor, making it possible to take pictures in dimmer light. You can think of an aperture as the pupil of your eye. When it's bright outside, your pupils contract (and you squint), letting in less light. When it's dim, your pupils dilate.

A narrow aperture reduces the amount of light that can reach the sensor, letting you avoid overloading the imaging device in very bright light. These lens openings are used in tandem with shutter speed (the amount of time the sensor is exposed to the light) to control the exposure. Your digital camera needs a selection of lens apertures (f-stops) so that you can take pictures in a broad range of lighting conditions.

F-stops aren't absolute values; they're calculated by measuring the actual size of the lens opening as it relates to the focal length of the lens, using a complicated formula. The easiest way to visualize how f-stops work is to imagine them as the denominators of fractions. Just like 1/2 is larger than 1/4 or 1/8, f/2 is larger than f/4 or f/8. The relationship is such that as the amount of light reaching the sensor is doubled, the f-stop increases using an odd-looking series of numbers: f/2 is twice as large as f/2.8, which is twice as large as f/4, and so on through f/5.6, f/8, f/11, f/16, and f/22 (which is just about the smallest f-stop you'll encounter in the digital realm). Figure 1 shows a lens opening that's partially closed.

If you're taking photos in automatic mode, you don't need to know what f-stop you're using because the camera selects it for you automatically. Your digital camera probably displays the f-stop being used, however, either in the viewfinder or on an LCD panel, and the information can be helpful. Just remember the following things (see Figure 2):

• The larger the f-stop (the smaller the number), the more light that is admitted (faster). An f/2 lens (small number, large f-stop) is a fast lens, whereas one with a maximum aperture of f/8 (larger number, smaller f-stop) is slow. If you need to take photos in dim light, you want to buy a camera with a fast lens.

• The smaller the f-stop (larger the number), the more of your image that is in sharp focus.

• As the f-stops get smaller (larger number), exposure time must be increased to let in the same amount of light. For example, if you take a photo at f/2 for one-half second, you need to double the exposure time to one full second if you stop down (reduce the aperture) to f/4.

Typically, you'll find that among non-SLR digital cameras, the speed or maximum aperture of camera lenses is smaller than is common among 35mm film cameras, and the range of available apertures is more limited, too. For example, even an inexpensive snapshooter 35mm film camera might have an f/1.9 lens (pretty fast), and serious photographers with 35mm SLRs probably own f/1.4 or faster normal and wide-angle lenses. Although zoom lenses usually have smaller maximum apertures, in the 35mm film world, f/2.8–f/3.5 are common numbers.

In the digital camera realm, things are a bit different, primarily because the very short focal lengths of the lenses are more difficult to design with large lens openings. So don't be alarmed if your favorite digital camera has an f/4 or f/5.6 maximum aperture. You might even find that the lens is labeled f/4–f/5.6 because the effective widest opening can vary as a lens is zoomed in and out. A lens might have an f/4 opening when zoomed out to 38mm but only f/5.6 at its maximum telephoto setting of 152mm.

To make things really interesting, your digital camera lens might have only a limited number of different f-stops available — perhaps f/4, f/5.6, and f/8, or maybe none at all. You won't miss the lack of f-stops: Modern electronic shutters are fast enough to provide the proper exposure without smaller lens openings. Furthermore, in 35mm photography, smaller lens openings have been used to increase the range of sharpness (called depth of field). However, the short focal length of non-SLR digital camera lenses usually means that just about everything is sharp, anyway.