Aperture – controlling the light

The aperture of a lens is often intended to mean the lens’s maximum aperture – that is, its speed. In this sense the term is used to designate a major lens characteristic, as when a lens is described as having, for example, a focal length of 135mm, an angle of view of 18 degrees, and an aperture of f/3.5.

On the other hand, aperture is used in a broader sense to indicate simply the working aperture, or f-stop, at which the iris diaphragm in a lens is set. It is this second meaning that we are more concerned with here, for it points out one of the way you can control the amount of light reaching the sensor in your camera. When the diaphragm is open all the way, the greatest amount of light passes through the lens. But as the diaphragm is gradually closed down, less and less light reaches the sensor. Since some method is needed to keep track of exactly how much light is getting through, the f-stop system was invented.

An f-stop is a number indicating the ratio between the focal length of any given lens and the diameter of its diaphragm opening. For example, if a lens has a 50mm focal length, and its diaphragm is open to 12.5 widths, the focal length is four times as great as the lens opening. That translates into a lens “stop” of f/4. With a 100mm lens and a 25.5mm opening, or a 200mm lens and a 50mm opening, the aperture is still f/4 and will pass for all practical purposes, the same amount of light. By means of this system of relative apertures any lens of any focal length set at a given f-stop will admit the same quantity of light as any other lens set to the same number.

Of course, you don’t actually have to measure diaphragm openings and do the calculating necessary to arrive at the f-stops you need. Everything is done for you by the lens maker when all f-numbers are inscribed on the barrel. As further convenience the numbers are arranged so that each one in turn changes the amount of light reaching the sensor by a factor of two. That is, as you move from the largest opening (a small number such as f/2, for instance) to the tiniest opening (perhaps f/22), each number represents only half as much light as the previous one. Conversely, when you start with the diaphragm closed down and then gradually open it, each successive number doubles the light admitted.

If you are wondering why large f-numbers mean small opening and vice versa, there is a simple explanation. Think of each f-number as the denominator of a fraction. We know that a fraction with a large number in the denominator such as 1/16 indicates a lesser quantity than a fraction with small denominator such as ˝. The same logic hold true for f-stops. Bigger numbers mean smaller openings and consequently less light.

Many photographers are uncertain about the benefits of owning a fast lens – one with a large maximum aperture. There are some advantages of course. In dim light situations where a tripod or flash is impractical, a fast lens will still allow you to take hand held pictures at reasonable shutter speeds. In addition, the greater light gathering power permits you to focus more easily. Not only is the finder image brighter, but it snaps in and out of focus more readily because depth of field is shallower. The decreased depth of field is also convenient when you wish to blur an unsightly background behind a portrait subject.

Unfortunately, a fast lens has disadvantages too, the prime on being higher cost. Along with that may be greater weight and slightly reduced optical performance at the largest aperture. However, the standard f/1.4 lenses furnished with many current cameras are not only fast, they are excellent optics as well.


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