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In addition to the focus ring, lenses have a ring that determines how much light is let into the camera. This ring controls the size of the aperture or the iris-the opening in the lens that allows light to pass. In addition to focal length, lenses are classified according to the amount of light they let into the camera. Fast lenses allow more light to enter the camera while slow lenses allow less light to enter the camera. Because the amount of light entering the camera is the single most important element in taking the pic¬ture, this characteristic of the lens is an essential factor in the ultimate qual¬ity of the television picture. The iris diaphragm, generally called the fstop, is the opening that can be made larger or smaller to regulate the amount of light admitted to the sen¬sitive surface of the imaging device. It is ad¬justed according to the amount of light on the set or on the object being televised. A series of numbers, such as filA, fl2.8, fl4, etc., marked on the lens ring indicates the size of the iris opening. The lower the num¬ber, the wider the opening and greater the amount of light entering the camera. The smaller the iris opening, the larger the f-stop number. In other words, the size of the f-stop number has an inverse relation to the size of the iris opening; for instance, an
f/3.8 lens has a smaller opening than an fl2.5 lens. Lenses usually range from fll.O to f/22.0. Figures 2.7(a), (b), and (c) show how exposure can affect a picture.
An f/1.4 lens is considered a fairly fast lens because it can open up wide enough to let in a good deal of light. An f/4 lens would be a slow lens because it can let in much less light. It is important to practice adjusting the lenses with different amounts of light to determine the best opening for a clear picture. If we have to open the lens very wide to get enough light, it will be much harder to focus.
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