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The lens determines the quality of the images that are formed inside a camera. The lens and its quality determine what kind of image your camera can collect and to a great degree set the ultimate quality of picture you can hope to take.
You can take photographs without a lens—that's the principle behind pinhole photography. But nearly all cameras have lenses for one very good reason: A lens collects a lot of light. It captures more photons. That gives the camera more light to work with and makes exposures quicker. The lens also affects the view, what the camera sees. It even alters the aesthetics of the image you make.
Literally speaking, an aperture is nothing more than a hole, and that's what it means for cameras. The aperture is the hole in the lens through which light can get to the image sensor. In photography, digital or otherwise, the aperture is more important than a mere hole. It's a variable hole, one you can make larger or smaller to let more or less light reach the film or sensor. A larger aperture lets more light in; smaller lets in less.
Varying the size of the aperture helps a camera of any kind cope with light conditions. When light is too bright, it can overwhelm the image sensor; too dim, and the image sensor might not be able to find enough photons to make an image. To prevent these problems, most cameras use wider apertures in dim light and smaller apertures in bright light.
The mechanism for setting the aperture is termed the iris of the camera, and it corresponds to the iris of the human eye. By sliding thin plates called iris blades—in manual mode typically by rotating a ring around the lens termed, appropriately enough, the aperture ring—you can adjust the size of the hole between the blades and thus the aperture.
The size of the aperture is measured as an f-stop. Most commonly, the f-stop is a number in the geometric series 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22, 32. The series is designed so that the next higher stop cuts the light transmitted through the lens to half the value of the previous stop. An f-stop setting of 8.0 allows half as much light into the camera as a setting of 5.6. (The sequence is simpler than it looks. Each f-stop differs from its predecessor by the square root of two, the results rounded.) Table 23.1 lists the ISO standard (nominal) f-stops, the actual (computed) f-stops, and relative light values.
Although the sequence of numbers is now an arbitrary sequence, the value of the f-stop is scientifically defined. It is the focal length of the lens divided by the apparent aperture of the lens (not the actual size of the hole in the iris but the size of the hole visible through the lens—the glass in the lens can magnify the aperture or even make it appear smaller). For example, a lens with a 50-millimeter focal length set at f-stop 4 would have a visible aperture of 12.5 millimeters. This relationship leads to the common way of writing f-stop settings. A setting of four is usually written as f/4. In other words, it is the focal length divided by four.
Lenses are usually described by the widest aperture at which they can be set (for example f/1.4 or f/2.8). Sometimes the widest aperture setting falls between the numbers of the standard f-stop sequence (for example f/2.3), but the value represents the same concept.
Many zoom lenses are marked with two f-stops (for example f/2.8–4.3). The two values do not represent the total range of stops available from the lens. Rather, they represent the range of minimum f-stop values. The formula for determining the f-stop of a lens requires the size of the aperture to vary with the focal length of the lens at a constant f-stop setting. A longer lens requires a wider aperture for the same f-stop setting.
Zoom lenses are able to vary their focal lengths to change the size of the image they make in your camera. Nearly all zoom lenses automatically change the aperture as you zoom to maintain a constant f-stop setting. The physical diameter of the lens limits its maximum aperture—the hole in the iris can't be bigger than the lens itself. When the focal length is set shorter, however, the largest possible aperture represents a wider (lower value) f-stop, which is desirable because it allows more light into the lens so that you can take photographs in dimmer light. Consequently, lens-makers let you take advantage of the wider f-stop settings at shorter focal lengths, and the minimum f-stop value varies with the focal length setting of the lens. The highest f-stop number represents the widest setting at the longest focal length setting of the lens. The lower value represents the widest f-stop setting possible at the most favorable focal length setting.
When using a lens, the aperture or f-stop setting can have a dramatic effect on the final image beyond setting exposure. The f-stop determines the depth of field (more correctly, the depth of focus) in the image.
Technically speaking, the focal length of a lens is the distance from its nodal point to the plane of focus of its image of an object an infinite distance from the lens (that is, where the image appears sharpest). Although this highly technical concept appears to have no practical value in judging cameras or lenses, focal length has an important ramification. It determines the field of view a lens provides or, the corollary, the size of the image. As a practical matter, a lens with a short focal length provides a wide field of view and makes things look smaller and farther away. A long lens provides a narrow field of view and large images that look closer. There are other aesthetic considerations we'll discuss in the chapter about using a camera.
Wider, smaller, larger, and closer are all relative concepts. With lenses, such terms relate to a "normal" lens. For some reason not readily apparent, the photographic world has decided a normal lens has a field of view of 46 degrees. That is, the angle between the camera and the left side of what it sees is 46 degrees from a line drawn to the rightmost side of the image. On a 35 mm camera, that means a 50 mm lens is "normal."
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