There is no such thing as correct exposure, and certainly no correct depth of field. Nevertheless photographers can hope to achieve both exposure and depth of field that satisfy perfectly their requirements. Both are elements of the creative process and can be manipulated to achieve our objectives.
Many images include wide ranges of brightness and colour, and ideal exposure varies across the frame. Exposure meters are therefore designed to provide averaged measurements for defined areas. They are calibrated using a standard grey card that reflects 18 percent of the light falling upon it. The assumption is that when all the variations of a real image are stirred together the result will be 18 percent grey.
Unfortunately designers cannot know how a camera will be used, or where in the frame a subject might be placed. The picture area is therefore commonly divided into zones, and separate exposure determinations are made for each one. These zone exposures are then combined into a compromise value using algorithms incorporating typical picture compositions. Zones are arranged in a number of different ways. Centre-weighted systems, useful for portraits, assume the subject occupies the centre of the frame and give greater priority to exposure in this area. Multi-segment or matrix systems use at least five zones, four quarters and a reserved central area, and give excellent all-round results. Spot-metering systems measure exposure exclusively for a very small and sometimes moveable area, usually close to the centre of the frame, and are useful for measuring shadows and highlights.
Once exposure has been determined, the information must be used to ensure that the film or digital sensor is exposed to the appropriate amount of light. Cameras have two basic mechanisms to control this - a shutter and an aperture. A shutter is a blind that opens and closes to admit light for a precisely controlled period. An aperture is a hole of variable size that controls how much light passes through the lens while the shutter is open.
Various mechanical and electronic systems have been employed to create shutters, but all are basically the same. Thin, lightweight blinds of fabric or metal are held in place to block light entering the lens aperture. When the shutter is released, the blinds are rapidly withdrawn to expose a section of film or a sensor. When the period of exposure has elapsed, the blinds are returned. Preset exposure periods, or shutter speeds, are arranged in a convenient and reasonably precise doubling sequence. Standard speeds are 1 second, then 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500 and 1/1000. These are marked as reciprocals for convenience and hence shown as 1, 2, 4, 8, 15 etc to 1000.
Apertures are controlled by an iris-like diaphragm. A series of crescent-shaped blades forms a near-circular hole at the centre of the lens. The diameter of the hole varies as the blades slide over each other under the control of an external ring. The aperture can be varied from a tiny opening a few millimetres in diameter to the maximum diameter of the lens. The aperture control ring is calibrated using a series of standard numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 and 32. These numbers are a measure of the lens aperture, known as f-numbers or f-stops, each step on the scale halving or doubling the area of the aperture and hence the amount of light that passes through the lens. Each stop is in fact the number by which the focal length of the lens must be divided to give a particular aperture diameter. For example, in a lens of 200mm focal length, 4 (usually written f/4) represents an aperture diameter of 200/4 = 50mm.
The beauty of the ratio-based f-stop system is that it is independent of focal length. Lenses can therefore be interchanged without altering exposure settings. A 50mm lens set to f/4 passes the same amount of light as a 300mm lens set to the same aperture. The doubling sequence also complements the parallel scale of shutter speeds. Consequently, changing shutter speed and aperture the same number of steps in opposite directions leaves exposure unchanged - 1/250 second at f/4 gives the same exposure as 1/125 at f/5.6.
Shutter speed and aperture are set using a number of different exposure modes. Manual mode requires each to be set independently. Shutter-priority mode requires the photographer to select the shutter speed and leaves the camera's automatic exposure system to set the appropriate aperture. Aperture-priority mode allows manual selection of aperture with the corresponding shutter speed set automatically. Various program modes set both parameters automatically according to convenient algorithms, leaving the photographer free to fire at will.
Exposure modes are chosen to suit subject and circumstances. If a photographer needs a high shutter speed it can be set directly in shutter-priority mode. If a particular aperture is required then aperture-priority mode may be preferred. Either way the exposure is the same - only the means of selection and the degree of manual intervention change. A key advantage of automatic exposure modes is speed of operation.
Despite all the technical precision and clever design, it is important to realize that there is no correct exposure. On many cameras it is possible manually to apply small steps of exposure compensation. These are added to, or subtracted from, the automatically calculated value. Compensation is useful for creative purposes, and in difficult light conditions. A photographer seeking to achieve an airy atmosphere dominated by lighter tones may choose to over-expose an image. A degree of under-exposure might be used to produce a preponderance of darker tones and hence a gloomy atmosphere. In difficult light conditions the photographer may choose to bias the automatically calculated exposure in a particular direction knowing that it will not otherwise satisfy requirements. Some cameras even provide for automatic bracketing, where exposure is varied incrementally in cycles of a few frames. The photographer can then obtain rapidly a number of slightly different exposures and choose the most pleasing image.