Depth of field is greater when the lens aperture is smaller. In a scene of great depth the use of an aperture of f2, for example, allows either foreground subjects (left) or background subjects (right) to be brought into focus, but not both.
The main areas of practical camera handling in photography concern sharpness control, exposure, and lighting.
The image on the film is sharpest when the lens is focused to the exact object distance. Usually, however, a scene includes objects at varying distances from the camera. Various factors affect the sharpness distribution in a picture of such a scene.
Movement of the subject while the camera shutter is open for the exposure leads to a blurred image. The exposure time must therefore be short enough to keep the blur within acceptable limits. The shutter speed required depends on the movement speed of the object, the scale of the image (movement blur becomes greater the nearer the subject or the longer the focal length of the lens used) and the movement direction; movement across the direction of view produces the most blurring.
Movement blur can be reduced, even with comparatively slow shutter speeds, by moving the camera (panning) to follow the subject during the exposure. This records the moving object comparatively sharply against a blurred background and emphasizes the impression of speed.
Camera shake through unsteady support during the exposure also creates image blur—over the whole picture in such cases. Hand-held shots generally demand shutter speeds of 1/30 second or shorter. For longer times a firm camera support—such as a tripod—is essential.
The correct exposure (aperture and shutter settings) can be derived from tables or calculators or by direct measurement of the subject luminance with a light meter.
The selection of an appropriate aperture and shutter speed among equivalent camera exposures depends on depth-of-field and subject-movement requirements. Some automatic cameras simplify this by selecting just one such combination at each exposure level (program automation).
Most current electronic flash units incorporate a sensor cell that measures the light reflected from the subject and controls the flash duration (and hence the exposure) accordingly. In certain cameras in which photocells measure the light reflected from the film, the same cells can similarly control the flash duration of suitable dedicated flash units. Lacking these provisions, flash exposures may be determined by measurement or by guide-number calculation.
Special meters can measure flash light quantity on a scene during a test firing of flashes; these are used extensively with more elaborate studio setups.
Some cameras use this principle for semiautomatic flash-exposure control: the aperture adjustment is coupled with the distance setting on the lens (or with an automatic rangefinding system) so that the lens aperture gets larger with increasing distance. This coupling is adjustable for different flash guide numbers.
The ideal negative exposure records the darkest subject shadows as a just visible density. More exposure yields a denser negative, which, however, can still give an acceptable print by appropriate print-exposure adjustment. This range of usable negative exposures, the exposure latitude, depends on the film and the subject. This latitude is greater the lower the subject contrast and the greater the film's exposure range (and, generally, the lower the film contrast). Because of exposure latitude, simple cameras with limited exposure adjustability can still yield acceptable pictures under differing light conditions.
The kind of lighting on the scene governs the way in which the picture reproduces the subject. Orientation of the subject—as in taking a portrait—with respect to the light direction can often control the effect. Lighting from behind the camera (front lighting) gives flat effects, light from one side yields depth and modeling, while the principal light from behind the subject produces dramatic against-the-light effects of high contrast. Artificial light setups in the studio, with tungsten lamps or electronic flash, offer the greatest flexibility. Under such conditions the photographer can arrange two or more lamps for various lighting effects.
Photographers have a wide variety of light sources at their disposal. The sun itself remains the most richly varied and versatile source of outdoor lighting. They can work with the blazing light of the noonday sun, the silvery illumination of a foggy day, the soft lighting of an overcast sky, or the dramatic hues and shadows of a sunset.
Using fast film, photographers can take pictures indoors—by normal indoor lighting or by sunlight entering through a door or window—or outdoors at night by the artificial glow of street lights. Sometimes, however, an adjustable or portable artificial light source is required. One such source is the photoflood, which is powered by ordinary house current and provides a continuous source of illumination over its lifetime. At one time flashbulbs were widely used. These disposable bulbs produce a bright momentary flash of light that is synchronized with the opening of the camera's shutter. Today they have been largely supplanted by the electronic flash.
Electronic flash units are available in a range of sizes from miniature versions built into small cameras to large, powerful studio units. These too must be synchronized with the camera's shutter, but they provide many thousands of flashes without the need of replacement. Specialized units having a very brief flash duration are used to stop fast action.