Focus can be achieved in a number of ways. A lens may be adjusted manually while the photographer observes the image, or set directly to a predetermined distance using a calibrated scale. In daylight, autofocus systems achieve precise focus in a split second. When ambient light is insufficient for normal operation, an autofocus assist illuminator function of a suitable flashgun may be required to increase subject contrast. The focused distance is then maintained while light pressure remains on the shutter release. Some systems can also track continuously a moving subject while maintaining focus.
When focusing on a subject the photographer must bear in mind not only the focused distance, but also the depth of field - sometimes called the depth of field envelope. Depth of field is always important, but particularly so when a subject is close to the camera. If a typical 85mm lens is used at a distance of about one and a half metres from a subject, the depth of field at f/5.6 will be approximately ten centimetres. At f/2 this reduces to four centimetres. Clearly the depth of field envelope must be carefully positioned.
Focusing a lens at the hyperfocal distance maximizes depth of field. This is the shortest distance from the lens to an in-focus subject, for a particular aperture, when the lens is focused at infinity. When set to the hyperfocal distance depth of field extends from about half the hyperfocal distance to infinity. This can be useful in wide-angle environmental portraiture where there may be a requirement for both subject and background to remain sharp.
Nikon's defocus image control (DC) lenses, specifically designed for portraiture, facilitate the movement back and forth of the depth of field envelope relative to the focused distance. For example, it is possible to defocus the foreground or background, more or less than normal, while maintaining focus on the eyes. This makes it easier for the photographer to maximize use of the available depth of field.
When working close to a subject, photographers often have to decide which features of an image should remain in focus and which should, by necessity or design, be allowed to go soft. In most cases it is important to keep the principal subject sharp. Background detail will probably be defocused. With this in mind it is important to "buy" the appropriate depth of field when choosing film for close work. A product rated at ISO 200 has the potential to give greater depth of field than one rated at ISO 100 because a smaller aperture can be selected. Of course shallow depth of field is not always a problem. A large aperture might be chosen specifically to minimize depth of field, exaggerate the impression of depth and consequently highlight the key elements in an image. This selective technique is known as differential focusing.