Fixed Focus Camera
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On many of the cheaper cameras, there is no focusing movement and the
lens is at a fixed distance from the film. With such fixed focus
cameras, all objects beyond about 7 feet from the camera are in
reasonably sharp focus.
The success of fixed focus relies on the depth of field of the lens. For cheapness, such cameras are fitted with lenses of relatively small aperture and proportionately great depth of field. A typical example would be a 2 1/4 x 2 1/4 inch box camera fitted with a 7.5 cm. lens with a working aperture of f11. If such a lens is focused on 23 feet, the depth of field at f11 extends from 11 1/2 feet to infinity. In practice the definition would be considered sufficiently good from about 8 feet. It is thus possible to use such a camera for making reasonably good pictures by simply avoiding subjects closer than 8-10 feet. Many cameras of this type can be used closer than this because they are fitted with lenses with a working aperture even smaller than f11. Others have a lens of shorter focus (e.g., 5 cm) with a correspondingly greater depth of field.
Fixed Focus Setting
Any focusing camera can be converted at once into a fixed focus
camera for snapshooting by focusing the lens at the hyperfocal distance
for the aperture in use. If the lens has a depth of field scale, setting
it for fixed focus working simply means adjusting the focus so that the
distant limit of the depth of field for the working aperture coincides
with the infinity mark. Obviously, the smaller the working aperture, the
closer the near limit of the zone of sharp focus to the camera.
By adding supplementary lenses fixed focus cameras can be used for subjects closer to the camera than the near limit of the depth of field.
Fixed focus makes the camera easy to use and eliminates the risk of
getting the subject out of focus. But while it gives a sharp picture of
the subject (so long as it is not too close), it also gives a sharp
picture of everything else more than 8 feet or so in front of the
camera. This general sharpness is sometimes undesirable. It is all very
well for a street scene or a landscape. But trouble arises when the real
subject of the picture is viewed against an unimportant or ugly
How to Subdue the Background
The owner of a focusing camera can deal with the unwanted background
by differential focusing. This cannot be done with a fixed focus camera,
so the background must be played down in one of the following ways:
- By choosing a plain background such as the sky, or a stretch of sand, grass, or snow. In this case the eye has nothing to focus on but the subject, so it no longer tends to wander away from the point.
- By shooting from a low viewpoint. This cuts out the competing objects near the subject and shows it against more distant things which may be lighter in tone and may also be out of focus. Best of all, it may show the subject against the plain blue tone of the sky.
- By choosing a contrasting background. A light-toned subject will stand out if it is photographed against a dark-toned background or an area in shadow. In the same way, a dark subject will stand out from a light-toned background.
- By swinging the camera. If the camera follows a subject that is moving across the line of view, the subject will be sharp and the background blurred.
- By using flash. So long as the background is considerably farther away from the camera than the subject, it will be too dark to attract attention. This method can only be applied in poor light, and it works only if the background itself is comparatively dark in tone.
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