Fixed-focus lens

Fixed focus lenses have a fixed focus, so a steady distance setting.

The basic idea

Lenses form, strictly speaking, only objects in a plane sharply, but you can accept a certain amount of imprecision, so that, the depth of field results in practice, a focus range. In a miniature camera is tolerated, for example, blur circles up to 1/30 mm. The depth of field depends on the focal length and the aperture value, the shorter the focal length and the higher the aperture value, the greater the depth of field. This can be exploited for a so-called snapshot setting. Hides one example, a lens with 50 mm focal length 8, and adjusts the distance of 6 m, then the range of 3 m is mapped to infinity. Thus, for photographing moving objects quickly without being stopped by the focusing.

Snap

The idea of the snapshot setting can be used for simple cameras to dispense entirely with a distance setting. Condition are faint lenses, bringing a range of about 2.5 m at can be detected. This method has been utilized to the box cameras used. Ideally, one supporting the project with a moderate wide- angle lens, telephoto lenses, however, are unsuitable in principle for a fixed focus.

The limit on the light intensity, up to the fixed-focus lenses can be realized depends on the film format. With box cameras it is to f/11, with miniature cameras around f / 9,5 and 8 mm cine cameras at f / 2.8.

Use with CCD sensors

CCD sensors are a fixed focus especially helpful because they have an extremely small screen format. This results in very short focal lengths, which in turn have a large depth of field result. So simple webcams can easily be equipped with a fixed focal length lens. Should they, however, penetrate into the macro range, one focusing ( an autofocus system ) can also not handle.