Focusing movement
Focusing movement | ||
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Cheap simple cameras have lenses which are so-called 'focus-free'. In practice this means the lens is fixed in position to sharply image subjects about 2.5 m from the camera. The assumption is that this is a typical situation for snapshots, and items slightly nearer or further away will appear reasonably sharp due to depth of field. Most lens units include some means of adjusting its position forwards or backwards for focusing subjects closer or more distant respectively. Typically the whole lens shifts smoothly by a centimetre or so within a sleeve (or internal elements alter position). Focus is manually adjusted by rotating the lens barrel, or via a motor under the control of a sensor which detects when the image is sharp. Often point-and-shoot autofocus compact cameras show no distance settings on the lens. Lenses on cameras offering greater control, all single-lens reflexes for example, show a scale of subject distance which moves against a setting mark. All lenses can be set to focus for infinity. The closest subject distance offered depends on a number of factors. For example, the lens may not maintain the same high image optical resolution at close distances and mechanically it may be difficult to shift the lens further forward. The longer the focal length the greater the physical movement needed for adjusting focus settings. Again, close-up focusing may be purposely prevented because the lens is part of a camera with separate direct viewfinder. This grows increasingly inaccurate in framing up your picture the closer you work. Normal lenses for large-format sheet film cameras need more focusing movement to cover a similar range of subject distances, owing to their longer focal length. The whole front unit of the camera moves independently of the back, the two being joined together by bellows. There is seldom any scale of distances on lens or camera body; you focus by checking the actual image on a ground glass screen at the back of the camera. | ||