Compact cameras use both the 135 and APS formats, while digital cameras use focal plane arrays of much smaller formats, typically from 2/3 to 1/4 inch diagonal. Apart from small formats, most of these cameras do not use reflex viewfinders but rely instead on separate optical viewfinders. Both factors influence lens design in that the rear element can be very close to the focal plane, and that a useful zoom range is possible with only a few elements, especially if the maximum aperture is modest, perhaps f/4 at the most. The lens must also telescope down into the camera body for storage, to help keep the size of the camera small. For the 135 format, long range zoom lenses of 38 to 200 mm are used, although the aperture reduces to some f/11 at the long focus setting. The lenses are non-interchangeable and the collapsible telescoping barrel may cause some optical misalignment.
The basis of such compact designs was the Zeiss Biogon wide-angle lens which has a very short back focal distance (BFD). From this was derived a 'telephoto wide-angle' lens (see Figure 7.15a), a short focus lens using both telephoto configuration to reduce its physical length and internal focusing to keep its external size constant. This led to compact zoom lenses by using a moving element or group to alter focal length (Figure 7.15b) and moving the lens to give unit focusing under autofocus control. By use of technology such as plastic elements and aspheric lens surfaces, as few as four elements could be used for a satisfactory zoom or varifocal lens with a 2:1 zoom ratio. The lenses may suffer from noticeable curvilinear distortion at both ends of the zoom range and possibly some vignetting. The modest apertures available limit the useful range of an integral flash and low light level use may cause camera shake due to the slow shutter speeds needed, unless a fast film of 400 to 800 ISO is habitually used. The lenses usually lack a filter thread to take accessories such as lens hoods, filters and converters.
Figure 7.15 Zoom lens design for compact cameras. (a) Wide-angle telephoto configuration (35 mmf2.8) as precursor to a zoom lens. (b) A 38-110 mm zoom design. Glass element A is aspherical
Many digital cameras feature a 'digital zoom' feature. This is not an optical zoom using a lens, but by a digital image processing routine to magnify the central portion of the image stored as digital data. A bigger image is given but with reduced resolution and too much 'zooming' causes unpleasant pixellation of the image.
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