Afocal Teleconverters for Fixed Lens Cameras

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Small cameras and less-expensive cameras usually do not have interchangeable lenses. Therefore, telephoto and wide-angle attachments for these cameras must be attached in front of the camera lens. This is analogous to using a fixed-lens camera to take photos through the eyepiece of a telescope, thus replacing the human eye with a camera. For this to work with any camera lens, an afocal arrangement (shown in Figure 12.8) is required. The magnification in this arrangement is given by m = f / / = feS / /camera when the camera is focused on infinity. For closer distances the actual magnification measured from the image size on the sensor depends somewhat on the focal length of the camera lens. The same principles can be used to make a front-mounted wide-angle converter.

A focal teleconversion lenses can be quite satisfactory for point-and-shoot cameras and camcorders. They also work well on lenses designed for DSLRs, but tend to be bulky, and they offer no

Front Lens Converter Diagram
FIGURE 12.8. Front-mounted teleconverter.

advantage over the rear-mounted teleconverters. In fact, there is a serious problem for close-up photography. In contrast to rear-mounted teleconvert-ers, the front-mounted units greatly increase the minimum focusing distance. So there might be an advantage when photographing a bird at 10 ft., but at close distances the maximum magnification is actually decreased. In addition, afocal teleconvert-ers usually lack threads for mounting polarizers and filters, and they may put unacceptable stress on lens barrels. In particular, they are not recommended for lenses with rotating front elements.

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