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Thermodynamics

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Distortion

The failure of a lens to form a point image due to a point object is due to the presence of spherical aberration, coma and astigmatism. The variation in the magnification produced by a lens for different axial distances results in the aberration is not due to the lack of sharpness in the ‘image’. Distortion is of two types viz. (a) pin-cushion distortion and (b) barrel-shaped distortion. In pin-cushion distortion, the magnification increases with increasing axial distance and the image of an object appears as shown in fig. On the other hand, if the magnification decreases with increasing axial distance, it results in barrel-shaped distortion and the image appears as shown in fig.

In the case of optical instruments intended mainly for visual observation, a little amount of distortion may be present but it must be completely eliminated a photographic camera lens, where the magnification of the various regions of the object must be the same. In the absence of stops, which limit the cone of rays of light striking the lens, a single lens is free from distortion. But, if stops are used, the resulting image is distorted. If a stop is placed before the lens the distortion is barrel-shaped and if a stop is placed after the lens, the distortion is pin-cushion type. To eliminate distortion, a stop is placed in between two symmetrical lenses, so that the pin-cushion distortion produced by the first lens is compensated by the barrel-shaped distortion produced by the second lens. Projection and camera-lenses are constructed in this way.