Dec 13 2007
Geometrical Distortion is form of optical aberration which is normally observed in stereoscopic microscopy. Geometrical aberration is the result of changes in the image shape instead of the color or sharpness spectrum. The two most common types of geometrical distortion are referred to as positive and negative distortion. The positive and negative distortions are also sometimes called pincushion and barrel, respectively.
Geometrical distortion can occasionally be found in very sharp pictures which are, in other respects, amended for chromatic, comatic, astigmatic and spherical aberrations. For this instance, the object’s true geometry is no longer preserved in the image.
Geometrical distortion is caused by the differences between the off-axis image distance and the lens’ transverse magnification. Geometrical distortion arises because of the differences in focal lengths and magnifications through different part of the lens. This occurs when the distance drifts from the distance estimated by the theory of paraxial for constant transverse magnification.
When other optical aberrations such as chromatic or spherical aberrations are absent, geometrical distortion is demonstrated by a misshaped image, although each of the image point is in distinct focus.
Normally, geometrical distortion will not be present in thin lenses. However, geometrical distortion can occur in thick simple lenses. Positive and negative lenses will show positive and negative geometrical distortions, respectively.
Geometrical distortion occurs in systems employing compound lens systems which incorporate lenses such as meniscus, telephoto, retrofocus, double gauss, zoom and fisheye lenses. Oculars in optical microscope are the primary cause of geometrical distortion. Some lower quality objectives also lead to geometrical distortion.
To monitor geometrical distortion in an optical microscope, one can use an imaging crossed grating lines in the wide-field mode. The lines should be seen as parallel and straight over the entire image field when looked through the eyepieces.
Source: AzoOptics