Abstract
Lenses are fixed before the face, eyes move behind them. This causes astigmatism due to obliquity of the pencil of rays, distortion, apparent movements due to varying prismatic effects, changes in perspective and altered focal surfaces. These undesirable effects may be diminished by careful choice among the surfaces giving the required lens effect. The relative influence of point focal toric, periscopic, piano and biconvex or concave lenses is explained.
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