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The Erfle Eyepiece
This eyepiece has an extra positive element, and the form of the lenses is different (Fig. 11.9). It has a concave surface near to the intermediate image. This surface reduces Petzval sum, and it also helps to achieve a longer eye relief by moving the chief rays away from the axis. The semi-field angle for this design is 30 deg (Fig. 11.10). In this example, the lateral color is not corrected, because the eyepiece is intended for use with a prism. This is a typical eyepiece for use in afocal systems of reasonable quality.