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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.

Erfle eyepiece

Figure 11.9 The Erfle eyepiece.

Erfle specs
(Click to enlarge.)

transverse ray aberrations

Figure 11.10 Transverse ray aberrations of the Erfle eyepiece. (Click to enlarge.)

M. J. Kidger, Fundamental Optical Design, SPIE Press, Bellingham, WA (2001).

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