Eyepieces

The eyepiece for many optical instruments is a simple magnifier. Because of aberrations, a single lens is not adequate for the eyepiece of a high quality optical instrument, and many lens combinations have been developed to minimize the aberrations. A sample is shown here.

Ramsden Eyepiece

Eyepieces Ramsden Eyepiece Orthoscopic Eyepiece Erfle Eyepiece

In the Ramsden eyepiece, the two lenses are made of the same kind of glass, e.g., spectacle crown. The lenses are of equal focal length and their separation is equal to the focal length. The field lens coincides with the first principal plane, which is convenient for the placing of crosshairs or reticules, but inconvenient in that it highlights any dust on the lens.

This eyepiece is effective in reducing chromatic aberration , doing a better job with longitudional achromatism than with lateral. Its approach to minimizing chromatic aberration is based on spacing the lens pair so that dependence upon the index of refraction is minimized. It is quite effective in reducing spherical aberration and distortion, and removes the problem of coma.

In standard use, the objective lens will form an image at the first surface of the field lens.

Orthoscopic Eyepiece

Eyepieces Ramsden Eyepiece Orthoscopic Eyepiece Erfle Eyepiece

This eyepiece is characterized by its wide field of view and high magnification. It finds application in high-power telescopes and range finders. It corrects well for distortion. The triplet corrects well for chromatic aberration.

Erfle Eyepiece

Eyepieces Ramsden Eyepiece Orthoscopic Eyepiece Erfle Eyepiece

This eyepiece has two achromatic doublets, even three in some variations. Its wide field of view (up to 70°) enhances its use in high quality astronomical telescopes.