1. Field of the Invention
The present invention relates to an eyepiece lens which can be applied to a telescope, a microscope, a viewfinder of a camera, or the like, or can be used as a magnifier.
2. Description of the Related Art
As a simply structured eyepiece lens applied to a telescope, a microscope, or the like, Ramsden's and Kellner's types are well known. An eyepiece lens of these types of eyepiece lenses comprises an eye lens positioned on the side of the observer's eyes, and a field lens positioned on the side of an object or an object image to be magnified. Ramsden's type comprises an eye lens, and a field lens. Kellner's type comprises an eye lens as chromatic doublets, and a field lens. Further, as an eyepiece lens using a new lens element, Japanese Published Patent Application Publication No. Sho 62-32414 discloses an eyepiece lens using a radial gradient-index lens. This publication discloses two embodiment. One is an eyepiece lens comprising two radial gradient-index lenses. The other is an eyepiece lens comprising a radial gradient-index lens, and a homogeneous spherical lens.
In addition, J. D. Forer et al. describe an eyepiece lens applying an axial gradient-index lens to Ramsden's or Kellner's types in Journal of Applied Optics, Vol. 22, No. 3, pp. 407-412, (1983). In Ramsden's type, the eye lens is composed of a homogeneous lens, so that chromatic aberration, field curvature, and spherical aberration are large. Even if an aspherical surface is introduced in this type of lens, the chromatic aberration, and the field curvature cannot be compensated. Compared with Ramsden's type, the eye lens of Kellner's type is composed of the chromatic doublets, so that the chromatic aberration and the spherical aberration are well compensated, but the field curvature is not sufficiently compensated. In addition, since the chromatic doublets are very expensive, it is difficult to reduce costs in Kellner's type.
On the other hand, in the eyepiece lens using the radial gradient-index lens disclosed in Japanese Laid-Open Patent Application Publication No. Sho 62-32414, the spherical aberration and the field curvature are large. Moreover, the chromatic aberration of the eyepiece lens is not particularly referred to and it might not be considered at all. Further, in the eyepiece lens disclosed in Journal of Applied Optics, since the gradient-index lens used is of an axial type, the aberrations are on the same level as those of Ramsden's or Kellner's type are compensated by introducing an aspherical lens, and thus the chromatic aberration and the field curvature cannot be sufficiently compensated.