1. Field of the Invention
The present invention relates to an objective lens for use in an endoscope wherein a transverse chromatic aberration is corrected.
2. Description of Related Art
An endoscopic objective lens is provided at the front end of an endoscope together with an illuminating optical system, and air or water supply channels. Accordingly, the diameter and length of the objective lens for an endoscope must be as small as possible.
A conventional objective lens for an endoscope is usually comprised of three lens groups consisting of four lenses. For example, an objective lens may consist of a first lens group having a single negative lens, a diaphragm, a second lens group having a single positive lens, and a negative third lens group having a cemented positive lens consisting of a positive lens and a negative lens adhered thereto, as viewed from the object side.
However, in conventional endoscopic objective lenses having three lens groups consisting of four lenses, achromatization is carried out only by the cemented positive lens adjacent to an image to be taken. Consequently, chromatic difference in magnification (or transverse chromatic aberration) will not be satisfactorily compensated.
The chromatic difference of magnification in the endoscopic objective lens should be as small as possible depending on the diameter of an optical fiber incorporated in the endoscope. If the chromatic difference of magnification fails to be sufficiently eliminated, an image (i.e., color) to be transmitted is dimmed particularly at the peripheral portion thereof, thus resulting in deteriorated image quality. This might lead to a misdiagnosis.
In general, the diameter of an optical fiber used in an endoscope was approximately 10 .mu.m, but recent progress in manufacturing technology has made it possible to produce optical fibers having smaller diameters and higher resolving power as an image guide. To respond to this, the chromatic difference of magnification of the objective lens for an endoscope must be restricted as much as possible so as to provide high contrast and resolving power to thereby obtain a high quality image.
The primary object of the present invention is to provide a compact objective lens for an endoscope in which a chromatic difference of magnification can be sufficiently eliminated without the need for substantial modification of an existing objective lens system.