PURPOSE: To provide inexpensive rigid endoscope observing optical system having well compensated chromatic aberration and a small number of lenses by including a diffraction type optical element generating a positive on-axis aberration in an objective lens system or a relay lens system.
CONSTITUTION: An objective lens system 1 is composed of a first lens group of a plano-concave single lens and a second lens group of a bi-convex single lens. A relay lens system 2 is composed of a bi-convex single lens and a diffraction type optical element r9 and relays an image only once. An eyepiece lens system 3 is composed of a bi-convex single lens. By arranging a field lens on the object side apart from the object image I, flaws and dust on the surface of the lens are not projected. A negative on-axis chromatic aberration generated by a lens of the optical system other than the diffraction type optical element r9 is canceled by means of the positive on-axis chromatic aberration generated in the difractive type element r9 of the relay lens system 2. Consequently, the on-axis chromatic aberration in the whole optical system is excellently compensated.
ONO KATSUYA
IGARASHI TSUTOMU
