PURPOSE: To achieve constitution which reduces the outside diameter of a front end, provides the back focus suitable for insertion of a prism for changing optical path and is compact by forming an optical system of which the objective lens system satisfies the specific conditions.
CONSTITUTION: The preferred 1st condition to form an objective lens having a desirable back focus length by reducing the outside diameter Φ1 of the front group lens is 8.0<f1/f ≤1.5 if (f) is designated as the combined focal length of the entire system, f1 is the combined focal length of a front group divergent lens group L1 and f2 as the combined focal length of a rear group convergent lens system L2. The preferred 2nd condition for avoiding excessively long overall length of the objective lens system by reducing the outside diameter Φ1 of the front group lens is 1<f2/f≤1.5. Economization of the optical path length by increasing the refractive index Np of a prism 2 for changing optical path is preferred for the purpose of compactifying the front end part of an endoscope and the preferred 3rd condition is Np>1.7.
KISHIKAWA TOSHIROU
SUNAGA YASUMASA
MIYANO TAKASHI
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