To provide a monochromatic image observing optical system in which dust and dirt stuck to a monochromatic filter are inconspicuous on an image forming plane and a whole size and the size of image are made large compared with a general monochromatic image observing optical system composed of an objective lens and a monochromatic filter.
This system is composed, in order from the object side, of an objective lens 10 composed of a first group 11 and a second group 12, a negative lens group 20, a monochromatic filter 30 and a positive lens group 40. The monochromatic filter 30 is a narrow band interference filter. Both the negative lens group 20 and the positive lens group 40 are constituted by single lenses. The negative lens group 20 has a power for diverging the converged light beam transmitted through the objective lens 10 and making it almost afocal luminous flux and the positive lens group 40 has a power for negating the negative power of the negative lens group 20.
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