Patent ID: 8780445

Claim:
A rigid-scope optical system comprising: an objective optical system; and a plurality of relay optical systems that relay an optical image imaged at a primary imaging plane at the entrance side onto a secondary imaging plane at an exit side; wherein each of the plurality of relay optical systems is formed of a positive front group, a middle group, and a back group in this order from an entrance side, wherein the middle group of one of the relay optical systems that is disposed at an extreme exit side is provided with a diffractive optical element having a diffractive surface, and wherein the diffractive optical element receives an incident light beam, at the diffractive surface, whose axial chromatic aberration between at least two wavelengths has been corrected by an optical system other than the diffractive optical element, and the diffractive optical element corrects axial chromatic aberration related to another wavelength contained in the light beam.