Relay optical system, and optical system for rigid endoscope and rigid endoscope using the same

A relay optical system, comprising:

• a cemented lens in which a first lens having a positive refractive power, a second lens having a positive refractive power, and a third lens having a negative refractive power are cemented,
• wherein:
• the cemented lens is disposed in an optical path of the relay optical system, which is formed by an object-side optical path and an image-side optical path, and
• the first lens is a meniscus lens which is adjacent to the third lens,
• a dispersion and a partial dispersion ratio differ for the first lens and the third lens,
• in a rectangular coordinate system in which a horizontal axis is set to be νdLA and a vertical axis is set to be θgFLA,
• when a straight line expressed by θgFLA=α×νdLA+βLA is set, where α=−0.00163,
• θgFLA and νdLA of a medium of the first lens are included in an area determined by the following conditional expression (1) and conditional expression (2), and
• the following conditional expression (3) is satisfied:
• 0.67≤βLA (1)
• νdLA<50 (2)
• −1.4<mg<−0.6 (3)
• where,
• θgFLA denotes a partial dispersion ratio (ngLA−nFLA)/(nFLA−nCLA) of the medium of the first lens,
• νdLA denotes Abbe number (ndLA−1)/(nFLA−nCLA) for the medium of the first lens,
• ndLA, nCLA, nFLA, and ngLA are refractive indices of the medium of the first lens for a d-line, a C-line, an F-line, and a g-line respectively,
• mg denotes a magnification of the relay optical system,
• the object-side optical path is an optical path positioned on an object side of a center of the relay optical system, and
• the image-side optical path is an optical path positioned on an image side of the center of the relay optical system.