| 摘要 |
A projection optical system has, from the enlargement side to the reduction side, a first, a second, and a third lens unit, and an aperture diaphragm. The negatively-powered first lens unit includes only a negative lens element and has at least one aspherical surface. The positively-powered second lens unit includes at least one positive lens element. The positively-powered third lens unit includes a lens element. The aperture diaphragm is disposed at the front focal point of the third lens unit. The lens element of the third lens unit is made of a material of which anomalous partial dispersibility and Abbe number are defined.
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