| 摘要 |
A focus detection optical system includes a condenser lens positioned behind an expected image plane of a photographing lens, an auxiliary lens positioned behind the condenser lens, and a separator lens including a pair of lenses behind the auxiliary lens. An object image formed on the expected image plane is divided into two images by the pair of lenses and reformed on a pair of areas on a sensor. The auxiliary lens is a plastic lens having a negative refractive power. The separator lens is a glass lens or a hybrid lens consisting of a glass substrate and a plastic lens having a curved surface which is layered onto the glass substrate, and the focus detection optical system satisfies the following condition (1): 0.068<m1/m2<0.090  (1), wherein m1 and m2 designate magnifications of first and second surfaces, respectively, of the auxiliary lens.
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