| 摘要 |
A varifocal camera objective is adjusted in its telephoto position, by an axial shift of one of its movable components or by the insertion of a negative lens member, to extend its overall focal length with consequent increase in its back-focal length, that increase being compensated by the emplacement of a supplemental positive lens member in first or second position. If the movement of the shiftable component is controlled by a cam track, the axial displacement thereof beyond the varifocal range may be normally blocked and may be unblocked only when a feeler detects the presence of the supplemental positive lens member. A similar feeler mechanism may be used to prevent the emplacement of an attachment bearing such a supplemental lens member unless the shiftable component is enabled to move into its off-normal position. A focal-length-increasing negative lens member may be carried on an eccentric disk for selective interposition into the path of incident light rays just ahead of a reflex prism. The supplemental positive lens member may have an aspherical surface approximating a paraboloid of revolution.
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