| 摘要 |
PURPOSE:To suppress and compensate the deterioration of focusing accuracy at the time of converting a focal distance to telephoto by controlling an axial luminous flux of a photographing optical system to satisfy a specific condition, when an internal extender is installed. CONSTITUTION:A photographing optical system is constituted of a first lens of positive refracting power, a second lens 2 of negative refracting power, a third lens 3 of negative refracting power, and a fourth lens 4, and in a fourth lens component, an internal extender 5 is provided so as to be attachable and detachable. In this state, when a composite focal distance and a full-aperture number of the photographing optical system before the internal extender 5 is installed are denoted as fa and Fa, respectively, and a composite focal distance and a full-aperture number when such extender is installed are denoted as fb and Fb, respectively, by satisfying a conditional expression (1), an axial luminous flux of the photographing optical system is controlled. Therefore, even if a focal distance is lengthened by inserting the extender 5, the focusing accuracy can be held in an allowable error range of the photographing optical system. |
