Abstract: An optical instrument, comprising an optical system including a focus lens group, an aperture that restricts light flux that passes through the optical system, an aperture motor that sets opening amount for the aperture, a memory that stores first information that represents a relationship between position of the focus lens group and appropriate aperture value and second information that represents a relationship between the aperture value and opening amount of the aperture, and a controller that, when controlling opening amount of the aperture using the aperture motor in accordance with position of the focus lens group, calculates an appropriate aperture value based on the first information and position of the focus lens group, calculates opening amount of the aperture based on the second information and the aperture value that has been calculated, and controls the opening amount to the opening amount of the aperture that has been calculated.