An optical instrument including an AF circuit 12 for obtaining focus detection information, focus position detecting means 14 for detecting a position of a focus lens, an extender optical system 9, position detecting means 16 for detecting a position of the extender optical system 9, and a lens control portion 13 for generating a focus driving signal from the focus detection information, the focus lens position information and the position information, wherein driving of the focus lens is controlled by the focus driving signal via a focus motor 20.