PURPOSE: To incoporate the photometer lens barrel into an existing lens barrel by simple constitution without generating a trouble by forming an image on a measuring field stop itself from a surface partially formed like a mirror or not processed by reflection preventing processing and arranged in the front of an object lens side entrance of a division prism in order to reflect the stop by autocollimation.

CONSTITUTION: The photometer lens barrel has the prism 3 for dividing an optical path from an object lens 1 in the microscope between an eyepiece 4 and a photoelectric photodetector 10 and a device for reflecting the measuring field stop 6 arranged in the front of the photodetector 10 to the eyepiece 4. The surface 27 which is partially formed like a mirror or not processed by reflection preventing processing is arranged in the front of the object lens side entrance of the division prism 3 in order to reflect the stop 6 and an image is formed on the stop 6 itself by autocollimation based upon the existence of the surface 27. Consequently an existing lens barrel is easily modified to the photometer lens barrel.