PURPOSE: To exactly decide the effective diameter of an optical system by deciding the effective diameter based on object information in an extreme state to be handled by that optical system.

CONSTITUTION: The main part of an effective diameter deciding system 4 is composed of a central arithmetic processing unit 1, external storage device 2, keyboard 3 and display device 6. In this case, the lens configuration of the optical system to be an object is set, and the instruction of its lens characteristics (focal system or afocal system) is inputted. Next, the lightness information of that optical system (such as FNO at the time of object distance being infinite, effective FNO, image side NA, diaphragm size, etc.,) and seven kinds of brightness deciding instructions are inputted. Further, the object information such as an object distance, image angle, image plane position, beam ratio or zoom state for deciding the effective diameter is inputted. Thus the object information concerning an object at an object point in the extreme state to be handled by the optical system is inputted and while using this object information, the effective diameter for each plane of the optical system is decided.