3M-5CA MC 500mm f/8 Lens USSR MTO-500 Maksutov Mirror Telephoto M42

Constant unchanged diaphragm. The focal length is 500 mm, the fixed aperture is 8 (in practice, they say, one step is more). The lens has a built-in lens hood, which is not only convenient, but also necessary (it is impossible to be guided to sharpness without pushing the lens hood).