The C91004 is the Pentax Close-Up Lens #2 for 40.5mm filter-screw size. It changes the focal length of the camera lens by +500 millimeters. In effect it makes the object much larger and much more detail is evident for critical evaluation.
In the Box
Pentax C91004 #2 Close-Up Lens Adapter for 40.5mm Filter CS-Mount Lenses increases Focal Length by 500mm