The C91005 is the Pentax Close-Up Lens #3 for 40.5mm filter-screw size. It changes the focal length of the camera lens by +333 millimeters. In effect it makes the object much larger and much more detail is evident for critical evaluation.
In the Box
Pentax C91005 #3 Close-Up Lens Adapter for 40.5mm Filter CS-Mount Lenses increases Focal Length by 333mm