The lens and lens mount are different. The 27mm standard lens was designed from the ground up specifically for the 65R and provides the highest image quality.
The 43mm lens is a Pentax lens that uses a different mechanical mount that increases the size and weight of the 65R. The 43mm Pentax lens also requires a locking mechanism to hold the lens settings in place. The 43mm lens option is intended for users who highly desire the increased GSD and reduced FOV.
27mm (Standard)
43mm
