The KIPON Pentax645-GFX 0.8x Adapter allows the use of Pentax 645 medium format lenses on smaller sensor FUJIFILM GFX-mount bodies.
The adapter virtually eliminates any crop factor or image loss and maintains a diagonal angle of view. It reduces the focal length by 0.8x, and it also multiplies the focal length by 0.8x so that the effective focal length becomes shorter, and the shooting range is larger.
So, the adapter's built-in three groups of five optical lenses increase the aperture by one stop so that f/2.8 becomes f/2, while the reduced depth of field produces a desirable blur. The KIPON optic design lets light come to the digital sensor vertically as far as possible, actually improving the performance of some lenses.