Erik (back office) working on the 42mm f/0.75 TV Heligon. He was drilling and applying loctite as can be seen in the images.
Bjørn (front desk) explained that it's f/0.75 at infinity, however, when focused to its only focus distance, the effective lens aperture becomes approximately f/3.8 with the D750.
EDIT: I didn't mention the the images were taken with 85/1.4 AF-D and the dedicated 1.4 II TC as an extension ring that Erik (back office) modified and gifted to Bjørn (front desk) as a birthday present.
I need to get myself a TC and let Erik (back office) remove all the glass elements from it in order to have a universal extension ring