In certain cases lens constructor is not accurate in declaring the flange focal distance at infinity.
If your focus at infinity is not completely sharp, you can easily fix it.
There are few methods to calibrate at infinity the focus ring.
The best one (which is the more accurate) is the following:
A. Get an analog or digital camera (which we call camera X) with a normal or tele lens (from 50mm on). If you use a digital camera it could be better because you will be able to zoom the target.
B. Focus the camera X at infinity (make sure to disable the autofocus feature).
C. Get the Malefic camera. Remove the top cover. Simulate a well inserted film (it could be also a paper or cardboard) and mark it with a cross or paint dot – in picture we used a printed cardboard. Anyhow anything that could be focused is ok. Use a led torch to illuminate the target if needed.
D. Set the focus ring of Malefic at infinity.
E. The two cameras must be positioned face to face at very close distance.
F. Point the camera X to Malefic camera mounted lens. Open the shutter of malefic lens.
G. At this point, you should see the target screen from camera X viewfinder.
H. Adjust the Malefic focus ring as long as you get the sharpest target subject in viewfinder.
I. Congratulation! You found the exact focus at infinity.