As a Nikon D40 user, I find this lense and viewfinder combination on Tokyo Camera Style very flash!
I'm supposing the D40 needs to be in mirror lock up to accommodate the Voigtlander SL 15mm f4.5 (Sypal has incorrectly identified the lens as 12mm but, no matter, still love his stuff).
The lense has a huge back end.
My D40's mirror does go up in mirror lock up but the shutter stays open as well. The specs say the MLU is for sensor cleaning only.
Hmmmm. Mystery. I've read elsewhere about users placing a foam bumper on top of the rear element so the mirror hits this instead of the lens. The lens must come awfully close to the shutter!!!
My combo is less exotic but more safe plus I like using the Nikon E Series lens.