So I've never owned a nikon camera before and have just attached my new f1.4 50mm to my new Letus Extra, hurrah. I want to take it off, I don't know how.
There is a little metal tab on the Letus lens mount - if you are looking directly at the lens, it should be in about the 5 o'clock position. Press it in and hold it, and then turn the lens clockwise. After about a 45 degree turn, it should disengage.
Ah I figured it out, I had taken my metal tab thing, the locking pin, out when fiddling with the mount when I got it. Then the metal pole catch thing had got twisted, and the little hole into which the tab goes had turned and there was no way to release the pin.
I had exactly the same problem when i first used my LEX. It was all due to the little locking /release knob on the lens mount, i took it out, screwed it back in and pushed it back as per normal and it worked fine. I guess they may just get stuck sometimes.