Originally posted by ryan s So short as the image is "in focus" 45.46mm behind the lens
Are you sure? Pentax has 45.5mm (K mount and M42 screw) registration distance. Still, even most FA/m42 lenses have their rear element inside of the camera.
And you could probably go even deeper with APS-C camera. Since the lens would be DA anyway, there is no need for it to be mountable on film cameras because of the missing aperture ring and it would vignette being wide-ish.
So i did some measurements:
I just measured the maximum absolutely safe distance (from the mount to the edge of mirror in up position - most closest point to the mount). It's 8.84mm, this gives us 45.46 - 8.84=36.62mm fl (x1.5=54.93).
Unfortunately it would have to protrude even more, because it is somewhere in the centre of the whole lens assembly from where you can measure the focal length.
For example M50/1.4 protrudes 7.96mm in the camera behind the mount, when focussed to infinity. Some primitive logic shows that it has some (50-45.46)+7.96=12.5mm's of glass "behind" it's focal length.
Applying the same value for our minimum distance 36.62+12.5=49.12 (x1.5=67), you get the same 50mm.
What's the situation with 40Ltd. Is it retro focus and how much it protrudes?
Probably you can go even deeper, since the mirror is nearest at its top and goes away when swinging down.
Most likely if you're not making F1.4 lens, and unless there are other compatibility issues (like SDM contacts) i think it could be possible to make a bit shorter and faster (like 33/2) "normal" pancake, with not so much glass (read less price, weight and more quality) as in retro focus design.
Since i've stumbled on such dumb ideas i guess it' s time to bed. Have a good talk!