This seems like as good a place as any to share some freelensing photos! A few years back I was experimenting with this technique using some Canon FD mount lenses held in front of a mirrorless camera (GH3 in this case) to play with the focus plane and deliberate light leaks. It definitely took some practice to get focus where I wanted it and strike a balance on how big of an impact the light leaks have on the final result. Now that I have cameras with focus peaking, I think I'll try this out again. If memory serves, the lens was a Canon FD 35mm f/2 shot at f/2.