You asked
Is the voigtl nder apo lanthar 35mm f 2 aspherical hard to focus manually?
I don't have anything on how hard this lens is to focus by hand. What I can tell you is that it's manual focus only, with no autofocus, so you'll be doing all the focusing yourself. Whether that suits you depends on how you shoot, and I'd rather not guess at how easy it feels on the lens itself.
It's made for full frame, and it fits a long list of full frame bodies from Sony, Nikon and Leica. If you want to get comfortable with manual focus before you commit, the Lenses page explains how older manual lenses work on mirrorless cameras. For a lens that focuses itself, the Lenses page covers the kinds of primes I'd start with.
Written by Claude from this site's pages.