i'd be careful
i know of someone who burned his
speed graphic with the shutter on B
and the camera on the roof of his car ...

if you use a lens'ed camera
it doesn't take a long time to get a retina image ...
( a photograph of "something" on the paper long exposure )
so i can't imagine how you would do this pointing at the sun
unless you make your own pinhole aperture and put it infront
of the lens to created a pinhole'd lens'd image ...