the first Single Lens Reflex (SLR) designed cameras the lenses were made with what
is called a preset aperture. Because the light coming
through the lens is redirected via a mirror to the view
finder, when the aperture is set to the working aperture,
(as in the picture f22) the amount of light coming through
the lens is reduced to the point that it is difficult
to see and focus on the subject. The view finder gets
darker the more we stop the lens down.
lenses were made with 2 rings one the photographer set
to the working aperture say f22 and the other set to the
maximum aperture of the lens in this case f8.
During focusing the lens was left open on f8 but just
as the photographer was going to take the photograph they
would turn the ring hard round until it stopped or locked
at the preset working aperture and this would leave it
set at the desired aperture for the exposure. They would
then have to move the ring back to f8 to focus on the
As you can imagine they were not that easy to use.