The design that makes them black out is the same thing that makes them
work. These split prisms essentially look out of different sides of the
lens. This is the same thing as a rangefinder, though, I think the
separation would increase with the size of the entrance pupil. The
angle of the split prism (the difference between the two halves)
indicates how *accurate* they are but also how quickly they black out.
When one side (or both with slow lenses) blacks out, essentially you are
looking at the sides of the lens, that is, the insides of the barrell,
rather than through to edges of the lens as you are supposed to. This
is because the angle of the prism is too wide for that lens and is
directing your vision into the side of the lens.
The problem I personally have with split prism screens is that they are
not a ground glass in the split prism area. That means that nothing
ever looks blurry when it is out of focus. It just looks split, but sharp.
Mike
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
