Using the split prism on the 1-13 screen is not possible at or beyond
f5.6 because it blacks out (or depending on exactly where your eye is
one half does). As you get beyond f4 it becomes increasingly
critical exactly where your eye is placed to get the prism to
function properly. This much I understand. However, I've noticed
that if you try focusing at apertures close to f5.6 (from maybe f5 or
f4.5 depending on where your eye is) the two images merge on the
split prism when the lens is out of focus. You can try this by
shutting down a lens, focussing, and then openning it up again.
The reason I bring this up is that it would make me wary of f5 lenses
with this screen (even though they were designed for it). It may
also explain one comment made a month or so ago on the list that
someone had got back pictures taken with the 70-210 f4.5-5.6 'Zuiko'
zoom and had found many of them out of focus. Clearly a variable
aperture lens is particularly sensitive to this problem as it will be ok
at one focal length and not at another. This is likely to lead to a
lot of dissatisfied OM users of this lens with older cameras with the
1-13 screen since the aperture covers this 'danger area' almost
exactly.
Can anyone explain why this is the case - I thought that the split
prism just let you see the image as seen through different parts of
the lens - unless the lens has aberations (which it cannot have to
this degree) it ought to work ok as long as the prism is clear. Does
the 2-13 suffer from the same problem or does it focus to a smaller
aperture? If the OM2000 does this it is particularly bad since the new
zoom is designed to go with it.
Clearly the problem can be overcome by using a matt part of the
screen (or by changing to a completely matt one) but it's hard to
ignore the split prim if it LOOKS as if it is working ok.
Christopher (UK)
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