On Sat, 27 Nov 1999, Lou Riina wrote:
> Dear all:
> Happy Thanksgiving
> Pardon my elementary questions but......When I look through my OM1N
> Viewfinder the subject is often very dark and the light meter is hard to
> see. Also there are several marks present. The circle that one focuses
> within is also hard to see clearly. Is this the focusing screen mentioned
> in previous recent emails and can it be changed and if so what is
> recommended for my macro/portrait medical work?
> Lou
Lou,
I've read some of the other posts on this one as well now, so I think
I've got some idea of the context and I'll take a shot at explaining
this.
It's not that elementary a question.
Your camera uses the lens to focus an image on the focusing screen in
much the same way that a slide projector focuses an image of the slide
onto a screen. Actually, it's more like a back-projection screen since
you see the image transmitted through it rather than reflected from it.
In order for this to work properly, the focusing screen has to diffuse
the light, so that the image only appears sharp when your eye is focused
on the screen and the image of your subject is also focused on the
screen. This diffusion is what makes the image on the matte area of the
focusing screen dimmer than a directly viewed image.
If you have a clear screen, the image of your subject could be focused
by the lens somewhere either side of the screen and your eye can still
see it sharply because your eye can adjust it's focusing to adapt.
Because there is no scattering from the screen, the image you see is
much brighter, like in a telescope. The split prism/microprism devices
also view the image directly, hence they always appear brighter & in
focus.
It is possible to buy clear screens for your OM, with a focusing aid in
the centre, designed for astrophotography, photomicrography etc. I
think I read that you were trying astrophotography, so one of those
would probably help a lot. You could also use it for portraits, but
you'd have to specifically use the microprism to focus the shot - the
main part of the image will always appear focused (unless you're using a
really big telephoto, but let's not go there just at the moment).
Presumably, the Canon AF bodies you looked at were fitted with a
clear-field screen.
I hope that was comprehensible.
--
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