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RE: [OM] focussing aid/screen Q

Subject: RE: [OM] focussing aid/screen Q
From: "Windrim, Brian" <brian@xxxxxxxxx>
Date: Wed, 1 Dec 1999 14:09:11 -0000
Hi All,

I'm going to have a crack at answering Acer's questions. Please bear in mind
that
I don't have a lens in front of me so this is really a thought-experiment on
my
part.

This is a really long posting so feel free to skip it if you aren't
interested. I've
just spent my lunchbreak writing it and don't have time to condense so I'm
going to
post the lot.

Acer asked:

> However, what's unanswered is this: when I view thru a telescope (2000mm
> f:10), using various eyepieces, the image is very bright, but becomes dark
> on the matt standard-issue screen in my OM1n. Not only that, but it's only
> on the matt surface that the image is dark--on the split-image/microprism
> aid, it's as bright as thru the tele's eyepieces. Ditto for various
> lenses--no matter how slow or fast they are, the focussing aid's image is
> bright as ever, and even perfectly sharp and focussed, stopped down lens
> or no. It's probably a virtual image?

Correct. Or rather, it's a real aerial image which is probably what you
meant.
A real image is one that *can* be projected onto a screen, a real aerial
image
could be so projected, but isn't.

Try this. Take a 50/1.8 off the camera. Hold it in front of you, rear
element towards
you, about 10 inches from your eye.

You should be able to see the image of what's on the far side of the lens,
upside down
and bounded by the diameter of the rear element, but sharp and bright. This
is the real aerial image.

If you put a finger of you other hand (or a piece of paper, or whatever)
next
to the lens and move it towards you until you can focus your eye on both the
finger and the image at the same time, you should find that your finger
is approximately the same place that the film would be (and the focussing
screen, via the mirror) if the lens were on a camera.

Now if you move you head slightly from side to side you will see that the
image stays
static in relation to your finger. If there are both near and far objects in
the image
then their lateral relationship may shift slightly as the viewpoint changes.

If you push the stopdown button on the lens then the visible extent of the
image
will shrink with the diaphragm aperture, but the remaining part of the image
will
remain just as bright and sharp.

Now imagine placing an OM 1-4 focussing screen (plain matte) where your
finger is (assuming
a dark enough room to see the image). If you did this quickly, without
moving your eye,
you would see that the projected image superimposes *exactly* on the aerial
image, but
is much dimmer on account of the scattering of the image by the screen.
However, this
scattering will now allow you to see parts of the image which were
previously clipped
by the edge of the rear element or the diaphragm.

Now bore a 5mm hole through the centre of the focussing screen (I said this
was a
thought-experiment :-). This will allow you to see an aerial image in the
middle,
always bright and (to your eye) in focus, while the surrounding screen
"fills in"
the rest of the image and shows the state of focus as it would appear on
film.

This is equivalent to a matte screen with clear centre spot. Olympus don't
make one
of these, but now you know how to make your own :-)

> How does that work? How do the focussing aids work?

Remember how the relationship between near and far objects shifted as you
moved your
eye from side-to-side? Or rather, as you looked at the objects through a
different part of
the rear element and hence through a *different part of the front element*.

Instead of allowing you to look directly ahead along the lens axis, as our
hypothetical drilled screen does, the two clear-glass prisms of the
split-image deflect
the view sideways (in opposite directions) so that you are seeing the
subject via the
perimeter of the rear element. This is why the split-image goes black with
certain
lenses and/or apertures, the deflected view is looking past the edge of the
rear
element or the diaphragm.

By placing two different viewpoints adjacent to each other in this
way, differences in object distance to be made visible as lateral
displacements of the
(aerial) images. Because the prisms are located exactly at the desired place
of focus
the images co-incide for objects that are at the correct distance for focus.

The microprism is just the same thing, but smaller and repeated so as to
break up
the image.

> So there seem to be an inverse relationship between finder brightness and
> fosussing ease. If that logic hold true, then the 2-series Oly screens
> will make for harder focussing under certain conditions, yes?

Well, if you imagine (again, sorry :-) a screen that could be varied between
completetly
clear and completely matte by changing the degree of scattering by means of
a magic dial,
you could select a ratio of the properties of the aerial and projected
images
described above.

The clearer the screen became, the brighter it would be near the centre due
to the aerial
component and this bright, always sharp, aerial image would overwhelm the
scattered light
from the projected component, making it hard to distinguish focus. At the
same time,
however, the corners of the image would become dark where the aerial image
was absent,
due to cutoff by the rear element.

But clever, microscopic, shaping of the surface of the screen could aleviate
this
darkening by deflecting the view *towards* the centre of the lens in much
the same was
as the split-image deflects it away from the centre. In effect the screen
would be composed
of thousands or millions of tiny prisms, each re-directing a part of the
*aerial* image
towards the eye of the viewer. This is similar to a fresnel lens, and indeed
a very fine
fresnel surface could achieve the same effect.

I think that this is what the modern, ultra-bright focussing screens do. By
cunningly
favouring the aerial image over the scattered component, they greatly
increase apparent
brightness while making it more difficult to perceive focus.


This is all guesswork on my part, but I think it holds together.

The thing that I don't underestand is this:

Olympus makes focussing screens (1-5, 1-6, 1-7, 1-12) that have *no* matte
portion,
i.e. there should only be an aerial image. How come you don't see the edge
of the rear
element, as you would with *no* screen installed in the camera?

Is this due to the rest of the viewfinder optics? Is there a fresnel
component to these screens? If so, are the fresnel rings visible through the
viewfinder?

As you'll gather, I've never actually used one of these, only seen the
description
in the literature.

Answers (or comments/criticisms) on a postcard, please.

Hope no-one has fallen asleep reading this.

-Brian

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