Re: [photo-3d] Silver screens

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

Glad to hear that your painted screen worked out so well.   Some paints,
especially some of the newer ones, have been formulated specifically for
single coat applications.   They have a special additive to the vehicle that
tends to keep pigment particles in suspension.

The amount of tilting required is very slight.  Observing the light pattern
on the back wall of the room from the screen position while moving the
screen, you can "sweep" this pattern all the way from the ceiling to the
floor with a very small amount of adjustment.

If you find that the depth of field of your projection lenses is so shallow
that this very small difference in projection throw visibly affects
sharpness, you are probably going to have an even greater problem with the
position of the slides themselves, since a shallow depth of field means that
you will also have a shallow depth of focus (the distance from the nodal
point of the lens to the slide plane).

The magnification ratio is so great that an inch difference at the screen
would have the same effect as a fraction of a millimeter at the slide.
Slides in different slide mounts (cardboard, plastic, aluminum, between
glass, of different brands, etc.) would have to be individually refocused.
Not to mention the slide carrier itself, which might not always seat in
exactly the same position, or hold the slide precisely in the same plane.

If this is of concern, and I think it should be, check this out on your
projector.  Get a resolution test slide (from large camera stores, also some
mail order houses like Porter's).  If you intermix slides in different kinds
of mounts, you should be prepared to remount the test slide in each type of
mount that you will be using.

Be sure to check the ability of you slide carrier to return to the same
position, by cycling it in and out of the projector several times while
carefully observing the screen.

Either have a sharp-eyed assistant stand close to the screen to report
results, or equip yourself with a quality "binocular" or "monocular"
telescope to accurately judge critical focus from the projector position.

Conscientious projectionists (is that an oxymoron these days?) in theaters
will keep some type of telescope in the projection booth for the very
purpose of insuring critical focus on the screen.

You may be surprised at how small the amount of change in the position of
the slide will affect sharpness, while how great
a change in the position of the screen is required to defocus the image by
the same amount.

This is because the backfocus (distance from the nodal point of the lens to
the slide) is very small, while the frontfocus (distance from the nodal
point of the lens to the screen) is very large.

JR

----- Original Message -----
From: "John Goodman" <jgood@xxxxxxxx>
To: <photo-3d@xxxxxxxxxxx>
Sent: Sunday, October 29, 2000 5:19 AM
Subject: [photo-3d] Silver screens


> John A. Rupkalvis wrote:
>
> > All silver screens are more directional than their
> > counterparts ... If you can, tilt the screen such that the
> > brightest reflection is on the center seat...
>
> I've been pleasantly surprized at how well my home-made
> silver screen works. Mr. Rupkalvis previously posted, most
> helpfully, that "several very light "dusting'" coats applied in
> different directions crossing each other" are called for, however
> I read this a couple of days too late. I have pro-type paint
> spraying equipment, and applied a single heavy coat of
> aluminum-flake oil paint to a sheet of Melamine-surfaced
> plywood.
>
> I can't tell if the cross-talk is any worse than factory-made silver
> screens, but there is a rather narrow zone of optimum
> brightness, right along the axis of projection. With the projector
> at navel height and centered on the screen, the best view is
> right behind the projector, or else when sitting on the floor in
> front of the projector. Then, any slight darkening of the upper
> portions of the view, due to the lower viewing angle, is not
> usually an aesthetic problem.
>
> I particularly like the lack of texture offered by a silver screen,
> which increases apparent sharpness, but the depth of focus
> with the projector lenses seems quite narrow. Very slight focus
> adjustments make a lot of difference in terms of viewable
> detail, so it would seem desirable that the screen be in exactly
> the same plane as the projected slide. If the screen were tilted
> out of the plane of the slide, in order to shift the zone of
> brightest reflection to the audience, wouldn't this be at the
> expense of critical focus?
>
> One thing is certain. Painted silver screens are as fragile as
> front surface mirrors. Any contact whatsoever with the surface
> is apt to be deleterious.
>
> John Goodman
>
>
>
>