Re: Keystoning in projection

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

Shab wrote:

> If you need to raise your projector above the audience anyway, why not
> project down at an angle and tilt the screen to remain perpendicular to the
> film? Would that tilt be too disturbing to look at? What will that angle be
> anyway in your case?

Heck of a good question.  About 66" of the screen width is usable and a 
7-perf has a 28 mm wide aperture and a FED has a 38 mm long lens.  So I
want to sit (38/28)66" = 7.5' from the screen.  Considering the projector
is 20' (horizontally) away, you'd probably have to have the projector 
10' above the screen to miss the audience.  OK, quit being silly and 
figure it out.  Person's head is an 8" diameter sphere located half way
up the screen and 7.5' from the screen.  Call the screen height 56" because 
of the aspect ratio of 7-perf.  That means the projector has to be 4'10"
above the center of the screen (13.6 degree angle from center of screen 
to center of projection lens.  If I have an 8' ceiling in my viewing room, 
the audience will have to sit on the floor to get their eyes down to an 
elevation of 3'2".  Bean bag chairs?  Lounge chairs?  8-)

Now try 5-perf format.  Screen height goes from 56" to 66".  New figures
are 15.5 degrees and 5'6" elevation gain.  Even worse.

I should figure the keystoning.  I know people can only tolerate about
5% keystoning in flat movies.  Later for that calculation.  I suspect
I'd have to use lens fall to make this work.  Well, aside from ability
to tolerate heat, camera lenses and projection lenses are pretty similar.
So I could go to a pair of 50 mm perspective control lenses.  Right, that's
well within Bill Gates' budget, just not mine.  8-)

I think tilting the screen doesn't really control keystoning but I 
ought to figure that too.  It would sure fix focus, though.

John B


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