P3D Comparing projectors/screens

[Les Gehman <lpg@xxxxxxxxxxxxxxx>]

The other night Tom Schmerge and I spent an evening comparing different
projectors and screens.  We compared three different projectors and five
different screens.  The three projectors compared were a TDC 716 with
4" lenses and 750W bulbs, a TDC 116 retrofitted with 75mm EL-Nikkor enlarging
lenses and 500W bulbs, and a FED/BOY projector with 80mm lenses.  Of the
projectors, the 116 w/EL-Nikkor's and the FED/BOY were virtually identical
in sharpness.  Both were slightly better than the 716 w/4" lenses, but not
as dramatic of a difference as we had expected.  In comparing the projectors,
we moved them as needed to make the (identical) images equal in size.

However, we did find a very great difference in screens.  One of the screens,
a Da-Lite, was about 1.5 stops dimmer than the others, including other
Da-Lites.  This was measured by projecting identical slides onto two screens
with the FED/BOY and 116, which were very close in brightness, then stopping
down the enlarging lenses until the brightness matched.  Unfortunatly, the
dim screen is the largest screen either of us own, which we had planned to
use for the inagural meeting of the Northern Colorado Stereo Club.  Our other
four screens varied from each other by about half a stop and all looked much
better than the dim screen.

While the lenses in the 716 were not as sharp as either of the others, the
750W bulbs, or possibly the lenses, did make for a brighter image which, at
least with the slide we were comparing, seemed to make for a more desirable
image.  This makes me think that once the image quality is "good enough",
whatever that means, brightness is next thing to work on.  

This whole experiment raises the question of how do we tell in advance if
a screen will be bright or dim?  Do they lose their reflectivity with age?
I've been thinking of buying a 70" Da-Lite Silver Picture King ($170 from
B&H), but now I'm not sure I want to chance buying a dimmer screen than
what I already own.  I'd sure like to hear from anyone else that has done
some screen comparisons.


Les Gehman
lpg@xxxxxxxxx


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