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This topic comprises 2 pages: 1 2
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Topic: Off Centre for Digital Projectors
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 07-03-2019 08:34 AM
Almost no movie projector is exactly in the center of the screen, not vertically and many also not horizontally. Perfect vertical alignment is almost impossible, because the beam would probably impact the audience before it reaches the screen. And horizontally, it's quite common to have multiple projectors aimed at the screen, which makes perfect horizontal alignment impossible.
All DCI compliant projector/lens combinations allow for a certain amount of lens shift. The larger your lens shift will be, the more side effects you'll get though, like non-uniform focus and optical aberration.
Then there are the digital tools, that allow for trapezoid corrections called keystone corrections. I consider those options last-resort options, because they digitally alter the image, thus lowering the potential resolution and often leave some nasty artifacts.
Also, in order to correct focus issues due to extremely high projector placement, like in rooms with very steep stadium seating or balconies, a common trick is to tilt the screen a bit.
I know about one auditorium that is located in the basement, which has the booth located on the left side (when facing the screen), this sub-optimal placement is due to height restrictions. This leaves some noticeable issues with focus on screen.
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Jim Cassedy
Phenomenal Film Handler
Posts: 1661
From: San Francisco, CA
Registered: Dec 2006
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posted 07-03-2019 11:01 AM
I worked in a screening room where the NEC projector was located way off
center using an old spotlight port because we wanted to keep our two 35mm's.
But the picture always bothered me. I didn't have a trouble with image focus,
but trying to get an even light across the screen took a lot of work, and even
though it "looked ok", it wasn't ideal, and it bugged me.
I also had to play with the internal projector masking quite a bit to hide the
fact that the image was trapezoidal. Corporate customers who put power-point
graphs or charts on screen often commented that our projector "wasn't level" ,
and I'd have to explain what was going on & why it looked that way. Enough
of the image was masked that we were just barely "DCI legal".
I finally had enough, and early this year I got them to spend the money to
move the NEC to position between the two 35mm's. This involved buying a
spacer to bolt onto the existing projector pedestal to raise it about another
foot or so, and to re-do some of the power & and signal wiring.
The whole thing cost just under $1000, but IMO it was worth every penny.
My tapazoidal image is gone and screen illumination uniformity is now no
longer a challenge to achieve.
The NEC is now located between the two 35mm's, and shoots out of one
of the projectionist viewing ports. I can still do 35mm change-over shows.
I can easily squeeze between the NEC and the left 35mm projector, with
no problem, and I can still see enough of the screen to focus & catch catch
the cue-marks. It's a bit of a tight fit, but I'm a skinny guy & have no
trouble. If "the other guy", who is a lot larger than I am needs to do a
35mm show, the NEC is on wheels, so we just roll it back a bit so he
can fit in there. I made sure there was enough slack on all the cable
connections to do this when we moved the NEC to the center position.
Long story short- - IMO it was well worth the effort to get the projector
to its' new position which is now almost dead-on center screen, and I
sleep a lot better at night knowing that I'm no longer forced to work
with what I always considered a 'compromised' image at that location.
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 07-03-2019 01:26 PM
quote: Marco Giustini
Those are not available on DCI projectors thought so if you run out of lens shift you'll see vertical/horizontal keystone.
It's not certain we're talking bout DCI projectors here. But, DCI projectors indeed don't allow keystone corrections, but almost all other digital projectors do. Not all digital projectors do have lens shift though. (Interestingly though, DCI still allows for scaling, but that's another discussion.)
I had a special case of a laser scanning projector once, that did have a pretty interesting trapezoid correction configuration, by adjusting the laser scanning pattern itself.
quote:
PS: I find that not many techs like to adjust the focus uniformity by playing with the aforementioned tools!
It's a pretty time consuming process and you need the proper tools to do it right, like at least some good binoculars.
The problem with scheimpflug/boresight adjustments is that when you over-use it, it seems to cause some additional optical aberration, especially in the edges of the screen. (You can try to compensate that by adjusting the convergence a bit... ![[Smile]](smile.gif) )
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