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This topic comprises 5 pages: 1 2 3 4 5
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Author
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Topic: 3-bladed shutters
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Michael Schaffer
"Where is the
Boardwalk Hotel?"
Posts: 4143
From: Boston, MA
Registered: Apr 2002
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posted 07-21-2004 04:42 PM
quote: Matthew Jaro
If anyone has adjusted multi-sync computer montitors, they will note that 60 Hz refreseh rate is intolerable and they will raise it. 48 is horrible.
Yes, but you cannot directly compare these. A monitor is building up the image line by line, and while the film projector also "builds up" the image by progressively revealing it, when the shutter is open, the image is statically present for a moment.
It's hard to compare the two, but subjectively I would say the amount of flicker you see in 48 Hz projection is comparable to about 70-75 Hz on a computer monitor. My eye needs about 85 Hz for a stable monitor image, but at 72 Hz on the big screen I don't see any flicker anymore either.
I don't know about the contrast since I never had the opportunity to see an A-B comparison, but I did work a lot with 3-bladed FP30Es, and these had a very stable image indeed.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 07-22-2004 09:31 AM
My this is an interesting debate!
Here are some facts...3-wing shutters can have ZERO affect on contrast ratio. However, I read Sam's post as "perceived" contrast...that is what one thinks they are seeing and that can not be measured with a meter. The meter will still measure the bright areas and the dark areas and come up with a ratio. As Sam says...what is the equivalent to psychoacoustics for vision?
However...while I'm on that subject...contrast on 3-wings. I have not observed the same thing. I have not seen contrast ratio change. I wonder when Sam did his experiments if his meter was calibrated to integrate properly for 72Hz as well as 48Hz to provide equivalent light levels.
I bring this up because of my work with Kinoton's "E" projectors. Most of the ones I work with have a shutter that operates as a 2-wing above 20fps and as a 3-wing below 20fps. It is one of the advantages of having the shutter as a separate motorized item and not tied to the gear train. There are merely 3 passes of the blades before advancement below 20 fps (note, one can have the machine configured for 3-wing at even higher speeds...it is all in how the software is programmed...they also have a "3-wing" verson of the projector that has a different shutter blade all together...it is actually a two-wing blade but with very narrow openings on the order of a Simplex or Century 3-wing...but I digress).
Now consider this...as one slows down the projector and the machine switches from 2-wing to 3-wing mode...I don't see any shift in light or contrast on the screen (perceived). If the perception of contrast was based soley on the number of flashes per cycle, then one should see the screen image dramatically change but you don't. The flicker just diminishes from not bad to not there (you can hear the blade speed up when this happens).
BTW...if you do need to run a mix of silent and sound films...there is no substitute for me now...there is only the Kinoton "E" series of projectors. There is no switching of shutter blades...it is just done is software and it works flawlessly. It is really cool watching stuff at 16fps without flicker.
I will say, I have yet to meet the person that installed 3-wing shutters in a screening room or small screen venue that didn't think they made all the difference in picture quality.
Sam is also correct on his statement on the light loss being more than just 33%. A typical Simplex shutter has a 84-degree shutter (closed portion). This leaves us with a 53% shutter. A 3-wing shutter will also have 84-degree shutters but three of them. Leaving us with just 108-degrees for the three open portions or just 36-degrees each. The math is such that the true light is an integration over the entire cycle of the shutter blade. The three wing shutter is only 30% efficient leaving us with about 56% of the light of our 2-wing counterpart. Again presumming the closed portion of the 3-wing blade is still 84-degrees for both 2-wing and 3-wing.
On those machines with a 90-degree shutter with 50% efficientcy, the numbers fall such that a 3-wing shutter has a 30-degree opening so the efficientcy is only 25% which is only 50% of that of the 2-wing...as Sam stated above.
As to the rise and fall times of a 2-wing over a 3-wing...there is ZERO difference. The rise and fall times are a function of the shutter speed and the radius of the blade...period. A small shutter (i.e. Christie) will have a more pronounced flicker due to its small shutter than a large shutter (e.g. Norelco AA2 running a double speed). The frequency of the blades in both machines is the same...48Hz...the velocity of the blades as they cross the aperture are not. This is easily observed using John's solar cell connected to an O'Scope. Personally, I find it easy to observe on the screen. Century used a double shutter to increase the velocity of their shutter (double it) without increasing its size. One can also decrease the shutter closed portion as well since there are now two blades to further increase the light. However...there is a penalty...each shutter will have some bounce to it due to gear lash and the reversing shutters goes through several extra gears...since each blade is bouncing differently at different times, there will be a bouncing hot-spot on the screen. This is further accentuated since the center is the last part of the frame to close and the first to open. A light weight aluminum shutter will have a violent bounce...a heavy steel shutter will have a more damped bounce....I've seen several mis-diagnosed xenon lamps (flicker) that really was due to shutter bounce on a double shutter.
Now, consider this. I've had some of the same comments about 100-degree steel shutters (Century part C1-D-50S) as some have made here about 3-wings increasing the perceived sharpness. This is also in a screening room like environment with a relatively small screen and low wattage lamps.
Steve
[ 07-22-2004, 04:33 PM: Message edited by: Steve Guttag ]
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