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Topic: SMPTE std's for cropping and light
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 06-07-2003 10:05 AM
As far as cropping goes...in the SMPTE it is a tough nut to crack. Generally, less than 5% is desired and 10% or more is considered not good enough.
The industry has a growing problem though. As focal lengths are getting shorter, the fringe area is getting wider/taller (the shadow area created by the distance of the aperture to the film plain plus the thickness of the plate itself). You'll lose 5% or more trying to put that fringe/shadow area on the masking.
Also, the SMPTE is not specific on the amount of overscan that can be counted on. That is, if the maximum width is .825-inches, then how far can the aperture plate be safely overcut before disaster results? Can you cut out to say .840" and not see any DTS track on the left or clear/blue lines on the right? It isn't defined. As such, .825" can only be truely interpreted to be the MAXIMUM opening and cropping measurement starts there.
So, lets say you cut your aperture plate so you just barely start to see the .825" line...then close the masking down to hide the shadow and present a nice crisp edge...then fin that you are now only showing the audience out to .800 inches. There is 3% right there. Now what about keystone? If the aperture plate must also take into account keystone issues, it is quite easy for one to exceed 5% cropping.
I would therefore dismiss any percentage cropping requirement (and have done so). It is something beyond the installer's control. The design of the theatre and equipment is not in the installer's perview. Thus, you can't hold them accountable for that which they can not control.
Now, not setting bypass levels is another story....that is something they clearly should and within their control.
Steve
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John Walsh
Film God
Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999
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posted 06-08-2003 10:58 AM
From what I can research;
SMPTE 196M-1995 defines the type of light meter, where in the auditorium to measure the screen light, and what the screen brightness should be. Interestingly, it allows as much as 22fL for theaters, which seems too much, but I'm sure rarely happens anyway.
SMPTE 195-1995 gives the projected image dimentions, but doesn't seem to actually state min/max (croping) values. At our theater chain, we'd liketo see no more than 3%, with 5% the maximum. Of course, we do have a few, "oops!' screens to fix...
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Frank Angel
Film God
Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 06-15-2003 10:51 AM
As far as how brightness standard levels are arrived at, that was a question someone once asked me years ago and I really didn't have any answer. How did the SMPTE come up with 16ftL? And wasn't there an SMPTE Journal article a few years ago where studies were done with images of much higher brightness 22-27ftL? They found that test subjects found the brighter screens preferable, even though increasing brighness increases the perception of flicker? Yet, there is projectionist I know who vehemently hates bright screens....says brighter screens "look horrible."
So is a lot of this subjective? Did a few SMPTE folks sit down and look at various brightness levels and say, "Gee, that looks good" and then measured it, found it was 16ftL and bingo, they had a standard? I say this facetiously, knowing that the screen brightness is the end of a long chain of meticulous processes designed to match what is seen by the camera, through emulsion types, the printer settings, etc, to what is eventually seen on the screen. But in the end, someone sitting in front of a screen has to say, "Yup, that's it....that brighness looks perfect." And that last, final evaluation has to be fraught with subjectivity, as was proven by ordinary people in the test who seemed to prefer images brighter than the 16ftL standard.
Frank
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John Pytlak
Film God
Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000
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posted 06-16-2003 07:19 AM
It wasn't always 16 footlamberts! American Standard PH22.39-1953 specified "The brightness at the center of the screen for viewing 35mm motion pictures in theatres shall be 10 +4/-1 footlambert when the projector is running with no film in the gate." But by American Standard PH22.133-1963, the standard for review rooms had been established at 16 +/- 2 footlamberts.
I recall that Glenn Berggren and Bobby Pinkston were very involved in writing the current standard, which allows up to 22 footlamberts. In the early 1980's, some favored LOWERING the aim from 16 footlamberts, to accomodate theatres who were not meeting the standard, but "Brighter is Better" prevailed. Almost everyone agrees that anything lower than 10 footlamberts greatly compromises image quality. The upper limit is really defined by the perception of shutter flicker, which becomes objectionable for most people at about 25 footlamberts (2-blade shutter, 24fps). Of course a three-blade shutter or higher frame rates would allow even higher screen luminance without flicker.
I have projected "normal" prints (timed at 16 footlamberts) at 60 footlamberts using a three-blade shutter, and the images are amazingly good. But for most theatres, the low efficiency of a three-blade shutter makes achieving these high luminances on a large screen impractical.
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