Film-Tech Forum ARCHIVE: 16 foot lamberts, for all ratios?!

This is topic 16 foot lamberts, for all ratios?! in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Duncan Smith (Member # 1556) on 01-31-2003, 01:40 PM:

I'd like to see what opinions and experience there is out there on this topic.

I have worked in a few theatres now, one of which had the policy of ensuring the light on screen was always at this level. This theatre was equipped with virtually all current 35mm projection ratios: 1:33, 1:38, 1:66, 1:75, 1:85, 2:35 and 70mm amongst others. The policy there was to take light readings on screen for each ratio and adjust the amperage of the Xenon lamp so as to achieve 16 ft/l. Depending on the size of image, lens length and speed, aperture size etc differing amounts of light reach the screen for each ratio necessitating this adjustment. Your average multi can only project 1:85 and 'scope so they usually operate at a level of compromise regarding luminance for each ratio resulting in one being too dark or too bright.
What are your thoughts on this procedure of adjusting the xenon output?
I find it to be effective and professional and continue to practice it where possible although I am aware that it can stress a xenon lamp more than just letting it run at a constant amperage for it's entire life.

Posted by John Pytlak (Member # 331) on 01-31-2003, 01:54 PM:

Standard SMPTE 196M specifies an aim screen luminance of 16 footlamberts, with an allowed range for theatres of 12 to 22 footlamberts. If you have an accurate screen luminance meter, and the time to change the lamp current, setting the luminance to exactly the 16 footlambert aim is a fine idea -- as long as you stay within the specified operating current range of the lamp. In general, the 1.85:1 "flat" format will be the least efficient and will need the highest current.

Lamp focus is sometimes used to "tweek" screen luminance. Within a small range, you can defocus slighty, reducing brightness, but improving illumination uniformity. But you need to be careful to avoid misfocusing so far as to get a "hotspot" or a "doughnut" illumination pattern.

Another thing that can be done is to use internal lens "stops" to reduce light levels of the more efficient formats to match all the formats shown. Stopping down a lens decreases light level, but increases depth of focus. Optimum location of the lens stop (aperture) varies with the lens design, and should be done in consultation with the lens manufacturer.

Posted by Steve Guttag (Member # 268) on 01-31-2003, 08:31 PM:

With common height masking, it is pretty easy to have FLAT and SCOPE both fall well within SMPTE guidlines with Scope slightly brighter than FLAT (1.85). You can split the difference and have two very nice pictures.

ISCO offers studio stop down rings for their Ultra-Star PLUS lenses (the RED ones). They have f/2.4 and f/2.7. Using these to balance your light between formats has the advantage that you also pick up depth of focus on the stopped down lens too.

Schneider showed at ShowEast and is working on bringing out a line of lenses with a variable speed (like an SLR lens) so you can adjust it for perfect light balance between the formats.

Steve

Posted by Larry Shaw (Member # 388) on 02-03-2003, 07:02 PM:

Kinoton has an option that allows automatic adjustment of bulb current with turret change.

Posted by John Wilson (Member # 269) on 02-04-2003, 04:55 PM:

On one of our smaller screens, our 1.85 is brighter than our scope. So far, no tech has been able to fix the problem.

Any ideas?

Posted by Greg Mueller (Member # 17) on 02-04-2003, 05:13 PM:

Maybe the anamorphic is sucking light?

Posted by John Pytlak (Member # 331) on 02-04-2003, 06:24 PM:

I agree with Greg that it's likely due to poorer lens efficiency. What image sizes? What lenses? What f/stops?

Posted by John Walsh (Member # 168) on 02-04-2003, 09:58 PM:

"On one of our smaller screens, our 1.85 is brighter than our scope..."

Do you had adjustable top (up/down) masking that forces you to use the framing knob to position 'scope films out of the lens center.

Posted by Rick Long (Member # 213) on 02-04-2003, 10:57 PM:

Larry Shaw said "Kinoton has an option that allows automatic adjustment of bulb current with turret change.

So does Strong.

I would be interested to know what luck you have had with these over time.

Thanks

Posted by Frank Angel (Member # 248) on 02-05-2003, 02:58 AM:

But varying the lamp current wouldn't give you the advantage of greater depth-of-field as would stopping down the lens.

Frank

Posted by Paul Mayer (Member # 355) on 02-05-2003, 04:31 PM:

Big Sky offers an automated lamp focus feature (helped with the installation of four retrofit kits here last month).