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Author
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Topic: Problem with lenses??
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Antonio Marcheselli
Phenomenal Film Handler
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Posts: 1260
From: Florence, Italy
Registered: Mar 2000
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posted 11-28-2000 05:31 PM
Hello everyone!I have ISCO lenses and during credits I can see a blue/purple halo around words. This halo can be noted only if you are close to the screen. My screen size is 36 feet long and the distance from booth is about 166 feet (if I converted from meters correctly!!). Is it normal? Thanks Antonio
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Steve Guttag
We forgot the crackers Gromit!!!
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Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 11-28-2000 09:42 PM
Depending on your vintage of lenses...the fringing may be chromatic abberations caused by the lens design (all lenses have this abberation to some degree...typically one of the big reasons for increasing the number of elements in a lens to to minimize it).Most of the time, if the fringing is to the point of being objectionable, particuallary if it isnt' consistant across the entire image, (check with 35-PA and look at all the little squares), it is a lamphouse alignment issue rather than the lens.... The current crop of ISCO and Schneider lenses are really very good to the point where fringing shouldn't be noticable unless you have a lamp alignment problem...I'd look there. Steve ------------------ "Old projectionists never die, they just changeover!"
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John Pytlak
Film God
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Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000
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posted 11-29-2000 06:46 AM
I agree with Steve that modern lenses are usually very well corrected for chromatic aberration. Lamphouse alignment can be an issue --- use a laser alignment jig, and be sure the mirror to aperture distance is set correctly.If this is a "scope" lens, have you set the distance ring properly? If not, astigmatism will cause the horizontal and vertical lines to focus at different points, aggravating chromatic aberration. See the procedure for properly setting distance in the "Tips" section of Film-Tech. If the images were good before, and now have deteriorated, what has changed? Lamphouse alignment and damage to the lens are the things to check. BTW, sometimes there is slight color fringing around the words in credits on color prints, due to the lenses used in optical printers, or "halation" from overexposure. You should use the SMPTE 35-PA (RP 40) test film, which is a black-and-white silver image, to check for chromatic aberration. ------------------ John P. Pytlak, Senior Technical Specialist Worldwide Technical Services, Entertainment Imaging Eastman Kodak Company Research Labs, Building 69, Room 7419 Rochester, New York, 14650-1922 USA Tel: 716-477-5325 Fax: 716-722-7243 E-Mail: john.pytlak@kodak.com
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Randy Stankey
Film God
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Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99
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posted 11-29-2000 07:27 PM
I didn't mean to insult. It's just that almost EVERY theatre I've been to has had lenses that are dirtier than I'd like to see.Personally, I don't clean lenses all the time. I prefer to do it on a case-by-case basis. If I see one that's dirty I clean it. I'd rather have a couple of dust specks than have them all scratched up from poor cleaning technique.... but what I've seen in RIDICULOUS!
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John Pytlak
Film God
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Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000
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posted 11-30-2000 06:28 AM
Randy:Usually, a dirty lens causes haze, reducing the contrast and sharpness of the image. Bright objects or titles may have a "halo" around them, and the dark areas of the scene will go "smoky". Dirt or scum on a lens would not normally cause colored fringes or a "blue/purple halo around words", which sounds more like chromatic aberration or color fringing due to halation in the printing process. Dirty lenses are a real "contrast killer": http://www.kodak.com/US/en/motion/newsletters/reel/december99/pytlak.shtml I agree with you that lenses should be kept clean, but care is needed not to damage or scratch the lens surfaces by improper cleaning. Loose dust is best removed by a clean camel's hair lens brush. Fingerprints, oil droplets, scum, or dog saliva require careful use of liquid lens cleaner, being careful not to get liquid into the lens. ------------------ John P. Pytlak, Senior Technical Specialist Worldwide Technical Services, Entertainment Imaging Eastman Kodak Company Research Labs, Building 69, Room 7419 Rochester, New York, 14650-1922 USA Tel: 716-477-5325 Fax: 716-722-7243 E-Mail: john.pytlak@kodak.com
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John Pytlak
Film God
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Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000
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posted 12-04-2000 09:31 AM
Oil on a film print shows up on the screen as "oil mottle" --- a blotchy or streaky appearance, most obvious in scenes with sky or plain backgrounds.Projecting oily prints may vaporize some of the oil, leaving an oily "scum" on the rear element of the lens, which will cause haze and degrade contrast. If so, careful cleaning of the lens with lens cleaner is called for. ------------------ John P. Pytlak, Senior Technical Specialist Worldwide Technical Services, Entertainment Imaging Eastman Kodak Company Research Labs, Building 69, Room 7419 Rochester, New York, 14650-1922 USA Tel: 716-477-5325 Fax: 716-722-7243 E-Mail: john.pytlak@kodak.com
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