Film-Tech Forum ARCHIVE: Problem with lenses??

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Posted by Antonio Marcheselli (Member # 373) on 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

Posted by John Eickhof (Member # 323) on 11-28-2000, 09:35 PM:

If this is happening on all films projected,
you should check the lens for cracked or
damaged elements. A halo effect can be caused by frefraction of light through a crack or chip. Also, if the anti-reflective coating has been burned, discoloration will occur.
If the lens is damaged, it is best to replace it.

Posted by Steve Guttag (Member # 268) on 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!"

Posted by John Pytlak (Member # 331) on 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

Posted by Randy Stankey (Member # 64) on 11-29-2000, 10:15 AM:

Please forgive me for asking this... Is the lens clean?

I've had people ask me to "fix" the picture on more than one projector. I went there and took one look at the lens. It looked like a DOG LICKED IT!

I just handed the guy a bottle of lens solution and a book of tissues and smiled at him...

Posted by Antonio Marcheselli (Member # 373) on 11-29-2000, 05:38 PM:

Ok, thanks for your suggestions.
The halo is present on scope (I will check on flat) with all prints. Lamps aligmnent should be OK, but I will keep on mind that something wrong with lamp can cause this problem.
Randy, you can bet that my lenses are very clean: I performed the annual clean few months ago...

(I'm kidding of course!!)

Thanks again, I will check with your suggestions and I'll let you know

Bye
Antonio

Posted by Randy Stankey (Member # 64) on 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!

Posted by John Pytlak (Member # 331) on 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.

Posted by Randy Stankey (Member # 64) on 12-01-2000, 11:09 PM:

You're right on the difference between the dirty lens and the halation/fringing effect. It's just that lately I've been having to do a lot of "Meatball Surgery", if you get the "M*A*S*H" reference. You have to ask those stupid questions to get the "junky" stuff out of the way before you can start to solve the "real" problems. I have learned to start by eliminating the simple solutions first and working up to the hard things.

I guess the next question I'd ask is, "Does this occur with all prints, or in other houses with the same print?" (If it's a multiplex) After that, I'd look into lens condition and age. I'd also quickly check the "astigmatism" adjustments if it's only on the SCOPE lens. You'd be surprised to find out how many lenses I've found that have their settings all catty-wompass.

Oh, Yeah! What about oil or some other foriegn (non-Filmguard) substance on the film?

Posted by Antonio Marcheselli (Member # 373) on 12-03-2000, 05:23 PM:

Randy,

Don't worry! I understand what you mean! I can't tell you how many dirt lenses I've saw...

Bye

Antonio

Posted by John Pytlak (Member # 331) on 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.