Film-Tech Forum ARCHIVE: Color shifting

This is topic Color shifting - lens problem? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Mike Carro (Member # 232) on 04-07-2004, 08:23 PM:

I have an ongoing problem with my presentation. In the vicinity of the center of the picture there is color fringes. By this I mean there seems to be a blue fringe on edges. Most noticeably on credits. The sides are fine. Nice and sharp. Now it does this in flat or scope. So, could it be an improperly focused lamp or is it the port glass that I'm using or is it the lens. Using newer Iscos. In fact brad helped me get those. So I know they're good but they are used. The port glass isn't optical glass but it is clear(not green if you look at the edges). So that's my question.

Thanks

Posted by John Pytlak (Member # 331) on 04-07-2004, 09:33 PM:

Sounds like a bit of chromatic aberration.

First, eliminate any effect from the uncoated port glass by running the evaluation WITHOUT the glass.

Looking at your critical scene content (titles), run through focus. Does the fringing shift across the image, where maybe the center has no fringing and the edges do? As you focus does the fringing turn from blue to yellow? If so, it's likely a bit of chromatic aberration.

This test is best performed with a B&W silver image alignment film like SMPTE 35-PA (RP40). A color print (even of B&W titles) may have a bit of color fringing from the printing optics.

http://www.vanwalree.com/optics/chromatic.html

quote:
Chromatic aberrations
When different colors of light propagate at different speeds in a medium, the refractive index is wavelength dependent. This phenomenon is known as dispersion. A well-known example is the glass prism that disperses an incident beam of white light into a rainbow of colors [1]. Photographic lenses comprise various dispersive, dielectric glasses. These glasses do not refract all constituent colors of incident light at equal angles, and great efforts may be required to design an overall well-corrected lens that brings all colors together in the same focus. Chromatic aberrations (CA) are those departures from perfect imaging which are due to dispersion. Where the Seidel aberrations are monochromatic, i.e. they occur also with light of a single color, chromatic aberrations only make their appearance in polychromatic light.

Posted by John Hawkinson (Member # 1135) on 04-08-2004, 04:15 AM:

You can always do a test with another lens, or walk the lens over to another projector. Even if the focal length is wrong for the throw, you should still see the same aberration if it is the lens...

--jhawk

Posted by Darryl Spicer (Member # 711) on 04-08-2004, 09:19 AM:

but in his original post he states that it does it in both flat and scope. I think it could possably be the port glass causing this. Do what John Pytlak suggests remove the port glass and see if the problem is still there. Only way I could see it involving both lenses is if the lamp projected through booth lenses with no film for a period of time and damaged them in some way.

Posted by Mike Carro (Member # 232) on 04-08-2004, 10:35 AM:

I'm leaning towards the port glass too. The fact that it's slanted might factor in too. It's gonna be a pain taking it out. I might as well wait until I get some optical glass.

Posted by Pete Naples (Member # 761) on 04-08-2004, 12:10 PM:

Lamphouse mirror?

Posted by Daryl C. W. O'Shea (Member # 1303) on 04-08-2004, 01:05 PM:

You don't absolutely need to take the port glass out to see if it's the problem. You could move the projector around (laterally and angle) to see if the problem moves with it.

Posted by Mike Carro (Member # 232) on 04-08-2004, 03:38 PM:

Move the projector........... you've got to be kidding. It weighs about 600 lbs. Plus it's aligned perfectly with the screen. And as Pete said, could it be the lamphouse mirror?
I'll figure it out one way or the other.

Posted by Daryl C. W. O'Shea (Member # 1303) on 04-08-2004, 03:47 PM:

I guess that's why I really like adjustable tables... and crowbars when necessary.