andre_noble3 Posted May 28, 2004 Share Posted May 28, 2004 I am having problems with the Lee polyester filters. I am getting weird circular rainbow/banding/interference type patterns across my images when using them with wide angle lenses. I was able to isolate this problem to the polyester type filters themselves. For example, I get banding in both the 4x5 Polaroid and 4x5 traditional negatives when using the Lee 23A Red and Lee 21 Orange, but not in an identical scene taken with corresponding B&W glass filters shot at the same moment. Furthermore, the polyester filters where used singly - not in conjunction with a polarizer or any other filter. The problem appears limited to use with wide angle lenses. (Unfortunately, much of my own photography involves these parameters). At time of use, there was no direct sunlight striking either the filter or the filter holder which my have caused unwanted reflections. Furthermore, the problem seems to relate to the angle in relation to the sun at which the image is taken. The camera was pointed appx. 180° away from the sun in the 35mm slides using a 20mm lens, and approximately 90° away from the sun in the large format images using a 90mm Nikkor f/8 lens and front rise. Finally, rotating only the filter caused the �interference pattern� to rotate around inside the viewfinder in kind. I also visualized the same problem with other Lee Polyester filters such as the Colour Temperature set. After a couple phone calls to Lee Filters in England, they deny having ever experienced such problems before despite possible evidence such as this thread on Photonet: http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005bh7 Anyone else have a similar experience with these (or their Calumet equivalents)? Note: I am specifically referring to the thin Lee Polyester filters. I do not mean the thicker Lee resin filters, nor am I referring to the thin Kodak gel filters, which are both manufactured using a different technology than the polyester type filters. Thanks in advance for any input you can share. Link to comment Share on other sites More sharing options...
michael_briggs2 Posted May 28, 2004 Share Posted May 28, 2004 <p>My memory of the instructions that come with the Lee thin polyester filters is that the instructions warn against using the filters on one side of a polarizing filter. Of course, you aren't using a polarizing filter, but this warning is a clue that the polyester filters can be birefringent. When certain plastics are stressed, or from stress created in the plastic when the item was made, they become birefringent, which means that two polarizations of light have different refractive indices in the glass.</p> <p>Skylight is naturally polarized in some directions. In fact, the two directions that you describe are consistent with directions of maximum polarization. It is normally stated that the polarization is maximum at 90 degrees from the sun -- this could be 90 degrees to the side -- this could be one of your cases. Maximum polarization can also occur at 180 degrees in azimuth from the sun, but above the horizon by an angle that makes the patch of sky 90 degrees from the sun -- this could be your second case. See <a href="http://www.polarization.com/compass/compass2.html">http://www.polarization.com/compass/compass2.html</a>. So I think this is a second clue that your colors are from some polarization phenomena.</p> <p>The phenomena of birefringence in plastics is best observed with two polarizers, one in front of the plastic to polarize the light transmitted through the plastic, and a second after the object to analyze the light. In your case, the partially polarized skylight may take the role of the first polarizer. I can't explain why you are seeing the effect without a polarizer after the polyester filter, nevertheless, from the clues I suspect that polarization and birefrigence is involved.</p> <p>Here are some explanations and example photos of stress birefrigence in plastics observed with polarized light: <a href="http://www.oberlin.edu/physics/catalog/demonstrations/optics/birefringence.html">http://www.oberlin.edu/physics/catalog/demonstrations/optics/birefringence.html</a>, <a href="http://www.devicelink.com/mddi/archive/99/03/008.html">http://www.devicelink.com/mddi/archive/99/03/008.html</a>, <a href="http://www.devicelink.com/mpb/archive/97/01/001.html">http://www.devicelink.com/mpb/archive/97/01/001.html</a> and <a href="http://www.fas.harvard.edu/~scdiroff/lds/LightOptics/Photoelasticity/Photoelasticity.html">http://www.fas.harvard.edu/~scdiroff/lds/LightOptics/Photoelasticity/Photoelasticity.html</a>. Does the effect you see look similar?</p> <p>From this analysis, one shouldn't use a polyester filter in front of polarizing filter, because skylight could act as a source of polarized light and the polarizing filter could act as the analyzer, producing a colorful display of birefrigence.</p> Link to comment Share on other sites More sharing options...
andre_noble3 Posted May 29, 2004 Author Share Posted May 29, 2004 This is exactly what is happening with my Lee polyester filters. The patterns in my photos look VERY simlar to the patterns in the protractor below. I will develop my remaining roll and post some actual examples later. A key quote from the Oberlin page states, "The protractor shows colored bands without being deformed because anisotropies arise when the plastic solidifies when manufactured. The colored patterns reveal internal stresses." Thank you Michael for taking the time to research the issue. Link to comment Share on other sites More sharing options...
andre_noble3 Posted May 30, 2004 Author Share Posted May 30, 2004 Birefringement phenom: <img src="http://www.photo.net/bboard/uploaded-file?bboard_upload_id=18185184"> Link to comment Share on other sites More sharing options...
andre_noble4 Posted May 30, 2004 Share Posted May 30, 2004 Here's a picture of the actual problem.<div></div> Link to comment Share on other sites More sharing options...
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