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Old July 10th, 2004, 04:40 PM   #1
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zebra questions

Dear folks,

Here are a couple of things I probably should know by now, but I don't so here goes. I was playing around with my XL1s the other day. Set the Zebra for
100 and then switched the camera to automatic thinking that the setting
would not allow me to overexpose the highlights.....but it did. So my question is,
should this happen. If I eliminated the zebra lines, then the rest of the image
seemed badly underexposed, which I suppose indicates that my contrast
ration is very high. So question two is, are there simple ways ( without
going into huge lighting setups ) to minimize the contrast in a shot. And finally
(this is a little off topic), does a polarizing filter increase contrast. Hope these
questions aren't too fuzzy, and as always, I appreciate everyone's help.
Glen
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Old July 11th, 2004, 07:27 AM   #2
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Controlling lighting is key, and a second factor can be to control the color/contrast of objects in your set. You can use reflectors, camera angle, and time of day to help manage natural light problems But no matter how you slice it, video has a limited contrast range and you may have to decide between highlight and shadow detail. Using a good monitor at the shoot can help you decide.
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Old July 12th, 2004, 09:48 AM   #3
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Someone please correct me if I'm wrong but I don't think turning on or off zebra stripes will affect your shoot at all, they're just a graphic representation of what the exposure is like...no?
TIA,
Randy
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Old July 12th, 2004, 10:41 AM   #4
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AFAIK that is the case IOW the zebra stripes simply show you what the "light meter" has done. In Glen's case it allowed bits of overexposure for the sake of keeping shadow detail and seems to do a pretty good job of it because if Glen fiddles with the EV control he gets a result he doesn't like as much.

As for a polarizing filter: it is used to enhance the contrast between, say, clouds and blue sky but can also reduce dynamic range under some circumstances. The most dramatic effect would, to my way of thinking, be seen if the brightest and darkest areas of a frame were emitting orthogonally polarized light. If the range of intensities were, say, 3 stops and the polarizing filter reduces cross polarized light by 1 stop then if the filter is adjusted to reject the darkest part of the picture it would be 4 stops darker than the brightest and the filter would have thus increased dynamic range by 1 stop. If, OTOH, it is set to reject the brightest part of the picture then this part is only 2 stops brighter than the darkest part and the dynamic range has been reduced by 1 stop.

In the real world you generally have a single linearly (or eliptically) polarized source you want to suppress while the rest of the scene is randomly polarized. In the case of specular reflection from water as these are probably the brightest things in the scene suppression of them could be expected to reduce dynamic range. Often it is the sky which is to be darkened. As it usually falls between the brightest (e.g. clouds) and darkest (e.g. shadows) part of the picture the dynamic range would be unchanged in this case though the difference in exposure of clouds and sky is increased giving the desired effect.

A.J.
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Old July 12th, 2004, 03:41 PM   #5
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AJ

You seem to have the stuff on polarization wired. What exactly is
orthogonally polarized light? Going back to my still photography days, I always
just slapped the polarizing filter on to cut glare or blue up the sky. I never did
really understand what it was doing. Do you know of any good readings on
polarization? It would be nice to actually understand it. Thanks, Glen
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Old July 12th, 2004, 05:32 PM   #6
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>>"As for a polarizing filter: it is used to enhance the contrast between, say, clouds and blue sky but can also reduce dynamic range under some circumstances."<<

AJ (or anyone else that may know), I wonder if the filter would help lessen the contrast of say a very dark skinned person in a white shirt?
TIA,
Randy
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Old July 13th, 2004, 07:59 AM   #7
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Gents,

For a basic explanation of what polarization is and how a polarizing filter works try http://www.colorado.edu/physics/2000/polarization/polarizationI.html.

To answer the specific question: light is a form of electromagnetic energy which propagates by exchanging energy between electric and magnetic fields. If the electric field is confined to a plane the light is said to be "linearly polarized". If light from one source has it's electric field in a plane that is at right angles (90 degrees) to the e-field plane from another linear source then the two sources are said to be "orthogonally polarized". In this case a perfect polarizing filter could block one source (a) completely while letting source (b) through by aligning it with source (b). If aligned with (a) the converse would be the case. If aligned half way between the (a) and (b) planes the two sources would pass with equal diminution. Thus this is the case where the filter gives the most dramatic control. You will never meet this case in the real world (unless you are in a laboratory).

Polarizing filters are useful when light is linearly polarized so that the filter can be used to control it. In general light is not linearly polarized - the e-field can be found aligned at any angle - so the filter is ineffective. I would expect the light scattered from a person and the person's clothing to be randomly polarized so that a filter would probably not work in this case but you never know - try it!

A.J.
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Old July 13th, 2004, 08:24 AM   #8
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Thanks AJ!
Randy
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