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Old Jun 11, 2011, 11:25 AM   #11
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What a great explanation. Thanks for the write-up.
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Old Jun 15, 2011, 3:48 PM   #12
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Another great use for polarizing filters is for skin in sunlight. Suntanned skin will be a little darker since the moist surface of the skin will be reduced by the polarizing effect. Im still unclear about linear vs circular polarizing filters. I saw your last post with the sky effect caused by circular filter on wide angle lens. And I was led to believe that linear filters dont even work on digital cameras for some reason. I will have to study up on this. And your thread here should be a special stickie of some kind since its very informative and more than a shot for a challenge.
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Old Jun 15, 2011, 8:29 PM   #13
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I've read a couple of things about the difference between linear and circular. I don't know the optic differences but I've had more problems with exposure when using a linear polarizer than I have with a circular polarizer. I've occasionally had some problems with AF, where the camera might not lock focus given a scene that it should not have a problem with, but that's been less of a problem than with the camera's tendency to over-expose. From what I've read, other cameras might have more problems with AF than I saw.

I'll chimp every shot when using a linear, and don't hesitate to change to manual exposure and adjust accordingly if I spot a problem. But after this day I'll be much more likely to use the linears than I have in the past, I'm much more clever about such things now than the last time I used one (not long after I bought my first dSLR). I wouldn't buy a linear polarizer since I didn't see any real difference in the actual picture, but since I already have them, I'm going to be using them more now.
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Old Jun 24, 2011, 10:17 AM   #14
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So thankful, you shared it with us. I learned so many things in this thread. Very useful! The photos are interesting!
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Old Jun 27, 2011, 3:00 PM   #15
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How do polarizers work, and why are circular polarizers recommended for SLR cameras, and not linear ones? I will try to venture an explanation which is based on my own understanding (and may therefore not be entirely correct).

My desktop has been in the shop and I have been using my portable for limited time on the forum for a while now, so I missed the filter discussion entirely. Perhaps I can add a little, if someone hasn't already done so in another earlier thread (I haven't reviewed many of them). As a microscopist, I have some experience with polarizing microscopes, and I can tell you that many other materials can polarize light as well as filters do. Calcite crystals were the usual polarizers used on microscopes before plastic polarizers were invented, but they were expensive and hard to obtain. Some microscopes substituted flat angled glass beneath the stage; today stacks of glass slides can be used for the same effect. Sheets of some clear plastics can also have polarizing effects .

In microscopes the mirror is beneath the polarizer and does not affect polarization, but in SLRs the mirror is behind the polarizer. Linear polarizers remove light at right angles to the linearity; mirrors also reflect light differently depending on the angle at which the light strikes them. Water reflects light in one plane, and a polarizer can remove that reflection by canceling it out by being oriented in the plane at right angles to it - think of venetian blinds - two blinds at the same angle would pass light; two at right angles would block light; in this way crossed polarizers cause complete extinction, (microscopes use two polarizers - one polarizer below the slide, the other called the analyzer, above the slide) to alter the degree of polarization by rotating one or the other. SLR mirrors are angled (slanted vertically) and may be affected by the angle of the polarizers transmission, so in the case of SLR cameras, the mirror is a first surface mirror, so no light actually strikes he glass itself, but the mirror is still angled. Horizontally oriented rays will be reflected in the same plane, and focus will not be affected; vertical rays, however will be reflected unevenly from the slanted mirror, the upper ones behind the lower ones, and sharp focus could be compromised. Circular polarizers have been provided for SLRs because they must in some way not allow the mirror to interfere with the planes of light reflected from the mirror, as a linear polarizer might (depending on its orientation) - its name implies it does not act in a linear fashion - I have no idea how it does this, but it must not affect focus as the liner can. Nor do I know (but will experiment sooner or later to try to find out) how the angled (slanted sideways) translucent glass in Sony's SLT cameras (which both reflects and transmits light) might be affected by by one or another type of polarizer.

This has been somewhat rambling and I hope without diagrams it has not been too confusing. I have not seen the SLR/circular polarizer relatinship explained in this fashion, but it seems logical, and I hope it is correct.
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Old Jun 27, 2011, 10:49 PM   #16
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It is very interesting. It also makes me wonder about where the exposure sensor is in relation to the AF sensor. As long as I had good contrast I didn't have much problem with focusing, but there were exposure problems - with the camera overexposing often. Perhaps the mirror was causing the meter to think the scene was darker (more light was being blocked) than it really was.

I'd be very interested in what you find out with using various polarizers and the Sony SLT cameras.
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