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Old Oct 25, 2006, 6:25 PM   #1
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Looking at photo sites, I'm glad I bought a polarizer. I've learned a lot, but have some questions.

It looks like linear and circular polarizers both rotate. I don't need circular on mine, but I've got it. Cost more, but I don't want return hassles.

When I took some test shots earlier today (right angle to the sun, angled upward about 40 degrees, on a tripod) it looked like I could darken the sky the most when the "notch" or "white marking" was at the 10 o'clock position and the 4 o'clock position. (camera's automatic mode may have changed exposure time and LCD viewfinder contrast, so I disregarded this experiment).

When I put the camera into "M" (manual), and shot 24 shots, "every half hour, around the clock", all were 1/400 sec. and ISO 80, it seemed like the darkest points were at 7 o'clock and 1 o'clock. (like before, tripod, 40 deg. angle, sunny, few clouds in the sky, I assume the auto white bal. chose the same balance for each shot.)

Do all polarizing filters have two "darkest" spots and two "lightest" spots?

Also, are the "lightest" spots in the rotation equivalent to not having it on at all?

I am thinking of leaving the polarizing filter on full-time. No indoor shooting is planned, ever. Your thoughts?

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Old Oct 25, 2006, 8:29 PM   #2
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Yes, there will be two "dark" and two "light" spots while turning the polarizer caused by the way the filter works. The "light" spots are where the filter is most aligned with the polarization of the incoming light and the "darK" is when it is at right angles however the light coming out of the filter is still polarized and there is still the overall light loss due to the darkness of the filter. http://clarkvision.com/photoinfo/eva...ers/index.html

There probably isn't much effort made to align the polarizing element and the marking on the filter ring so use it as a guide only.

As for leaving it on all the time, remember you are losing a couple of stops of light with it on. This will affect your shutter speeds if nothing else and make the viewfinder dark.

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Old Oct 26, 2006, 8:21 AM   #3
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If there is absolutely no polarised light around then the filter will block half your light (1 stop) and add absolutely nothing to your picture. You only need to use it when there is polarised light about and you want to reduce it.
Glare, either reflected off of water or from moisture in the air is one case where you really want one. Reflections off of glass are another. Snow too, if you're in a cold place. At all other times take it off. There is always some glare, but it has to be fairly bad to be worse than losing half your light.
If you are using a wide angle you may also find that the sky is uneven as light from the edges (or top and bottom depending on the angle of the filter) is hitting the polariser from different angles.
A circular polariser is a linear polariser with an extra layer. It turns the polarised light waves (sine waves) into something analogous to corkscrews (a corkscrew is circular when viewed end on, hence circular polarisation). It has nothing to do with the shape or use of the filter. You need a circular p. for certain autofocus systems.

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Old Oct 26, 2006, 8:11 PM   #4
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> ...overall light loss due to the darkness of the filter.

> ...then the filter will block half your light.

I won't be leaving it on all the time then. It came (Hoya HMC) in plastic packaging, I could store it there inside the camera bag. A couple of other questions:

Will clouds remove polarized light from the sky? Rain is forecast tomorrow, I guess I'll use the tripod on the porch and the histogram feature to find out. Or the polarized sunglasses.

Thanks guys!

> Glare, either reflected off water or moisture in the air....

Is that why the polarizer turns the sky such a deep blue? Moisture? Just wondering.

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