Degree of polarization?

I am on my way to getting a CPL and I've read a bit about DOP (degree of polarization) but I don't understand how one sets this? The CPL I am getting is a circle (duh)? So does it spin?

I know, dumb question but I'm a filter newb!

Yes, it does.

Thanks Jon

So how do I know how to spin it, will I see the difference through the view finder. How is the glass different in different parts?

Yes, you'll see the change as you rotate it. The glass isn't "different in different parts". Have you ever worn polarized sunglasses? It works just like they do.

Thanks again, I have worn polarized sunglasses but I just don't understand how spinning does anything (unless the glass is somehow different) or is it the distance from the lens that is changing?

Polarized glass, whether in sunglasses or a filter only passes light "vibrating" in one direction. As you rotate the filter, the direction of the vibration in the light that's passed will change. So you'll be admitting "different" light as you rotate it.
http://en.wikipedia.or​g/wiki/Polarization

Ah, I see. Thanks Jon!

I'll keep reading as well

Merlin, I too have a CPL and have enjoyed it's use for a number of years with very pleasing results. Jon has done an excellent job of explaining the science in a manner that I finally understood what was actually happening - thanks Jon .

Basically, once you thread the CPL onto your lens you will simply point the lens at your subject and slowly rotate the outer ring of the CPL as it is designed to do. If it's a sunny day and your FOV includes sky and clouds, you will see the sky visibly lighten and darken as you rotate the CPL. When the sky is at it's darkest blue the clouds will appear their fluffiest white and appear to almost pop out of the scene. And all of this will be at it's most dramatic when you are at right angles to the sun.

If you are shooting into the sun the beneficial/aesthetic effect will be minimal, but as you turn your body to a greater angle to it the sky will visibly darken, glare will reduced, color saturation will increase, overall picture quality will improve. Again though, you will need to be rotating the CPL to achieve the best effect. It will be immediately seen through your viewfinder.

I haven't heard of degrees of polarization, but I can tell you that in a polarizer, the lens crystals are aligned all in one dirrection. At zero degrees, they are horizontal. As you rotate the filter from zero to 90 degrees, you will see the image darken more and more to its darkest at 90 degrees.

What you're doing is getting rid of reflected, glare light, which comes off objects as mainly horizontal rays. At 90 degrees, the crystals in the filter are aligned completely veritcally, to remove all the horizontal rays.

On the brightest days, with the sun highest, the 90 degree point may be what you want...sky will be deep blue, there will be NO reflected light, but everything may actually look a little dull, lifeless, and though colors will be saturated, a bit dark.

Therefore, you can back off on the polarization by rotating the filter back to 85 degrees, 75, degrees, etc., until you get the life you want back in the photo, even if there is some mild reflection. That's how I would interpret "degrees of polarization."

You can find zero degrees by aiming at an area of reflected light, water, auto rear window, and rotating the filter until your viewfinder shows everything at it's brightest. 90 degrees from that point, everything will be at its darkest.

The idea is to use the filter when the sun is at 90 degrees from where you're shooting from, high noon, for example. I'm sure you know "Understanding Exposure," where there are more tips on using a CPL filter.

Great info sapearl and tvphotog! I am surprised I guessed that a CPL spins (just out of the blue, shows how much of a filter newb I am). Make sense to me. I am going to get a B+W one in 72mm size (use now on my 28-135) and later on my not-yet-acquired 15-85. Is there a reasonable way to buy less filters than lenses (for lenses with the same size filter)?

Thanks to all of you!

Well, you don't want a polarizer on your lens all the time - it costs you a couple of stops of light - so unless, like me, you don't like swapping filters every time you swap lenses you only need one filter for each lens size (or fewer if you don't mind buying the biggest filter you'll need and using step-up rings to let your smaller lenses fit it).

Ah, your last statement was kind of what I was thinking. So should I buy a 77mm and a 72mm step up? Does that make more financial sense in the long run?

Sorry if I'm being pedantic, but don't forget that even with the sun lower, say at 3PM, if you're still shooting at 90 degrees to where the sun is ( halfway between directly behind you and directly in front of you,) you still may want to try the CPL.

merlin2375 wrote in post #10751104
Ah, your last statement was kind of what I was thinking. So should I buy a 77mm and a 72mm step up? Does that make more financial sense in the long run?

Only if you're sure you're going to get a 77 mm diameter lens fairly soon.

do step down rings prevent you from using the lens hood?


J.