Originally posted by autumncat:
... Just trying to understand better - a circular filter will be different on one portion of the filter so that you need to spin it to effect different parts of the landscape? |
Not quite. A polarizer is the same all over, there is no one portion of the filter that polarizes more than any other portion. Spinning a polarizer has an effect on light because light particles are not all facing the same direction. Think of a childs toy top, spinning as it moves about; particles of light have a spin as well, but unlike several tops spinning together light may not all be 'standing' in the same direction. That is, their axis of rotation can be facing in any direction. A polarizer takes advantage of this by having many little 'bars' running thru it, much like the slats of a venetian (sp?) blind. These slats are very small and allow only those light particles that are 'standing' in the right direction. I always think of the old slap-stick TV shows with someone trying to take a long pole thru a door sideways -- it must be stood on end before it can pass thru the door.
Light, when bounced off of something becomes polarized to that object -- it doesn't matter if that object is glass, water, leaves or the blue sky. By turning the polarizer, the bars are turned -- say from verticle to horizontal -- which allows differently polarized light to come thru. When the polarizer is turned to refuse entry to those light particles refected from some object (such as glass) that light is not allowed in (thus no reflections!). The side effect of this is that it limits how much light is allowed in -- so, depending on the rotation of the polarizer, may reject anywhere from 1 to 3 stops of light.
Originally posted by autumncat:
Should I be able to see a significant (if any) difference through the viewfinder? |
Depends on the situation; if there is a significant amount of light with the same polarization then yes, since turning the polarizer would increase or decrease the contribution of that light. If, as is under many circumstances, the light is random, turning the polarizer will have no noticable effect since the same amount of light is coming thru regardless of how it is turned (just with a different spin).
Leaving a polarizer on all the time is probably not a good idea -- at best it adds another step in the composition of the shot (you must turn the polarizer to see if it's causing an effect or not) and also reduces the light available (which may not be a bad thing).
Probably over-simplified, but this is my understanding of it -- hope it helps. There are many articles online that go into much more detail (and even have pictures), if you need them. I believe some were linked earlier in the thread.
David
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