John A. Lind |
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Yonatan,
The key to using a polarizer most effectively is understanding how light is polarized as it reflects off of non-metallic surfaces. Jeff has explained its major application in outdoor landscapes and the optimal angle to the sun for enhancing blue sky.For other uses, such as eliminating reflection and glare from a smooth, flat water or glass surface, a polarizer has its greatest effect at "Brewster's Angle." This is the angle at which all of the light reflected from a flat surface is polarized in one direction, which allows a polarizer to block all of it as you you rotate the polarizer ring to its optimum position. Brewster's Angle depends on the index of refraction for air compared to the index of refraction for the reflective material. Each type of reflective material has its own Brewster's Angle. For glass, it's about 56 degrees (varies some by glass type). For water it's about 53 degrees. Measure this angle from pointed straight at the surface, or perpendicular to it. As you change the angle from Brewster's Angle, the amount of reflected light that's polarized goes down, reducing the effect a polarizer can have. Pointed straight at the surface (perpendicular to it) it's zero and a polarizer will have no effect at all. What does all this mean for using a polarizer?
When trying to eliminate reflection or glare from a smooth, flat surface of water, glass and most other clear reflective objects, don't aim straight at it. Move to an angle. About 45 degrees is close enough to Brewster's Angle for both water and glass to allow eliminating nearly all reflection and glare. For other materials (plexiglass, Lexan, acrylic, etc.) you may have to adjust the angle some. Start at 45 degrees and move to each side of it slightly looking for improvement. For some subject material such as fountains, waterfalls, choppy water with waves, or shiny leaves of trees and bushes, you cannot eliminate it all since it's not a flat surface. Moving to a different angle may not help, but you can eliminate some glare and reduce the level of bright highlights. In some situations, you might not want to eliminate all the glare from highlights, just some of it, because it would look unnatural without any. You can adjust this by rotating the polarizer ring. BTW, Jeff gave a great tip about using a polarizer as a neutral density filter. Don't forget that one. Comes in handy if you don't need a ND filter often enough to justify buying one. -- John
May 21, 2001
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