To All The question of Circular vs. Linear Polarizers has come up again, so I thought that I would repost this explanation. To those who have seen it, I apologize, to those who haven't I hope it helps. Terry ********************************************************************** Light always has a polarization direction, crossways to the direction it's travelling. You can think of it as being like arrows flying sideways, where the tip of each arrow is pointing in the polarization direction. (Actually, for this discussion, the arrows have tips on both ends.) For most light, the polarization direction is constantly changing in a random way, and we call it unpolarized light. When more of the arrows point one way than another, we call the light polarized. The _strength_ of polarization can vary - maybe there is only a slight excess of arrows pointing in some one direction, or maybe all of them point the same. Most of the things you take pictures of are emitting (or reflecting, or scattering) _unpolarized_ light towards the camera. But some things reflect preferentially at some polarization direction. Light from these subjects may be partially or completely polarized, and the amount of polarization may depend on the positions of camera and light source relative to the surface of the subject. Reflections from water, glass, and many other surfaces are partially polarized. Skylight is polarized, with varying strengths depending on angle from the Sun. A linear polarizer allows light that's polarized in its characteristic direction to pass through. Light that's polarized crossways to the polarizer's direction is absorbed. And light that's polarized in between is partially passed and partially absorbed. In this case, the part that passes through is weaker, but is now polarized in the polarizer's direction. Linear polarizers are used to eliminate the parts of the light that are polarized in some direction. So you can, for example, eliminate the reflections from a pool of water, or darken the sky, by using a polarizer. So where do circular polarizers come in? Modern SLRs, by and large, use components that are sensitive to polarization in both the light metering system and the autofocus system. So if you're using a polarizer to modify the photograph, you may wind up confusing the camera. This is particularly important in metering, since the strength of light reaching the film may be quite different than that reaching the meter. Autofocus systems will usually continue to work, but perhaps not at as low a light level as they otherwise would. The solution is to use a circular polarizer. This is a sandwich consisting of first, a linear polarizer, and second, a quarter-wave plate. The linear polarizer does all the good stuff you want a polarizer for in the first place, but the light coming out of it is polarized. This doesn't matter to the _film_, but will confuse the camera body electronics, as explained above. The 1/4 wave plate solves this problem. It is able to take those arrows that are coming out of the linear polarizer, and start them spinning, so their tips move in circles, like an airplane propeller. That way, the camera's internal systems get light that, _on average_, is polarized in every direction equally, and for the camera's purpose this is equivalent to unpolarized light. Circular polarizers are more expensive than linear ones because they have to include both the linear polarizer and the quarter-wave plate. To tell whether a particular polarizer is linear or circular, you can use any source of polarized or partially polarized light. If you don't have one, try using a reflection from a glass window at about a 45 degree angle. Look through the polarizer at the source. Orient the polarizer so your eye is on the same side as the camera lens would be. Rotate the polarizer, and you should see the reflection darken and lighten as you turn it. Now turn the polarizer around so that you're looking through it in the direction OPPOSITE to the way the camera lens would look through. Rotate it again, watching the reflection. If the reflection still lightens and darkens, you have a linear polarizer. If not, you have a circular polarizer. -- Terry Graham - Freelance Photographer & Windsurfer Graham Fine Art Photography mailto:[EMAIL PROTECTED] -- Terry Graham - Freelance Photographer & Windsurfer Graham Fine Art Photography mailto:[EMAIL PROTECTED]
