Craig Zendel wrote:
> Mike, > > Sorry, have to disagree with you. > > A circular polarizer does not "de-polarize", it rearranges the orientation > of the light waves. As these have already passed through the linear > polarizer they are all travelling in the same plane. When crossed, the > circular polarizer will allow through those waves at the same orientation. > Thus, the circular polarizer will pass some previously polarized light, > potentially effecting the metering. > > Try it. > > Regards, > > Craig Z Craig, Yes, it will pass some of the previously polarized light, but not in such a way as to "fool" the metering system as you seem to suggest. The metering system will show the proper amount of light entering the camera by metering the same percentage of that light it always does thus giving the proper exposure recommendation. How much this light is attenuated by the polarizing sheet is another story related only to the relative positions of the polarizing sheets. But once the light passes through the quarter wave plate, it is effectively no longer polarized (approximately equal portions are vibrating perpendicular to each other). The quarter wave plate (the additional component that makes a circular polarizer "circular") rotates a portion of the wave (approx 50%) so that they are out of phase from the light directed into the polrazer by a quarter of the wave length...essentially, the light that has passed through the polarizing sheet at front of the polarizer will be in a vertically vibrating plane (if the polarizer is oriented that direction), then it passed through the quarter wave plate, half of it will be vibrating vertically, and half will be vibrating horizontally. (Actually, this is too great a simplification, because the light that has passed through the polarizing sheet is not all vibrating in a single plane, but its amplitude is greater the closer it is to the orientation of the polaroid sheet). In any case, when light passes through the filter, it will be attenuated in relation to how closely it aligns with the polarizing sheet (for non polarized light, this is approx 50%, but for polarized light it would be between slightly less than all of the light or slightly greater than none of the light depending on the orientation of the filter in relation to the plane of polarization of the light hitting the filter), but then a portion of that now polarized and parallel vibrating light will be rotated 90 degrees out of phase with the rest. Then that light will hit the mirror in the camera and part of the light (the vertically polarized light, I believe) Will be reflected up through the eyepiece and to the metering system, while the horizontally oriented portion of the light (equal to the vertically oriented portion) will pass through the main mirror and hit the second mirror where it will be reflected down to the AF system. The fact that the light is already polarized in one orientation when it hits the circular polarizer makes no difference what *percentage* goes up to the metering system and how much goes down to the AF system--the percentage split will be the same as if no polarizers were involved (and the light entering the camera were not polarized either). In fact, if what you suggest were the case, the Circular Polarizer would not work at all for cameras that require them in the natural scenes where it is supposed to (because light from the area of the sky that darkens and light from reflections from non metallic objects is *already* polarized, just as if it had passed through a polaroid sheet). But of course, the total amount of light that will pass through the circular polarizer does depend on whether the source light is polarized. If the polarizer is oriented parallel to the already polarized light, most of it makes it through the filter, and if it is perpendicular, little makes it through. That is why we find a polarizer useful. Sorry to be so long, but the requirement that the light that has passed through the polarizer be "de-polarized" before it passes into the camera so that it can properly reflect off the semi-silvered mirror is what I was using that word to refer to in my original post. "De-polarizing" does not mean that it will somehow put back the light that was absorbed/reflected by the polarizing sheet and make the scene look like it did before passing through the polarizer if that is how you interpreted it. By the way, I did try it, by comparing a camera that can be fooled by a linear polarizer and one that cannot (and older camera without a semi-silvered mirror) and I am correct. Both cameras showed the same 5 stop difference through cross polarization indicating no metering error due to the cross polarization in the newer one. Mike > > Craig, > > > > As long as he has a Circular polarizer on the lens, the linear polarizer > > on the light table will not cause a problem. The Circular pol. > > effectively "de-polarizes" the light that reaches the metering system, > > eye piece and AF sensor. > > > > But you are right, two circular polarizers, if they are both orented > > with the quarter wave plate toward the camera will not work for > > photographing stresses in plastics. > > > > Mike -- Michael Shupe M.J.Shupe Photography Michigan Tech University www.northernlightsgallery.com * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
