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Why Autofocus Needs Circular Polarizing Filters

Why do autofocus SLRs require a circular polarizing filter? As I understand, linear polarizers cause the image to vignette. This property has nothing to do with the mode of focus. Manual or auto, if the lens is going to vignette, it will vignette, regardless of how it is focused. Thank you for your time.

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John A. Lind
BetterPhoto Member Since: 9/27/2001
  Circular versus linear polarizers have nothing to do with vignetting, which is an entirely different issue with using filters in general. Light, unless reflected off of a smooth, non-conductive surface (water, glass, etc.) is randomly polarized. The problem with using a linear polarizer on AF and some TTL AE SLR systems is these camera systems use a "beam splitting" mirror that reflects most of the light up into the prism (and to the viewfinder) while allowing some of the light to pass through to the AF and AE sensors behind the mirror. The "beam splitting" mirror works like a linear polarizer, with light in one polarization being allowed to pass through while the rest is reflected. If you look through two linear polarizers (stacked) and rotate one of them, you can effectively block out *all* light. This is the danger of using a linear polarizer on a system with a "beam splitter" for its AF and AE. If the linear polarizer is rotated into certain positions, the AF and AE sensors behind the mirror receive no light, which prevents the AF from working and throws the AE off.
A circular polarizer *is* a linear polarizer with a "quarter wave plate" behind it (usually cemented to the polarizer so it looks like one piece of glass). The quarter wave plate takes the linearly polarized light passing through the linear filter and gives it a circular polarization. To another linear polarizer (in this case the beam splitter) circularly polarized light appears to be the same as randomly polarized light. Regardless of how you rotate the circular polarizer, some light will always pass through the beam splitter allowing the AF and AE sensors to work correctly. The vignetting you spoke about is caused by the filter ring, not the filter, and is a result of stacking too much filter (or too wide a ring) onto a lens, especially if you stack on top of a lens hood (you should always stack the hood on top of the filter). Usually it is not a problem with a telephoto or standard lens, but can easily be a problem with a wide-angle lens. Try to keep yourself to no more than one filter plus the hood on top of a lens. If you must use more than one filter, then use thin filter rings and check through the viewfinder to ensure you are not vignetting the image.

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