Quote:
Originally Posted by OsmosisStudios
If you got a LINEAR polarizer (the front doesnt spin)
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The front of a linear polarizer spins the same as on a circular polarizer. The difference between the two is that in a circular polarizer there is a quarter-wave plate to depolarize the light that comes out from the filter.
"Useless" is a strong word, and the distinction isn't "digital", it's "autofocus SLR".
In SLRs and DSLRs, the (pretty much universal) approach to autofocus involves making the viewing mirror be half-silvered, or what is technically called a beam-splitter. Part of the light coming in from the lens is reflected up to the focus screen, while the rest passes through and is sent to the AF detectors.
In SLRs and DSLRs, the (pretty much universal) approach to in-camera metering is to measure the brightness of the image that is projected onto the focus screen. For autofocus SLRs and DSLRs, this requires that the percentage of light reflected by the viewing mirror be constant and known. For Canon SLRs and DSLRs, 60% goes to the viewscreen and 40% to the AF detectors.
Beam-splitters reflect a different percentage of the incoming light depending on the polarization of that light. With a linear polarizer, the light reaching the beam-splitter (viewing mirror) is still polarized, and the direction of the polarization depends on how you have the filter adjusted.
So you see the problem: adjusting the linear polarizing filter will change the percentage of light reflected to the viewscreen, which in turn will affect the in-camera metering.
But! The difference in the percentage that gets reflected is fairly low for beam-splitters built using an aluminum reflective coating, which is pretty much every SLR or DSLR beam-splitter ever made. It's probably less than 1/3 stop in most cases. Most people will never notice this difference.
So: for autofocus SLRs and DSLRs, the circular polarizer is in theory the correct choice. However, in practice a linear polarizer will probably work just fine. I've seen many postings from people using linear polarizers on their autofocus DSLRs, and none have reported any noticeable problems.