micro Loc: Texas

Yes, thanks to UHH I know the reason for needing Circular Polarizers for use with modern Auto Focus DLSR cameras.

But, I just went through my old SLR equipment bag and found one of my old Linear Polarized filters (circa 1975). I was amazed at how much more effective the old linerar glass was compared to the CP (just hand holding the filter and rotating).

Does anyone know if this is a restriction of CPs, or did I just buy inferior glass?

MT Shooter Loc: Montana

No, the Linear Polarizers do have a much stronger effect, thats why I still have and use one. But they are incompatible with AF systems and will also give false information to many DSLR metering systems. (The problem has NOTHING to do with digital sensors as some may try to lead you to believe.) You can still use them in manual mode with manual focus and enjoy their deeper effects.

snowbear

It has to do with the method that the AF systems work. Newer cameras have semi-silvered (partially transparent) mirrors and the AF grabs it's light through these. The linear polarizers are more effective, so they tend to block so much light it messes up the AF. This is the same reason a lot of AF systems don't want to work in low light conditions and can "hunt" before they zero in.

edit: MT Shooter posted while I was typing - I need faster fingers!

micro Loc: Texas

Thanks MT & Snowbear.
It's a little disappointing, compared to the dramatic effects of the older filters. I guess, not everything new is better!

fcrawley Loc: Melbourne, FL

The reason that the linears appear stronger to the eye are that they actually pass planar light. The circular ones have two filters in each: Front rotating one is a linear just like the old days and then behind it is a diffuser to convert the planar light back to random so that it will pass through the mirror. Remeber that any glass surface itself is a polarizer, so it will reflect any light in a certain plane, hence it will not pass certain planes of light so in order for the AF sensor to be accurate you must use random light instead of planar.