A CPL filter has two filters. The first is a rotatable linear polarized filter to block light entering which is at 90 degrees from the alignment of the linear polarizer plane. The second filter is a Quarter wave plate which retards/changes the polarized light into unpolarized light so that auto focus and metering can function properly. The film surface or digital sensor wouldn't know the difference. Don't ask me how quarter wave plates work. I just viewed several videos explaining the physics and my head exploded.

The purpose of a CPL is to give you control over strong reflected light entering the lens because reflected light is highly polarized. Think sun reflecting off a pond or off a glass window.

I decided to post these images because of recent discussions about whether or not people should use 2x extenders. I know these are birds, but putting them in the bird subsection will prevent the converter discussion.
The photos are taken with a Nikon 400f2.8s lens. The built in 1.4 converter is being used and coupled with a 2x converter. The wren also has a 5mm extension tube for closer focus.
So, the question is are these photos sharp enough, or should people not use the converters and attempt to gain the extra reach by cropping. I believe that any loss of image quality from using the converter is more than offset by the additional reach and the ability to come closer to filling the frame, but I thought it would be interesting to see where any discussion would go. Those of you who believe that no one should use teleconverters, go ahead and have your say. The photos will be posted in the first reply.