im trying to get my thick skull to understand...

What's the difference between a circular polarizer and a neutral density filter?

To my understanding, a CP is a special piece of glass (similar to high end sun glasses) that cuts down or eliminates reflections and glare. A ND filter is a regular piece of glass that is tinted grey, basically letting in less light, thus, allowing you to use a slower shutter in extreme conditions (broad daylight)...

Am I correct, or mildly retarted?

 

a CP reduces glare on glass, water, or any reflective surface which allows it to cut through. it also adds a slight amount of saturation. most CP are blocking off about 2 fstops of light.

a neutral density filter is basically a grey filter that blocks off a specific amount of light so that you're better able to control the exposure or aperture. for example, if you want a slow shutter speed to shoot a waterfall or river, it's difficult to do so on a sunny day without the use of a ND filter

 

To add to got_rice's post....

ND filters come in a whole bunch of flavors depending on how much light you want to stop. Grades run in 0.1 to 3.0 and higher. You also have graduated. I only have a 3.0 ND filter, so I can only speak toward that one - it reduces light by 10 stops!

You should pick up a CP - it is the one filter, I believe, every photographer must have. A lot of people recommend the slim ones, but I'm a fan of the larger ones. Because you have to twist/turn the CP filter, you're going to get your grubby hands on it, so get the one with the most lip to reduce how often your grime gets on the glass. They're not fun to clean.

 

I have a CP that fits all my lenses....I guess I should get a set of NDs then for the trip for running water purposes.

 

its not only running water, its good for taking portraits in daylight if you want to use a larger aperture to blur the background.

 

I knew about ND... but for some reason couldn't wrap my mind around the concept well enough to know why I need one.

 

Couple more things about CP's.

Since they block a certain direction of lightwave (definition of polarized) they will darken a sky differently depending on your angle to the sun. That can make CP's a difficult choice to add on a wide-angle shot with sky in it.

And they do nothing special to light reflected from metal surfaces. There is no additional polarity to that light compared to water or glass.

Also, since you're spending the money on good glass don't go too cheap on the filters. The optical chain is only as good as the weakest link and if you throw a $10 CP filter on it you might as well be using crap lenses too. There is room for debate on exactly how much you need to spend for quality but I'll go our on a limb here and say your CP should cost at least $85 for, say, a 62 mm size.

 

slim CPs are usually recommended for wide angle lenses to minimize vignetting. there's no real need for a slim filter on a telephoto lens.

I use a slim CP on the 16 35 and to be honest, it's a pain when I'm not using the camera. the 'cap' that B+W includes doesn't stay on it very well, so I just leave it off and hope the lens hood protects it