AnnieBananie wrote in post #12189107
...but I wanted to see exactly what that filter does, and here's the before and after... is there really much difference between this and a CPL?
Annie,
A Neutral Density filter's only purpose it to cut the amount of light coming into the camera. A PERFECT ND Filter would have no hue tint, no quality degradation, no chromatic aberration, no fringing, etc.. So, two pictures of static objects (without any motion like wind, or water, or clouds), one with and one without an ND filter, would look EXACTLY THE SAME. NDs are not perfect, but the quality degradation on the good ones is almost non-existent.
What are they used for? They have several key uses.
The first is to allow much slower shutter speeds for subjects where you want to emphasize motion, like a Waterfall. Lets say you aim at a waterfall during the afternoon and you get a 1/250th f8 ISO100 reading. You know that at 1/250th, water instead of looking smooth will look edgy and you don't want that. What are your choices to get slower shutter speeds? Closing the aperture is one way to go, but if you go further than f8, you risk degrading the IQ due to diffraction. You can lower the ISO, but you cannot use anything lower than ISO100. At this point, you have no option but to cut the light coming into THE LENS. An ND 8x (3 fstop) filter would allow you to shoot at 1/30th, allowing a much nicer silky look. An ND 64x (6 stop) would require a 0.4" of exposure giving you the perfect silky look.
There are other interesting uses. Lets say you are taking a nice portrait of a bride outside. Its bright daylight! Of course, you want shallow DoF so you go for your 50mm f1.8 and set it at f1.8 (this is just a 'for example' case, I wouldn't suggest f1.8 for that 
). You set it to highlight tone priority, so you don't blow the highlights so you have ISO200 available. Now, you take a light reading and your camera tells you 1/4000th is still not fast enough and you are still a couple of fstops overexposed (assume you are shooting in the surface of Mercury or on the Sun's own surface here ). You could, of course, close the aperture, but then you lose the DoF advantage! Once again, the only choice is to use an ND filter to allow for a 1/3200th and a correct exposure.
So, we usually have 3 ways of reaching an exposure. With Aperture, with Shutter Speed or with ISO. If one of them reaches its limit or your target mark (a specific f aperture or shutter speed) you still have the other two to play with. If the a second one reaches its limit, you are left to play with the third one. If that one reaches its limit, you need to resort to other means. When there is too much light, ND filters are one of the solutions, just like a Flash is the solution when there is not enough light.
So, you cannot compare images side by side, cause it will do no good. You need to compare them with the settings used. So, your first image probably uses a 1/60th or more, your second image probably got a 1/15th or less... I don't know.
To top all this, you have a GRADUATED ND filter... this follow the same idea but they have a different graduation on one side of the filter and, usually, no graduation on the other end. This allows you to filter light differently in one part of the frame than in the other. Their use is mostly for landscapes where you want the sky properly exposed AND the ground properly exposed as well.
So, I agree with Marubozo as to the fact that you cannot compare a CPL filter with an ND filter, they are not meant for similar results, and if you are not on those 'extreme' examples I pointed out earlier, the images between NDd image and noNDd image are going to be identical.
Hope this is more or less helpful? Also, nice to hear you are feeling better.
Rafa.
