For general photography in digital or on film there are several filters that are most frequently used to improve the rendition or alter the possible range of exposure values.
Make colors snap…
If I had to have only one filter it would be a circular polarizer. I use the polarizer on most of my nature photographs.
The only time that I do not use it is when I am photographing rainbows or handheld in low light. When photographing reflections the filter does not need to be removed just rotated to the correct orientation so that you get the desired degree of reflection. This can be useful in controlling the amounts of reflection from bodies of water as some reflection is nice but the natural amount is usually too much.
The polarizer will darken the sky a variable degree depending on its setting. Plants will be rendered with richer colors because the polarizer cuts the specular reflections off of the moisture in the leaves. Most colors will benefit from the polarizer’s effect.
This is different from turning on the vivid switch in your camera. In-camera processing is always a bad idea as it is better to enhance color saturation and sharpness interactively in post processing.
The exposure compensation for a polarizing filter is 1 2/3 stops. This has been standard across all the polarizers I have owned over the years.
The second filter I carry is an ND or neutral density filter. ND filters are marked with the opacity value as if they were being read on a transmission densitometer. .10 is 1/3 of a stop, .30 is one stop and .90 is three stops. I like the ND .90 as it gets me into a different range of aperture and shutter speed combinations.
Making the water move…
If I have two filters the second is a neutral density filter. At EI 100 the BDE, basic daylight exposure, for direct sun is 1/100 at f16. As many lenses only support f22 as the maximum aperture this makes the slowest shutter speed available 1/50 (at f22).
Good movement in flowing water happens at shutter speeds between 1/15th and 1 second. 1/15th gives you lots of texture and good movement. As you head towards 1 second the water becomes less detailed and more unsubstantial. At about 5s the water seems to be simply a cloud in the channel. With the preceding assumptions you will see that using a polarizer with its 1 2/3 stop compensation factor will give an exposure of 1/30 at f16 or 1/15 at f22. You are exactly at the top of the range of good motion effects.
Using the ND .90 and the polarizer I get an exposure value of about 1⁄4 at f16 or 1⁄2 at f22. This filter allows me to really hit that window of good motion that happens between 1 second and 1/15th second and to have some choices as to how I want to render the water rather then just being stuck at 1/15th at f22 with the polarizer.
I find that the ND filters will change the white balance of the picture slightly and I have a custom white balance that I use for the filter. This amount of color shift is certainly not much and can be easily corrected in post processing.
When you use more then one filter the old wives tale is that your focus will suffer a very small amount. I do not feel this is even noticeable and will not hesitate to use the second filter, especially as you cannot get to this range of exposure values without it....
The third filter is the UV0 or 1. I use these to protect an exposed front lens element. As I rarely remove the Polarizer this is only for that time. A good example is photographing in low light where the polarizers density makes it too hard to see through the camera and there is not a direct light source to produce specular reflections.
It is critical that whatever filter you are using that you use a lens shade. This prevents lens flare, which robs your pictures of contrast and color saturation. Unlike the difference between 1 and 2 filters, which I cannot identify in a photograph, I can usually tell you weather your picture was shot with or without a lens shade.
I use a lens shade and my fedora type hat, which I hold over the lens to cut that last bit of flare when looking into the light. If you look at most pictures of Ansel Adams working you will not that he is wearing a fedora, there is a reason for this…
When the wind conditions are to high for me to wear the fedora I carry an 8x10 piece of black mat board that I use as the additional shade. I cannot stress too much the importance of a good lens shade and making the extra effort to do some additional shading. You can usually see the effects of the additional shading in the finder because it really makes a big difference.
Watch The Quicktime Video - Part 1 Filters for Digital
Watch The Quicktime Video - Part 2 Filters for Black and White