Digital cameras operate in a different world with respect to color. Photographers can easily modify color in their cameras, or during post processing in their computers, via controlling white balance for scene color accuracy or effect.
Warming, cooling, and filters which convert fluorescent light to look like daylight, may be required for film, but digital cameras can achieve the same effects by their internal manipulation of the image’s digital data.
The use of traditional photography filters for modifying the color of the light, is unnecessary for digital photography, but other filters can work well for digital photography.
I have a UV filter screwed on to the end of every one of my lenses, not particularly to control light, but to protect my lenses from scratches, cracking, salt spray, dust and grime. I’d rather replace a $50 filter than a $1,000 or more lens. While walking near the Eiffel Tower a couple of years ago in a large crowd, my camera was knocked into a metal railing smashing the UV filter. The protected lens suffered no damage.
Be aware that the optical quality of these filters varies greatly. I purchase the higher end professional filters for my lenses. They don’t cost that much more. Inexpensive UV filters oft times cause optical problems for photographers.
Graduated Neutral Density (GND) Filters:
GND filters are used when the dynamic range of a scene exceeds the capability of the camera to record detail in both the brightest and darkest parts of the scene. They are often used in landscapes to allow the photographer to catch the detail in a bright sky while simultaneously catching the detail in the darker foreground. Another use is for sunrise/sunsets where the sky is far brighter than the land below.
They are primarily useful when the transition between light and dark is a straight line, something you often see in landscapes and sunrise/sunset photos. Please note the photo above on the right of a GND filter in a Cokin holder. You can see the transition line between the dark and clear portions of the filter. The entire filter is neutral density as it absorbs light of all wavelengths, thus not altering the color balance of the image.The holder is used to adjust the transition line, up or down, between the clear and dark areas of the filter to match the composition the photographer’s chosen.
These filters come in different stop levels. In other words, the dark portion can be purchased with different capabilities to reduce the amount of light coming through the lens. The graduation or transition between the two parts of the filter can be either “soft” (a gentler somewhat larger transition) or “hard” line (a sudden transition).
Neutral Density (ND) Filters:
ND filters reduce the amount of light equally of all wavelengths passing through the filter throughout the entire filter. These filters are often used so that even in bright light the aperture can be opened wide to reduce the image’s depth of field.
For example, on a bright sunny day an ND filter can be used to help obtain macro photographs of flowers in a garden with a blurry background to enhance the main subject of the image. An ND filter can cut the amount of light going through the lens to the sensor, permitting low f/settings which wouldn’t be otherwise possible. ND filters can be purchased at different stop levels.
Polarizing filters can darken blue skies, increase the contrast of partly reflecting subjects, and reduce reflections from non metallic surfaces like glass and water. If you only have enough cash to buy one filter, make it a polarizing filter.
National Geographic photographers have used the secret of polarizing filters since the magazine began publishing. Did you ever notice that virtually every one of their outdoor photographs have spectacular dark blue skies? You can use a polarizing filter to eliminate display case reflections in museums too.
Please note that all DSLRs will require a circular polarizer. Due to the partly reflecting reflex mirror in DSLRs they need a circular polarizer to avoid metering errors and auto-focus problems which linear polarizers would cause.
Polarization is most effective at 90º to the sun. The subject you are shooting will display maximum polarization at right angles to the sun's position. It the sun right behind you (180º), polarization is almost non-existent.
Please note in the photograph below of the Intermediate Egret, the effect of using a circular polarizing filter. The polarizing filter enabled the camera to pierce through the water to see some of the pond’s bottom, without removing all reflection, giving more depth to the overall photograph.