“Magic” filters for your digital camera

There is a special lens filter that can do wonderful things for your digital color photography. It’s called a polarizing filter and it is placed over the camera lens. It can do things that post-processing computer photo-editing software isn’t able to do.

Here are some of the filter’s “tricks”:

How does it work? A polarizing filter transmits light in only one plane. If you put two polarizing filters together, adjusted to two opposing plane directions, no light will pass through them. Together, the two will just look like a black disk.

Since light is normally bouncing around in all directions, the only way to see the effect of a polarizing filter on your photograph is to place it over the camera lens and view the results on a digital camera’s LCD monitor, look at the scene using a camera’s through-the-lens electronic viewfinder, or through a single-lens-reflex camera’s viewfinder. (You can see the polarizing effect by looking through the filter, but it may not be the same view from the camera.)

This is important because you have to rotate a polarizing filter as you aim the camera at a particular scene in order to see the maximum effect on the camera monitor or viewfinder. You can’t use the filter in the same position when you change locations and the light is different. Usually the effect is dramatic as the sky turns dark blue and white clouds stand out with an almost three-dimensional quality.
Although we don’t notice it, water vapor in the atmosphere greatly diffuses light. The polarizing filter only transmits light in a single plane, almost as if it were looking at a scene through Venetian blinds. This is what makes the difference — producing dark blue skies and eliminating the haze reflected from water vapor.

When it comes to glass reflections — for example in a storefront window, train or bus window, or museum display case — the filter reduces or eliminates the light bouncing off the surface of the glass.

Polarizing filters are usually manufactured in screw mounts with a filter that can be independently rotated. They are ideally used with cameras that accept screw-mount filters at the lens. Most high-end digital cameras, including SLRs and zoom-lens-reflex cameras have thread lens mounts. Many, but not all, point-and-shoot digital cameras have lens-mount threads.

Filters and lens mounts are identified by the filter diameter in metric millimeters. The symbol for the filter size is the Greek letter “phi” (ø), which looks like a small letter “o” with a slash mark through it. Look at your camera lens for such a mark and you will discover your filter size. For example, a lens mark may indicate “ø 49mm.” (If you don’t have filter threads on your lens, you can still use a filter by holding it over the lens, or taping the filter to the body of your camera.)

Depending upon the size of your camera lens, you should be able to purchase a polarizing filter for about $20 to $30 at your photo specialty store, or by mail order from an Internet photo accessory supplier, such as www.porters.com. Large-diameter polarizing filters for SLR lenses are more expensive.
Since we’re on the subject of filters, there are many other types:

There are many more filters for film cameras, but a great advantage of digital cameras is that photographers can adjust color tones through the white-balance setting on their digital camera and in the computer photo-editing program, making these color-correction filters unnecessary.