What Is a Polarizing Filter Good For?
Think of a polarizing filter as a sieve for light.
Certain wavelenghts are allowed to pass through the filter and others are blocked.
The sieve only has parallel lines, not a crosshatch pattern like something that you'd use to drain pasta.
Waves of light that are parallel to the lines of the sieve pass through, while waves that are perpendicular don't get through.
How It Works
Most polarizing filters that you screw onto the front of your lens contain 2 elements sandwiched together.
One part of the filter screws onto the front of your lens and is fixed in place.
The outside part of the filter can be rotated, which changes the orientation of the parallel filtering lines.
As you rotate the outside of a polarizing filter, you will see the effect that it has in the camera's viewfinder.
While rotating your polarizing filter might be more than enough fun to handle, it's important to know what effect it's going to have on your photos.
Minimize Reflections
So what does this mean for the photos you take with your digital SLR?
A polarizing filter turned to the right angle will block the light entering the lens from a reflective surface.
Water is a good example.
Imagine that you're trying to photograph some fish in a pond.
The angle of sunlight is such that all you get when you take the photo is a bright reflection off the surface of the water.
Add a polarizing filter, and now your camera can "see" through the reflections to the fish below the surface.
This reduction in reflected light also works on windows.
Perhaps your dog has decided to sit in the driver's seat of your car again, and you want to take shot through the windshield.
A polarizing filter reduces the reflection off the windshield's surface so that the dog can be seen in your photo.
Reduce Atmospheric Haze
Particles of dust and water in the air can also cause reflections.
These reflections typically make landscapes appear less clear and colorful as they might appear to your eyes.
Once again, a polarizing filter turned to the right angle will block out these unwanted reflections.
The most obvious side effect is that the color of the sky can be altered from light blue to dark blue.
This look can be overdone.
Landscape photographers rely on polarizing filters primarily to reduce reflections, and sometimes to mildly enhance the color of the sky.
Like any photography technique, the trick is to not get carried away with it.
The Angle of Light
A polarizing filter is not an x-ray device.
When you have the camera pointed directly at a reflective surface (like a window) no amount of polarization is going to reduce the reflections.
Polarizers work the best when they are at an angle to the reflected light.
With a window, you'll want to position the camera at an angle to the window between 35° and 60°.
For landscapes, polarizers have the most effect when you camera is at a 90° angle to the sun.
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