Of all the filters available, a polarizing filter is the first one you should buy to improve your landscape photography.
So What Does It Do?
The polarizing filter one thing and one thing well - it removes reflections from non-metallic surfaces:
By eliminating the reflection of light on the tiny water droplets present in the atmosphere the polarizer brings more saturated and slightly darker skies.
Water and reflective surfaces
By eliminating reflections, the polarizing filter will tend to make water and other reflective surfaces more transparent. The effect will also vary depending on the angle to the reflective surface. If you place your camera very low above a river, the effect will be very limited. If you shoot from a bridge above it, the water will look totally transparent.
The polarizing filter also reduces reflection from other surfaces such as foliage and thus makes their colors look more intense. It also tends to make shadow areas darker.
The main side-effect of the polarizing filter is that it absorbs 1.5 stop of light. So, if you shoot at 1/180th of a second and then add the filter, you will find yourself at 1/60th of a second. This happens whatever the orientation for a polarizer. In low light situations, some type of camera support, such as a tripod, will be needed.
Are There Any Pitfalls To Using One?
|I used a polarizer for this photo of sunrise from Mt. Sinai. Note the darker tone of the sky in the upper left corner which was about 40-45 degrees from the sunrise (just out of frame on the right).
Natural polarisation is uneven across the sky, with the most extreme effect when you're at 90 degrees to the sun. Using a polarizing filter on lenses wider than 24mm will make some areas of the sky much darker than others. Some pundits suggest not using a polarizer with ultra-wide lenses. No way. If you like the effect, go ahead and use it. Over-polarisation can also be a problem, particularly if you're shooting at high altitudes, or even at sea level if the sky is clear. This can turn the sky almost black - not a good look for many scenes.
Polarization is most effective at 90 degrees to the sun. That means that the subject that you are shooting will display maximum polarization at right angles to the sun's position. With the sun right behind you (180 degrees), polarization is almost non-existent.
An old trick for visualizing the maximum angle is to turn your index finger into a gun (like when you were a child), with your thumb pointing upward. Make as if to shoot the sun with your finger and your thumb will point toward where polarization is at its most extreme. Remember though that this isn't just at one angle. Rotate your wrist through 180 degrees (if you can), because the entire circle around the sun is equally polarized.
Polarization filters are, by thir nature, quite thick in comparison to other screw-on filters (after all, there are two pieces of glass in a polarizer). When used with a very wide-angle lens (e.g. a 24mm), the edges of the filter can actually block some of the incoming light and cause vignetting - a darkening in the corners of the photo.
To combat this, if you use wide-angle lenses a lot, you can buy a "thin" polarizer. These are made by B+W and Heliopan, among others. The downside is that they are even more expensive than their thicker counterparts.
Digital cameras give you the opportunity to examine your results where you stand, so if the polarization effect is too strong or weak you can always adjust the filter and take another photo. When you're looking through the viewfinder, actually look at all parts of the scene (in detail) and see how the polarization filter setting is changing how it looks. Don't just look at the "big picture".
If you take panoramic photos (i.e. several photos stitched together), then don't use a polarizing filter. Each photo will be unevenly polarized and the skies in each will be imopssible to match up.
How To Use A Polarizer
Before you start using your polarizing filter you need to check whether or not the front element of your lens moves when it focuses. If it doesn't you're in luck, as you can rotate the filter at any time and the effect will stay the same if you need to refocus. Many cheaper lenses aren't so forgiving though, and using a polarizing filter on a lens with a rotating front element takes a little more forethought. You'll need to set the focus before you start moving the filter - switching to manual focus can make it easier to keep the filter in the same position.
Whichever type of lens you have, you'll need to get used to seeing the effect of the filter as you change the orientation. The viewfinder can get dark with the filter in place making it hard to see the effect. You'll find it easier to see if you move the filter slowly and give your eye a chance to see the changes in the image. Look out for an increase in the contrast between the blue sky and clouds, and reflections on water disappearing.
When it comes to determining the exposure you can use the built-in metering with the filter in place.
Choosing Your Polarizing Filter
There are two types of filter to consider when choosing polarizers. The first are screw-in filters. These are ideal if you only use one lens or if all your lenses have the same filter thread. Hoya does a wide range of sizes at a reasonable cost.
The other option is to buy a polarizer to fit one of the square filter systems. For small diameter lenses the Cokin 'A' or 'P' ranges are ideal. For large diameter wide-angle lenses, or medium-format cameras, the Lee filter system is so adaptable it will fit almost any lens, although the polarizing filters for Lee's 100mm system are on the pricey side.
How Does A Polarizing Filter Work?
It might help to think of it like those children's toys where different shaped blocks have to go in the correct holes. Polarizing filters only allow certain light waves to go through the lens, while the other waves just won't get through. By rotating the filter, different ranges of waves are let through.
If you have an old manual camera you can often get better results by using a linear polarizing filter. However, these filters will confuse the TTL metering, autofocus and white balance on modern cameras. For this reason you'll need to use a circular polarizing filter on all automatic cameras.
The reason for this is because semi-silvered mirrors are used to siphon off some of the light coming though the lens. If that light is linearly polarized it renders either the metering or the autofocus ineffective. This means that you're going to have to buy circular polarizers unless you're shooting with a pre-1970's camera, or a view camera.
If you'd like a more in-depth explanation of what polarization is, pop over to this informative site. It has a Java applet that mimics a polarization filter. The page may take a bit of time to load, so be patient.