Why do I need a circular polarizer?
As you progress further into photography you might have gotten to a point where you're looking into how filters can improve your images. There are many different filters that can solve different problems. There are Neutral Density (ND) filters that can be employed to slow down your shutter speed. Maybe it’s a bright day and you want some motion blur on a waterfall, you could either use a ND filter or you could just forget the filter and shoot when the light is good. I know there are cases when you have to use an ND, but they can often be avoided by shooting at a different time of day.
There are Graduated Neutral Density (GND) filters which go from clear on one side of the filter and graduate to a ND. This allows you to tame a landscape where the dynamic range between the sky and the ground is too great. It allows you to capture the whole range in a single frame. Here we find ourselves in a position where you can generally reproduce the same effect in post production shooting RAW and just a few clicks with a GND tool. If the dynamic range is even greater, you can pretty quickly blend two different exposures together using layer masks in Photoshop or using HDR. For this reason, I don’t carry GND filters in my bag.
The last common filter you see today are Circular Polarizers. Most SLRs from the 70s on will not meter and AF properly with a Linear Polarizer. In a nutshell, light that’s reflected off of some surface is polarized and this filter has the ability to absorb that light. While its quite technical, you can read more about what exactly polarized light is and how it’s filtered out at the wiki. What we care about is how it affects our images.
What does a CPL do?
The first thing people discover they can do with their CPL is darken the sky to create more impact and make the clouds more dramatic. While this is useful, you have to be exactly 90 degrees from the sun in order for the effect to work evenly across the whole sky. You’ll see instances where only one corner of your sky is darkened when you’re positioned at a different angle. In my work, the composition is everything, so I’m not going to move around to get a weaker composition in order to maximize the polarizing affect on the sky. I prefer to darken the sky in post production by adding black to the blue channel. This trick looks natural and you aren’t plagued by it only working in a single corner. So I don’t usually use a CPL to darken my skies because I can usually do a better job in post.
The next thing people learn they can do is eliminate reflections on water. I shoot a lot of water and the impact of being able to see past the reflection showing the rocks and detail below cannot be replicated in post production. This is a great way to use your CPL and I use it for this all the time.
What you didn’t know a CPL can do
Now, lets talk about something most articles don’t teach you about CPLs. Light reflects off of all sorts of surfaces. Everything from rocks to leaves. If white light is reflecting off of the fall leaves or cobble stones at a beach, the original color of the object is getting drowned out with the reflection. This obviously hurts the color saturation and while you can always try to get that back in post, it doesn’t have the same natural feel of getting the color right off the leaves themselves. So use a CPL for color saturation.
Contrast is also compromised significantly. It’s common for an un-polarized image to have the shadows masked by reflections. This shifts the histogram towards the highlights. So by eliminating the reflections, more shadow detail is allowed to come through. This also relates to color saturation. In order for color to be rich and full of body, you need black. So use a CPL for image contrast.
Get out there and shoot with your polarizer and see how it changes the color and contrast of the image as you rotate it. Some times I’ve found the light was polarized in two different directions. In one CPL position, the reflections were filtered out in one half of the shot and in a second position the other half of the shot was filtered. So in these cases, I will take two shots. One with each half filtered and then manually blend them together using layer masks in Photoshop.
The last thing you can do with your CPL that you never thought of was boosting reflections. Sometimes when I’m shooting a body of water, like a lake, I may not want to see through the surface. Sometimes the water is calm and I want to capture the sky’s reflection on the water. In one CPL position, the reflection gets minimized allowing the light from the bottom of the lake to come through. In the other position, the opposite is true. The light from the bottom of the lake gets filtered which actually enhances the reflection. If the conditions are right, the water’s surface can turn into a perfect mirror which has tremendous impact on the viewer.
What kind of CPL do I use?
I shoot from the Nikon Platform so I have always shot with Nikon Circular Polarizers. Their optical quality is fantastic and can't be beat. The build quality is excellent. The outer element turns very smooth, it's very low profile so its pretty easy to stack with other filters without vignetting and the threads always thread in smoothly and never lock up on me.
I realize that all of you Canon shooters out there would probably never consider putting a Nikon anything on their rigs, so I also recommend B+W and Singh-Ray circular Polarizers. There's also a new up and comer, Breakthrough Photography who's filter products look great.