Circular Polarizers, A Filter You Need

There is the temptation to believe that you don’t need any filters for your digital camera, that you can do it all in Photoshop. But at least one is, in my view, essential for digitals, the circular polarizing filter.
In my film days my camera bag was filled with filters, and filter systems, such as the Cokin one. When you first switch to digital, or for all the photographers who are coming to it without a film history, there is a reasonable tendency to ignore filters. After all, you can do all of it in Photoshop, can’t you? Well, yes, for some filters. For example the camera has built in red, green and blue filters in the Bayer filter, so use of the Channels facility can replace these. But there are a whole variety of filters which are still very useful, indeed essential, to quality digital photography. So over the next few months we are going to work through these filters.

We start our series with the circular polarizer.

Let’s get our paragraph of technical stuff out of the way first, so if you want, skip to the next paragraph. Light has properties of both waves and particles. In the case of polarizing filters it is useful to think of it as a wave. These waves wiggle or vibrate up and down. Light reflected from most normal objects has light rays that are vibrating in all different orientations, some up and down, some left and right and some at all angles in between. Light reflecting off of some objects, like water, however, has all or most or the light rays vibrating in the same way. The light is said to be polarized. A linear polarizing filter can be thought of as being a railing fence that only lets through light rays that are vibrating in the same orientation as the slots in the fence. Rotate the polarizing filter and the light is blocked. Even with unpolarized light with light rays of all orientations, when you use a polarizing filter it will block the light not of the correct orientation, hence the general filter factor of a polarizer and the fact that your exposure needs to change by approximately two stops to compensate. Modern cameras (both film and digital, due to the exposure and autofocus systems) can have issues with linear polarizing filters. So we use a circular polarizer. This comprises two filters, a normal linear polarizer followed by an additional layer that depolarizes the light. Since the polarizing part has done all the work, the result is exactly the same as with a linear polarizer.

A polarizer filter will normally be a little thicker than other types of filter and has one part which screws onto your lens ad another which is rotated to change the plane of polarization that is let through the filter. Circular polarizing filters will normally be a bit more expensive than linear ones. As with all filters you put in front of your nice, expensive camera lenses, it is better to use a good quality polarizer to minimize distortions or aberrations being introduced by the filter.

The parts of a photographic scene whose light may be strongly polarized, and thus be most affected by the use of a polarizing filter, are:

• Reflections off of any non-metallic surface, including water, leaves and glass;
• The blue sky 90 degrees away from the sun.

So depending on the orientation of the polarizer, you can:

• Darken the sky;
• Cut out glare reflections when trying to shoot through a window;
• Reduce or eliminate reflections in a lake or sea;
• By removing all the tiny reflections off of things like leaves and grass, increase the color saturation of a scene.

Because the orientations of individual leaves and grass will all be different, you will get some improvement is color saturation no matter what the rotation of the filter because some reflections will be removed. So some glare is removed always when a polarizer is fitted.

A series of shots. The first has no polarizer fitted to the lens, the rest show the result as the polarizer is rotated by small amounts

The corresponding histograms show that the filter is significantly changing the color distributions within the image

Now, yes, there are plug-ins, actions and people who claim you can do all this in Photoshop. And you can, but there is a cost. The cost is that all the manipulations come at the cost of color resolution. So if you can do this before the camera you can maintain all the color resolution your camera gives you for other purposes, such as opening up your shadows or pulling extra detail out of the highlights.

In the series of images below we rotate the circular polarising filter from minimum effect, through partial to maximum. No adjustments have been made to the images, they are straight from the camera

Just as you use a lenshood on your lens (you do, don’t you?), you should still use one when using a filter, even the circular polarizer. Since your normal lenshood may not allow you access to the filter to turn it, you may need to use a different lenshood, such as those collapsible rubber hoods, screwed onto the filter.

Here we have the polarizer set for minimum effect

Set for maximum effect if pulls back the reflection in the river, even on a day that is quite overcast

A circular polarizing filter can hugely improve some shots. If you haven’t tried one, do yourself a favor and get one. Then use it. Experiment. Shoot images with and without the filter and with the filter rotated to different positions. Play with it, looking at various types of scene through it as you rotate it.