Using Circular Polarizing Filters for Digital Infrared Photography

We don’t normally think of using polarizing filters when shooting infrared but I decided to try it out.
Regular readers will know that I shoot a lot of digital infrared work, both with a converted for IR Canon 350D and with unconverted digital cameras that I have in for testing. I was testing circular polarizing filters for another article and I became curious to see if they would have any effect in infrared. So I went out with my converted 350D and a Hoya circular polarizing filter and tried it out.

In normal color photography a circular polarizing filter can reduce the specular highlights of reflections from non-metallic surfaces, such as water, roads, glass and leaves. It can also darken the sky, most noticeably the area at 90 degrees from the sun. You rotate the polarizing filter until you get the degree of effect you want. Generally there will be one point of rotation that has the least visible effect and another, 90 degrees apart, which will show the strongest effect.

In infrared I found it does exactly the same thing, though generally a bit more subtly. I also found that the visible positions of maximum and minimum effect also correspond to the minimum and maximum effect the camera sees, which is important from a use perspective.

The above two images were taken with the polarizing filter set 90 degrees apart for minimum and maximum effect

You can see from the accompanying images that it changes the distribution of tones in parts of the image. Since we are talking about an artistic interpretation of the scene there is no right or wrong, just whichever you prefer. In some images, for example, the image benefits from reducing reflections in the water and in others they do not. The same with extra sky darkening. What I also found is that, by making some parts of the image darker, the overall tonal range of the image is increased, from lightest to darkest. You see this as a wider histogram ‘bump’ in the image with the polarizer turned for maximum effect. This is a benefit, generally, as it means you do not need to stretch the highlight and shadow areas as much to get fill the range from black to white.

Two images with the polarizing filter set for the minimum and maximum effect

The histograms for the two images

Superimposed, you can see how the use of the polarizer can extend the actual range recorded

You can see the effect of the polarizer on the reflections in the water

The two original images with the polarizer set for minimum and maximum effect

The same two images with Autolevels applied. You can see, I hope, that the polarizer does allow you to change the look of the image at the time you shoot

Another two processed images

The effects can be subtle

Again the results can be very subtle, but still worth experimenting with

I really recommend that you try this for yourself, with your camera and your polarizing filter. I have tested this only with my Hoya polarizers. It may be that the filter material in other brands reacts differently with the IR. Though I would not expect this to be the case, you never know unless you test. I will get some other brand polarizers in and try this.