Taming those Annoying Highlights: Cross-Polarization Flash Macro Photography

Have you ever looked at your photos with dismay, wondering where all those bright reflections came from? You were set up on a beautiful flower with perfect dew drops. The light was low and you wanted to maximize your depth of field (DOF), so you used flash. Instead of the wonderful image you had envisioned, your photo shows grossly overexposed highlights. You see reflections from the flash such as those in Figure 1. Can these highlights be controlled?

Figure 1 – Moist, soft bodied creatures are prone to multiple bright highlights from flash photography. Here an Upland Chorus Frog is shown without and with the cross-polarization flash technique employed. The physics of light are beyond the scope of this article, but if we accept that light wave particles oscillate in particular planes, we can come home with the images we envision. This planar behavior of light allows us to use polarizers to filter out all but the scattered non-planar light.

The physics of light are beyond the scope of this article, but if we accept that light wave particles oscillate in particular planes, we can come home with the images we envision. This planar behavior of light allows us to use polarizers to filter out all but the scattered non-planar light.

How it’s Done

Using the appropriate polarizing filter (circular or linear) for your camera’s meter on your lens alone will help, but the real trick is to add polarizing films to the flash heads as well. Cutting the film to a size slightly larger than the head of the flash and using tape to secure it in place seems to work well. The plane of polarization of the film on the flash head must be 90 degrees or perpendicular to the plane of the polarizer on the lens for maximum effect. If you are using two flash units, each flash must have polarizing film and these must be aligned as close to parallel to one another as possible. Please see the example in Figure 2.

Figure 2 – Two-flash setup with polarizing film over the flash heads and polarizing filter on the lens. Notice that the films on each flash are aligned so that the light leaving both flashes is in the same plane of polarization. The equipment pictured includes a 35mm format camera, 180mm macro lens, 72mm circular polarizing filter, a 550 EX and a 420 EX flash with polarizing film taped in place over the flash head. The flash units are supported by Wimberley flash brackets consisting of a module 1 (quick release arm SS-010) and a module 4 (macro extension arm SS-040). All these components are supported on a Kirk BH-1 head and a Gitzo G 1410 tripod.

Using a flash exposure lock feature, if available, allows you to set and maintain a set flash value. With Canon, this is a Flash Exposure Lock (FEL) button on the camera, which locks the value for sixteen seconds; for Nikon, this Flash Value Lock (FV Lock) is available in some flash units and locks the value until it has been reset. It is important to set the flash exposure after you have rotated the polarizing filter on the lens until the reflections from the subject are minimal. If a flash exposure lock feature is not available to you, hold a piece of the polarizing film up to a light source in front of your lens in the same orientation as the film will be on the flash head. Rotate the polarizing filter on your lens until you achieve the maximum darkness or polarizing effect. Mark the top edge of the filter with a small dab of light-colored paint or nail polish so that you can reference this orientation in the future. Now assemble your flash(s) and set up as you would for any macro flash shoot. For small subjects, one flash may be all that is necessary. In this type of setup, the flash is often positioned just above the front element of the lens and produces a non-point source of light, even illumination without harsh shadows, with pleasing results. For larger subjects, this single flash technique may create a flat look. Using two flash units set to each side of the subject and in line with the front element of the lens will produce a more pleasing effect giving the subject more depth and texture.

How about exposure? This can be tricky. If your camera has through-the-lens (TTL) flash metering you are all set. If not, you will have to shoot a series of test exposures at your flash sync speed and various f-stops. Using only the scattered light reduces the available light by 3-5 stops. In order to maintain a large DOF, you will have to move the flash unit(s) as close to the subject as possible. Light from the flash falls off quickly when the subject is roughly fourteen inches or further away.

With the Canon EOS system, I am able to use the 550EX and the 420EX flash units on a Wimberley flash bracket system, a 180mm f/3.5 macro lens, ISO 200, f/29 and a shutter speed of 1/200 of a second. The flash units are positioned to line up with or to be just beyond the front element of the lens and pointed at the subject. The heads must be tilted so that the plane of polarization of the polarizing films over the heads is as parallel as possible (Figure 2). Using the FEL option also allows you to make certain that the alignment of two flash heads is perfect. While watching through the view finder, you can repeatedly press the FEL button and rotate the polarizing filter on the lens. Watch for the bright reflections to completely disappear. Any remaining highlights would be the result of improper alignment of the flashes.

Figure 3 – Comparison of plant material with a glossy cuticle. The only difference in these two images is that the circular polarizer on the lens was rotated 90 degrees from minimal to maximum effect. Notice the rich colors and hidden details in the vein structure seen in the polarized image on the right.

Some people feel that the lack of a highlight is “unnatural” or somehow looks odd. You can dial in just the amount of highlight that you want using this technique. Once the flash heads are aligned, simply rotating the polarizing filter on the lens will moderate the amount of highlight you get. Ninety degrees of rotation in either direction will go from maximum to minimal effect.

This may seem like a lot of work to set up and use, but you will be amazed at the results. Using polarized light helps to saturate colors and create depth for some subjects. In some instances with insect subjects, you will be looking through the exoskeleton and revealing underlying details. Using this technique with digital cameras opens a whole new world of possibilities; the instant feedback will allow you to control the effect in minute detail.

The polarizing films can be found at Edmund Optics.