Sometimes filters, during capture or in post production, have unintended effects.
During my recent trip teaching photography and image editing in Costa Rica, we happened across a multitude of rainbows on several different days. During one of the shooting sessions, one of the photographers on the tour turned to me and asked, “Should I be using a polarizer here?” I answered immediately, “No.” My answer was a gut response to my knowledge of physics than photography, but I think it bears a little more explanation.
Polarizers are generally used to provide emphasis in the sky details, making the clouds more defined, the blue of the sky deeper and, in general to provide better contrast. They also are very effective at cutting light reflections from water surfaces, wet stones, damp foliage and other bright spots.
Rainbows are a refraction of light through a field of mist caused by rain, waterfalls, waves or other water diffusion mechanisms. The light coming back to the photographer’s eye is not omni-directional as daylight but rather comes in at a specific angle. When you apply a polarizer to the light to punch up your sky, you have a problem with the rainbow. Consider these two photos:
The top photo is a shot taken without the polarizer on the lens. Notice the vibrant intensity of the rainbow. The bottom photo was taken with the polarizer adjusted to give a little more definition in the sky. Notice that the vibrant colors of the rainbow are now all but blocked by the polarizer’s selective passage of light. The clouds in the distance do show up slightly better but only at the sacrifice of the color.
Polarization works best when the sun is at an angle to the lens. Shooting straight into the sun or away from the sun yields markedly less effect other than foreground glare reduction. As rainbows form opposite the sun, the value of a polarizer is diminished for the landscape in general and detrimental to the rainbow in particular. Unless you have an extreme wide-angle view, the effect on the sky is probably minimal.
Bottom line, the best the rainbow appeared was without a filter. With a polarizing filter you could get it close to-but not quite as colorful as it appeared without a filter and you sacrificed a full stop and a third of shutter speed. In addition, spinning the filter just a little bit from it’s optimum angle, made the bow disappear back into the mist whence it came. For the rainbow, the polarizing filter is not a sufficiently sharp weapon in your arsenal.
We don’t often pause to consider the physical nature of the phenomena we encounter and then capture. What may seem like the right move won’t always work the way we expect it to work. Watch with your eyes through the lens and pay attention to your LCD display on the camera and see what other unexpected things may be happening.
Rikk Flohr © 2009