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Tuesday, April 29, 2014

Chasing Rainbows


How to predict where a rainbow will be and how to set up your DSLR to photograph it to create dramatic rainbow-scapes

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Rainbow over the Windows area at sunset, Arches National Park, Utah.
Here's an example. It's sunset, and the sun is due west, which means its bearing is 270º. The horizon antisolar point is directly opposite the setting sun, or at 90º (270º - 180º = 90º). The right limb of the primary rainbow will intersect the horizon at 132º (90º + 42º = 132º). The left limb will intersect the horizon at 48º (90º - 42º = 48º).

If the sun is more than 10º above the horizon, the situation is slightly more complicated. Find the altitude of the sun in your Heavenly-Opportunity printout or smartphone app, and find the correct row in the table for that altitude (see above). Add the number in the second column to the bearing (direction) of the horizon antisolar point. The right limb of the rainbow will intersect the horizon there. Now subtract the number in the second column from the bearing of the horizon antisolar point. The left limb of the rainbow will intersect the horizon there. The third column shows the altitude of the highest point of the rainbow.

Here's an example. Assume, again, that the sun is due west, at a bearing of 270º and its altitude is 25º. Consulting the table, you can see that the rainbow will intersect the horizon 35º left and right of the horizon antisolar point, or at 125º and 65º, respectively. The top of the bow will be 17º above a level horizon.

Now that you can predict where a rainbow will appear, let's talk about how to shoot it. A complete semicircular rainbow that arcs high into the sky and stretches from horizon to horizon is a spectacular sight, but its appearance in a print is often underwhelming. That's because the band of colored light is only about 2º wide. You'll need a 20mm lens on a full-frame sensor to include all of the primary bow. You can easily end up with a very thin line of color curving across your frame and a whole lot of boring, gray sky underneath. The best rainbow shots are often taken with a moderate telephoto and include only a portion of the rainbow.

Using a telephoto gives you another advantage: You can use a polarizer to enhance the rainbow. Light from a rainbow is tangentially polarized, meaning the angle of polarization is along a line tangent to the rainbow. For a full, semicircular arc, the angle of polarization is vertical near the horizon and horizontal across the top of the bow. Light from the background is unpolarized, which means it can be thought of as a mixture of vertically and horizontally polarized light. Rotate your polarizer to allow the light from a section of the bow to pass through, and you'll darken the background by blocking that portion of the background light polarized at 90º to the rainbow light. The rainbow will stand out strongly against the darkened background. This only works with a telephoto, however, because you need to isolate a portion of the bow that has approximately the same angle of polarization along its length. If you include the entire bow in the frame by using a wide-angle lens, you'll find that rotating the polarizer will cause one portion of the bow to strengthen while another part disappears completely.

Armed with your new knowledge of how rainbows form and how to shoot them when they appear, you'll be well equipped to chase rainbows the next time a crack of thunder announces the arrival of rainbow weather. As the sun drops below an altitude of 42º, look for a gap in the clouds to the west, calculate where a rainbow will intersect the horizon, and position yourself so your hoped-for rainbow will complete an already strong composition. Never again will you be like the 60-something student who came up to me after my rainbow lecture and said, "All my life, I thought that rainbows appeared randomly." Now he knows better, and his photos will be better as a result.

You can see more of Glenn Randall's photography, sign up for his monthly newsletter, read his blog and learn about his upcoming workshops at www.glennrandall.com.

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