There's nothing like dramatic light to take a scenic that's been shot thousands of times to a new level. Impending storms, shafts of sun that spotlight key compositional elements, fire engine-red clouds, all qualify, but what rides highest on many landscape photographer's lists of most dramatic, are rainbows. They have it all—prismatic color, a stormy sky lit by the sun, early morning or late afternoon light, and if you believe in folk tales, maybe a pot of gold. They're adrenaline-pumping phenomenons that rev up many a photographer's juices. With each rainbow I've encountered over the years, upon completion of the shoot I look to the sky, wink, say thanks, and rush back to the motel to clearly mark those files.
For a rainbow to materialize, a number of natural events need to occur. First off, the horizon where the sun sets or rises must be clear and the sun needs to be lower than 42 degrees in the sky. With your back to where it rises or sets, turn so your shadow falls directly in front of you. This places you 180 degrees from the sun. You now face what is known as the antisolar point. This is where the arc of the rainbow will appear, provided there is moisture in the sky. When all factors come together, a rainbow materializes. Understandably, they're not an everyday occurrence.
When you're out in the field and it's rainy, look toward the horizon of the setting or rising sun. If you see an opening in the clouds, look for a foreground in the direction of the antisolar point that has character and wait. Mount your camera to a tripod to ensure you get a sharp image. If a rainbow appears, make sure you capture it, exhausting all compositional possibilities. Shoot it vertically, horizontally, with a wide angle to take it all in and with a telephoto to sample portions with the most dramatic color. If a double rainbow puts in an appearance, break out the super wide to include both curves in addition to the full arc of both levels. Use a polarizer to enhance its color. Be careful how you orient it as an incorrectly spun polarizer can also make it disappear. Don't dwell on this as it's visible through the viewfinder. I suggest you resort to manual focus as the sensor of an autofocus camera may find it hard to distinguish contrast in the sky. If you waste a lot of time trying to autofocus, you may lose a great shot as rainbows often don't last a long time.