|Finding and Photographing Rainbows|
The span of a rainbow can be small (with high sun angle) or as wide as 84 degrees (with the sun at the horizon). The rainbow forms along part of a ring, and the angle of the sun determines what portion of the ring is visible above ground. If you're lucky enough to capture the maximum size rainbow, you'll need a 20-mm wide-angle lens. But a 28-mm will be plenty wide enough for partial rainbows or smaller spans. Refer to our Lens Angle-of-View Chart. If you're caught with a too-narrow lens, consider rotating the camera (in an artistic way, hopefully) so that the rainbow extends corner-to-corner across the frame. You'll get somewhat wider coverage and perhaps create an appealing image.
The colors of the rainbow can be rendered more intense and saturated using a polarizer (polarizing filter). The filter operates by reducing the scattered white light reflecting off the fronts of raindrops, mist, and background, leaving the colored light of the rainbow. This increases contrast and makes the rainbow appear brighter and more colorful. Be careful not to dial in too much polarizer, though, because it can filter out the rainbow itself. Try to strike a good compromise, leaving the rainbow bright while minimizing the hazy, white light surrounding it.
In case anyone is still shooting nature with film, use highly-saturated film for rainbow shots. There's no fear that the rainbow itself could be rendered unnaturally vivid... that's what you want. And the rest of the scene is typically dark and dull, and benefits from the color boost. A good choice here would be Fujichrome Velvia slide film for its high saturation and fine grain. Use ISO 100 film, or even Provia 400F, if there is motion in the scene (such as foreground trees blowing in the wind). The low-contrast storm scenes are ideal for slide film. But if you're capturing the end of the rainbow, the subject at that spot will usually be side-lit by some 40 degrees, and the shadow contrast may be high (except where the sunlit raindrops in the vicinity scatter light into the shadows).
On the contrary, waterfall spray rainbows may occur in a brightly sunlit scene that you want to capture with all its high contrast. In this case, a color negative film such as Fuji Reala 100 will capture more of the tonal range at the expense of reduced saturation and more coarse grain.
A combination of factors associated with storm rainbows lead to long exposure times, wide apertures, or both: (1) the dim lighting conditions under which most rainbows are visible; (2) the likely use of a polarizing filter at about 1.5 stops of light loss, and (3) the use of slower slide films. Since a passing storm (that produces the rainbow) might be followed by wind, your foreground may be in considerable motion. Yet, if you minimize shutter time to stop the motion, your large aperture will soften the focus on the foreground. This dilemma is solved, or at least avoided, by excluding the foreground and going for long exposures and smaller apertures, or including only foreground matter that is not in motion, such as a rock escarpment. For modern digital cameras, increase your digital ISO and reduce shutter speeds proportionately, but stay within your camera's low-noise ISO range.
Slight underexposure of a rainbow scene will tend to enhance the colors, giving deeper saturation. You can expose to the right with digital, but some cameras (or RAW converters) tend to wash out bright colors... experiment with some test shots to see where your color "sweet spot" exposure will be. Give yourself the best chances of a perfect exposure by bracketing exposures.
Fortunately, most rainbows are easy to meter and there are no special tricks to know. The low contrast of most storm rainbow scenes means that your camera meter will almost certainly give a good exposure. Just watch out not to include large areas of dark foreground or your meter may tend to overexpose sky areas. For bright daylight rainbows such as waterfalls, meter for the scene as usual and bracket to -1 f/stop, or rely on your preview histogram to capture the dynamic range.
Putting yourself out in the elements - especially setting up to shoot right after a passing storm - doesn't mean you and your equipment get thoroughly wet. In fact, you must avoid this unless you have waterproof gear.
A plastic poncho can be a great shelter in all but the heaviest downpour, and it allows you to keep some equipment sheltered as well (such as changing film). Wind can be the enemy of ponchos, though, and you may do better with a snug rain parka. Plastic rain pants are not the most comfortable but they will keep your pants dry and warm. If you do this a lot, get some breathable rain pants with gaiters to keep legs and shoes dry. Waterproofed boots are good for moving through wet grass or underbrush, even if the rain has stopped. A heavy-gauge plastic trash bag can be a temporary means of sheltering your camera bag from rain, and affords a dry place to put your kit after the rain stops. If you know in advance that you'll be waiting out the storm in the open, consider setting up a temporary emergency shelter - a tiny plastic tent. And please: always take precautions to avoid weather dangers such as lightning, flash floods, and avalanche.
If you'll be actually shooting in the rain, make a camera and lens cover using clear plastic sheeting cut to size and form fitted using clear packing tape - it's lightweight, relatively durable and rainproof, and certainly cheap. The heavier thicknesses are more durable and easier to manage; try six-mil (0.006-inch) thickness which is readily available in home improvement stores. Leave room for your hands to reach underneath for operating the controls and holding the camera. A visor (like a hood extension) of more rigid plastic can keep blowing rain from the front of your lens; check the visor in advance to ensure no vignetting on the corners of your image. Or for extreme conditions, buy a submersible plastic enclosure for your camera; they are available in flexible plastic and are not too expensive. Rotary controls can be difficult to operate with these enclosures.
To better understand what causes a rainbow, read Part 4. The Physics: How a Rainbow Forms.
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