Double Rainbow
by Dan Heim
Photographer Comments:
Optics: Zuiko 28mm
wide-angle lens at f10
Camera: Canon EOS 20D, exposures from 1/10 second to 1/4 second
at ISO 3200
September 1st, 2007, Heimhenge, AZ, 1800 MST
Late afternoon monsoon rains and a low Sun conspired to produce these spectacular double rainbows. Note the obvious increase in sky brightness (due to scattering) both inside the primary bow and outside the secondary. I got lucky in the second image and managed to catch a bolt of lightning striking the side of Gavilan Peak, though the image needed considerable digital tweaking to bring out both the bolt and double bow. The last image is a digital composite showing the full extend of the bow — nearly 180 degrees thanks to the low Sun. Unfortunately, a 28mm lens, plus the effective 1.6x magnification when focusing on a CCD, constrains my field of view to around 44 degrees.
To capture the full 94 degrees of a rainbow, and another 20 degrees for the secondary, in a single image, I need to use a 7mm ultra-wide-angle lens (essentially a "fisheye"). That lens (from Canon) is expensive. But I recently found one online for far less: Zenitar K2 Fisheye, made in Russia and compatible with my Canon. It's a "rectangular format fisheye," which provides 180 degrees diagonally, at f2.8 - 22. The last image is the first taken with the lens, on July 9th, 2008. Unfortunately, in my haste to capture this fleeting phenomenon, I left the aperture at f4 which was a bit fast (hence the washout). I'll keep trying, as we get these doubles quite often during monsoons. Next time I'll be ready.