A couple of days ago we had a spectacular afternoon double rainbow.
I was out planting grama grass seeds, hoping to take take advantage of
a rainy week, but I cut the planting short to run up and get my camera.
And then after shooting rainbow shots with the fisheye lens,
it occurred to me that I could switch to the zoom and take some
hummingbird shots with the rainbow in the background. How often
do you get a chance to do that? (Not to mention a great excuse not to
go back to planting grass seeds.)
(Actually, here, it isn't all that uncommon since we get a lot of
afternoon rainbows. But it's the first time I thought of trying it.)
Focus is always chancy when you're standing next to the feeder,
waiting for birds to fly by and shooting whatever you can.
Next time maybe I'll have time to set up a tripod and remote
shutter release. But I was pretty happy with what I got.
Double rainbow, with hummingbirds.
[ 19:40 Aug 09, 2016
More nature |
permalink to this entry |
The wonderful summer thunderstorm season here seems to have died down.
But while it lasted, we had some spectacular double rainbows.
And I kept feeling frustrated when I took the SLR outside only to find
that my 18-55mm kit lens was nowhere near wide enough to capture it.
I could try
it together as a panorama, but panoramas of rainbows turn out to
be quite difficult -- there are no clean edges in the photo to tell
you where to join one image to the next, and automated programs like
Hugin won't even try.
There are plenty of other beautiful vistas here too -- cloudscapes,
mesas, stars. Clearly, it was time to invest in a wide-angle lens. But
how wide would it need to be to capture a double rainbow?
All over the web you can find out that a rainbow has a radius of 42
degrees, so you need a lens that covers 84 degrees to get the whole thing.
But what about a double rainbow? My web searches came to naught.
Lots of pages talk about double rainbows, but Google wasn't finding
anything that would tell me the angle.
I eventually gave up on the web and went to my physical bookshelf,
where Color and Light in Nature gave me a nice table
of primary and secondary rainbow angles of various wavelengths of light.
It turns out that 42 degrees everybody quotes is for light of 600 nm
wavelength, a blue-green or cyan color. At that wavelength, the
primary angle is 42.0° and the secondary angle is 51.0°.
Armed with that information, I went back to Google and searched for
double rainbow 51 OR 102 angle and found a nice Slate
article on a
rainbow and lightning photo. The photo in the article, while
lovely (lightning and a double rainbow in the South Dakota badlands),
only shows a tiny piece of the rainbow, not the whole one I'm hoping
to capture; but the article does mention the 51-degree angle.
Okay, so 51°×2 captures both bows in cyan light.
But what about other wavelengths?
A typical eye can see from about 400 nm (deep purple)
to about 760 nm (deep red). From the table in the book:
|Wavelength ||Primary ||Secondary
|400 ||40.5° ||53.7°
|600 ||42.0° ||51.0°
|700 ||42.4° ||50.3°
Notice that while the primary angles get smaller with shorter
wavelengths, the secondary angles go the other way. That makes sense
if you remember that the outer rainbow has its colors reversed from
the inner one: red is on the outside of the primary bow, but the
inside of the secondary one.
So if I want to photograph a complete double rainbow in one shot,
I need a lens that can cover at least 108 degrees.
What focal length lens does that translate to?
Astronomical Adventures has a nice focal length calculator.
If I look up my Rebel XSi on Wikipedia to find out that other
countries call it a 450D, and plug that in to the calculator, then
try various focal lengths (the calculator offers a chart but it didn't
work for me), it turns out that I need an 8mm lens, which will give me
an 108° 26‘ 46" field of view -- just about right.
So that's what I ordered -- a Rokinon 8mm fisheye. And it turns out to
be far wider than I need -- apparently the actual field of view in
fisheyes varies widely from lens to lens, and this one claims to have
a 180° field. So the focal length calculator isn't all that useful.
At any rate, this lens is plenty wide enough to capture those double
rainbows, as you can see.
About those books
By the way, that book I linked to earlier is apparently out of print
and has become ridiculously expensive. Another excellent book on
atmospheric phenomena is
and Color in the Outdoors by Marcel Minnaert
(I actually have his earlier version, titled
Nature of Light and Color in the Open Air). Minnaert doesn't
give the useful table of frequencies and angles, but he has lots
of other fun and useful information on rainbows and related phenomena,
including detailed instructions for making rainbows indoors if you
want to measure angles or other quantities yourself.
[ 13:37 Oct 02, 2014
More photo |
permalink to this entry |