On 10/10/2014 6:34 AM, bj@xxxxxxxxxxxxxx wrote:
Chuck wrote
There are two more total eclipses coming up next year in April and
September so maybe you'll get another couple of chances. Until then you
can practice on the non-eclipsed moon which should be a bit easier.
The problem with this plan is that exposure must be much longer when the moon is in the umbra. There's actually a scale
for how dark any particular eclipse will is. <http://en.wikipedia.org/wiki/Danjon_scale> Practicing on the full moon
will give a false idea of exposure, by many stops.
This last one was not particularly dark: "Helio C. Vital leads a group of Brazilian amateurs who've carefully measured
the brightness of many past lunar eclipses. Taking into account the current aerosol content of Earth's atmosphere, he
predicts the October 8th event will be fairly bright, the Moon at mideclipse being roughly as lustrous as Jupiter
(magnitude –2 to –3)."
Unfortunately: "When the last of the moon enters the umbra, the total lunar eclipse begins. How the moon will appear
during totality is not known. Some eclipses are such a dark gray-black that the moon nearly vanishes from view. At other
eclipses, it can glow a bright orange. The reason the moon can be seen at all when it's totally eclipsed is that
sunlight is scattered and refracted around the edge of the Earth by our atmosphere. To an astronaut standing on the moon
during totality, the sun would be hidden behind a dark Earth outlined by a brilliant red ring consisting of all the
world's sunrises and sunsets."
So one can't know ahead of time what the exposure will be.
The exposure guide half way down this page shows how great the difference may be.
<http://www.mreclipse.com/LEphoto/LEphoto.html>
It suggests that for f5.6, ISO 200, if full moon exposure is 1/2000*, the brightest total eclipse would be 1 sec., while
a darker eclipse could go out in to minutes. If you look at his multiple exposure image above, you'll see that the guide
seems to be pretty accurate.
So, at least 11 stops darker! That means a lot of light must be gained using aperture, and ISO to retain a shutter speed
that won't blur noticeably, say 1/2 sec. Basically, it can't be done for any but a bright eclipse using my 1000/11 lens
without astronomical ISO or a tracking mount.
Soooooooo ... Shooting a full moon really doesn't prepare one for an eclipsed moon. Some combination of ND filters and
polarizers might allow checking out one's equipment and technique?
Darkly Loony Moose
* I know, Loony 11 suggests f/11 @ 1/200, or f/5.6 @ 1/800. My own best shots were f/11, ISO 200, 1/60. Pretty close to
Loony 11. As he says, use his table as a starting point.
--
What if the Hokey Pokey *IS* what it's all about?
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