I get a lot of questions from friends about how to take shots of the moon. The Super Moon of 2011 really invited a lot of shooting, as it visibly appeared 14% larger than a typical full moon due to it's orbit bringing it closest to the Earth. Spectacular. Unfortunately, many of those Super Moon shots generated disappointment instead of capturing the magic. Most unsatisfied shooters wondered why their compositions have big white dots where they saw our beautiful companion Luna. The simple answer - their camera exposed for night, but it should have exposed for a sunny day. Huh?
Moonlight is daylight
That big beautiful moon is visible and bright at night because its reflecting sunlight. It's just about the same distance from the sun as our earth, and relative to that distance, is really really close to the earth. The reflected sunlight is very near the same intensity as direct sunlight - so moonlight is reflected daylight. I know, the hardcore lighting wizards are thinking 'hey the moon is far away and the inverse square law says that the light intensity drops off alot when the light is far away'. Yeah, there's loss because the moon is some distance away from the Earth and by the way, it's not white. But the distance from the Earth to the moon relative to the distance to the sun is tiny, so that light fall off is pretty small. Tiny, in fact. So small that you should expose the moon as if it were a bright sunny day.
The camera sees night
My good friends are relying on the metering in the camera, and the camera's metering is averaging the light in the whole frame. The amount of bright moonlight compared to the amount of dark sky is really small. Even if you shoot with a big ole' 400mm lens, the moon is probably less than 10% of the frame. So the camera is metering for dark night because it thinks the overall scene is really dark. A really dark frame means long exposure, so that moonlight gets blown out and all the detail in the moon is lost. Instead of the smiling man in the moon, you get a big white dot.
If you have spot metering built into your camera and great zooming capability (so you can zoom in enough to fill in the spot metering zone with the moon, probably need at least 200mm), you can spot meter for daylight. That will ignore all that darkness. But what if you don't have that kind of horsepower? Go manual.
Sunny 16 Rule
There's a very old rule of thumb in photography from back in the film days before fancy built in metering existed. It's called the sunny 16 rule. It states that for a bright sunny day, set your f/ stop to f/16 and the shutter speed to match the ISO setting (which was the speed rating of the film back then). Say you have your camera's ISO set to 200, the sunny 16 rule tells you to set your f/stop to 16 and the shutter speed to 1/200th of a sec and you'll get a proper exposure of sunlit subject on a sunny day. This works for moon shots too, since moonlight is reflected sunlight.
The sunny 16 rule is a great place to start, but since it's cheap, take shots using both faster and slower shutter speeds, the keep the one you like best.
Use a tripod
Even though you're shooting at a reasonably high shutter speed like a daylight shot, hand holding will likely introduce blur, especially if you're using a long lens. The surface of the moon has exquisite detail, get every bit you can.
OK, but what about the Inverse Square Law?
I know what the numbers-types are thinking right about now - the light reflected from the moon has to go farther to get to the Earth than does direct sunlight because it has to hit the Moon first, take a left turn (i.e. reflect), and then travel from the Moon to the Earth. And the Inverse Square Law (ISL) tells us that the intensity of light will suffer as a result. This is physics after all. What will ISL do to the Sunny 16 rule of thumb? Not much as it turns out. Here's why.
Some numbers to ponder
|Earth to Sun
|Earth to Moon
If we look at lunar distance in terms of Astronomical Units (AU), (which just happen to be defined in terms of the avg distance from the Earth to the Sun) we see that the moon is only (on average) 0.26% the distance from the each to the Sun, or .0026 AU. What that means for moon shot photos is that the Sun's light travels 1.0026 AU when reflected off the moon (1 AU from the Sun and another 0.0026 AU from the moon to the Earth) :
Light-distance(sun->moon->earth) = SunDist(sun->moon) + MoonDist(moon->earth) = 1.0 AU + 0.0026 AU = 1.0026 AU.
From an intuition standpoint, think about standing on the Sun and looking at both the Earth and the Moon. They will be so close together as to be difficult to see them apart from one another due to the distance between them being only 0.26% apart w.r.t. the distance. That extra 0.26% is mighty small in the AU scheme of things. The Inverse Square Law is generally discussed in terms of distances being doubled or halved. 0.26% isn't quite 50%. In fact, its not even close (pun intended).
A little number crunching
The inverse square law states that Intensity is proportional to 1 / distance² , or in terms of AUs for the Sun and moon:
Sun Intensity: Isun = 1/(1AU)² = 1/(1*1) = 1.0
Moon reflected Intensity: Imoon = 1 / (1.0026)² = 1/(1.0026 * 1.0026) = 1/1.0052 = 0.995
So the intensity of light reflected from the Moon is 99.5%
of the light coming from the Sun.
Since an f-stop is a 50% change in intensity (i.e. light doubles with opening and halves when closing down a lens using the square root of 2 raised in increments of ~1.4.14 - check out wikipedia http://en.wikipedia.org/wiki/F-number
) ,we should be able to approximate the f-stop effect of Imoon by using the .005
drop in intensity ala:
f/Imoon = f/1/ √( 2.005 ) = f/1 / √1.0035 = f/1 / 1.0017 => f/1.0017, trivial difference
So, the ISL, being physics and all, does in fact steal a bit of light from us, but not much. Certainly not enough to worry about w.r.t. using the Sunny 16 rule as a starting point.
What's more important is to start somewhere and bracket, as what you really care about is a decent moon shot. Sunny 16 is a pretty good initial guesstimate. Unless your shooting film, bracketing costs very little, other than a few extra seconds of your time.