How to expose the moon?

Last night’s lunar eclipse occasioned a flurry of web traffic about how to set your camera to expose it correctly. I got to thinking – not always a good thing – about the problem, and the more I thought about it the harder it seemed.

Let’s assume that you’re making an image and you know the moon is the brightest thing in the field. Let’s make the further assumption that the moon is not large in the framed image; it’s only a component of the overall scene.

If you like to use your camera’s exposure meter – I don’t – you could set it to spot mode, meter the moon, and place it on Zone VII or (if you’re feeling lucky) VIII by opening up two or three stops from your meter reading. There’s a problem with this approach. Do you know that your camera’s spotmeter is taking its reading entirely from the moon, and not averaging in parts of the sky? If it is, it will think the moon is dimmer than it is, and you’re likely to have blown highlights in the final image. It’s actually worse than that; the light from the edges of the moon is dimmer than the light from the center, since the sunlight hits the edges at an angle, so maybe you should only open up a stop or two.

If you’re a fundamentalist photographer, you’ll note that the moon is a gray rock lit by the sun, and therefore, to place it on Zone V, or turn it into a middle gray, you’ll use the “Sunny 16” rule and set the f-stop to f/16 and the shutter speed to one over the ISO setting. Shooting digital, you don’t want a gray moon, you want an ETTR moon, so open up two, or, if you’re still feeling lucky, 3 stops. This is a pretty conservative way to go, since the moon’s reflectivity, at 12%, is less than an 18% gray card. This is the calculation that Ansel Adams famously muffed in exposing the negative of Moonrise, Hernandez, New Mexico. He did make a nice save, though.

The fundamentalist approach is useless during an eclipse, since you won’t know how bright the light falling on the moon is.

If, like me, you like to use the in-camera histogram, you could just make an exposure and look. If you’ve calibrated your camera’s settings using some variant of UniWB, the in-camera histo is a pretty good stand-in for the real raw histogram, and if you haven’t, you won’t blow the highlights, but you will probably not get a real ETTR exposure. However, there’s a fly in the ointment; the in-camera histogram is derived from the JPEG preview image, which is subsampled from the full-resolution sensor image. Unless the moon is reasonably large in the image, the subsampled JPEG is likely to omit the brightest pixel in the raw file. Even if it’s there, can you see one blown pixel on your camera’s histogram?

As far as I know, there’s no easy in-camera solution to getting a perfectly ETTR’d capture under the circumstances I’ve outlined here. You’ve got two choices: back off the estimated ETTR setting (probably the best move if you’re not fanatical about ETTR), or make a test image and look at the raw file (shooting tethered is a special case of this). A lunar eclipse takes long enough that that’s a viable option.

Or maybe you could slap on a long lens, make a test image, look at the in-camera histogram, and put your shorter lens back on the camera. Assuming similar T-factors in the lenses, that should work fine.

This may be a good example of analysis paralysis.