Photographing a Lunar Eclipse

The best advice for photographing a lunar eclipse is to take lots of pictures at many different exposures and throw most of them away. You never know just how bright your subject is going to be.

Most importantly, you'll need a telescope or telephoto lens that enlarges the Moon to a good size. For a typical lunar eclipse, the size of the Moon's image on film equals the focal length divided by 110 (with the result in the same units as the focal length). Thus an 8-inch f/10 telescope, with a focal length of 80 inches (2,030 millimeters), will produce a Moon image 0.73 inch (18.5 millimeters) across. This will easily fit on a frame of 35-mm film, which measures 24 by 36 mm.

An ordinary point-and-shoot camera with a lens of about 40-mm focal length will produce an image of the Moon barely 0.4 mm across, too small to record even as much detail as you can see with your unaided eye. If you have a point-and-shoot camera with a zoom lens, zoom the lens out as far as you can to get the highest possible magnification.

The minimum focal length for getting a good-looking Moon is about 300 mm. With a 500-mm lens and a 2-second exposure you can probably get away with a camera on a fixed tripod, but longer exposures or focal lengths will require a tracking mount to prevent blurring due to the turning of the Earth. A simple, do-it-yourself tracking camera mount is described on page 32 of the May 1996 issue of Sky & Telescope.

The full range of possible eclipse images — from sequences showing how the event unfolds, to close-ups of the shadow's edge on the Moon's surface — calls on a rather large bag of photographic tricks. Compared to recording a normal full Moon with a given camera setup, a lunar eclipse requires exposures perhaps 4 to 1,000 times longer — and everything in between! Much depends on the stage of the eclipse you're trying to record, the darkness of the shadow, and the pictorial effect you are after.

The shortest exposures are useful only during the partial stages near the very beginning and end of the eclipse, and they'll probably bring out little more than the smooth gradation in the shading of the penumbra. That's also what you'll get if you let a camera's built-in metering system take control of the exposure. To record the well-defined edge of the inner shadow, or umbra, which is so striking visually, you'll have to override the system and expose several times longer. Typically that means a ¼-second exposure at f/8 with ISO 400 film, "burning out" the Moon's brightest portion but getting sharp detail near the umbra's edge.

As for totality itself, there is no safe bet — the brightness of the fully eclipsed Moon is too unpredictable. Because film is cheap compared to the rarity of a total lunar eclipse, you can hardly go wrong trying every exposure setting you've got. There's plenty of time to experiment during this leisurely event.