technique - How do I meter for long exposures (10+ minutes)?

One of my longstanding interests is long exposure night photography.

However, many of the situations where I am shooting require exposure lengths on the order of 10 minutes, which is far beyond what the internal meter in my camera (5DII) can handle.

Right now, I take a test exposure, extrapolate from that to guess my exposure time, try again, etc...

This works, but since the test exposure alone can be 10 minutes, and it generally takes me a few shots to get the exposure right, it can take 30 minutes to an hour just to figure out exactly how long I need to expose for.

Obviously, this is really inconvenient (fortunately, long exposure subjects don't move much, most of the time).

For instance, recently I did a series of 13 minute exposures at f4, ISO125. It came out really well, but I would up spending 2 hours to get one shot.

Anyways, is there an easier way to determine how long an exposure has to be, preferably that doesn't take multiple test exposures? I've looked at light meters, but none of them make explicit statements about how low a level of light they can handle.

Answer

At light levels this low, you'll be much better off by taking some test shots and checking their histograms rather than just relying on a light meter (which is usually optimized for measuring light, not darkness). However, you can make the test shots take less time.

Perform the test shots at the maximum ISO your camera can handle (avoid the uncalibrated expanded ISOs though), and multiply the measured proper shutter time by the factor you decrease ISO for your real shots.

Learn how a histogram's right-hand end for shots underexposed by 1/3 steps, 2/3 steps, 1 step looks like, so you'll recognize them and need fewer test shots.

Take the test shots with widest aperture and multiply measured shutter time by the difference to real aperture squared (another way to say it: multiply by two for each stop you'll be closing the aperture).

For example, if your test shot showed that at f/2.8 and ISO 6400 your exposure should be 8 seconds, then at ISO 125 f/4 you will need to expose for 8 * (6400 / 125) * (4 / 2.8) ^ 2 = 8 * 51.2 * 2 = 819 seconds = 13 minutes 39 seconds.

Note that if you happen to be shooting film, you'll also have to adjust for the reciprocity failure of the film you are using.