How can I meter for very long night exposures without multiple 10-minute test shots?
Asked 5/15/2011
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I shoot long-exposure night scenes, and many of my shots need exposures of 10 minutes or more. My camera’s internal meter can’t really help at those light levels, so I’ve been taking a test shot, extrapolating, and trying again. The problem is that each test can already take many minutes, so dialing in one final exposure can take a very long time.
Is there a better way to estimate exposure for multi-minute shots without relying on several full-length test exposures? Are handheld meters useful at such low light levels, or is there a more practical approach?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
29
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.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
15y ago
0
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For exposures this long, trial-and-error with shorter test shots is usually the practical method. Camera and handheld meters are not especially reliable at extremely low light, so checking the image and histogram is often more useful than chasing a meter reading.
A good workflow is:
- Take test shots at a much higher ISO than your final image.
- Open the aperture as wide as possible for the test if needed.
- Use the histogram to judge how far off you are.
- Convert that result to your final settings.
Exposure scales predictably:
- Every 1 stop lower ISO = double the shutter time.
- Every 1 stop smaller aperture = double the shutter time.
So if a high-ISO, wide-aperture test gives a correct exposure in a short time, you can calculate the equivalent multi-minute exposure for your final ISO/aperture.
It also helps to learn what the histogram looks like when you’re under by 1/3, 2/3, or 1 stop, so you can refine more quickly. An exposure calculator app can speed up the conversions, but it won’t replace testing entirely.
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