How can I determine the right exposure for very long night exposures without repeated 15-minute tests?
Asked 8/26/2013
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I enjoy making very long night exposures that can take several minutes or more, but choosing the correct shutter time is difficult. My camera’s meter is often useless in very dark scenes, and bright point light sources can skew the reading even though the darker background is what I want exposed properly.
Right now I’m using trial and error, but that can mean several test shots at 10–15 minutes each. Is there a practical way to estimate exposure more quickly for extremely long night shots? Are there cameras or features that let you monitor the exposure building up during the shot, or is there a better workflow for finding the final shutter time?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
15
The Olympus OM-D EM-5 can display the cumulative collection of light in a long exposure. But there is another way to test long exposure that takes much less time and works with almost any digital camera.
Start out by setting your camera to the highest ISO setting it has, open the aperture of your lens as wide as it will go and start with short exposures until you find the correct exposure. Then reduce the ISO and aperture to what you desire and increase the exposure time by the same number of stops. Remember that ISO and shutter speed double/halve for each one stop interval up/down. Aperture is multiplied/divided by 1.4 (the square root of two) for each stop up/down, so every two stops is double/half the previous one.
For example, if you find that ISO 12,800 @ f/1.4 requires a 1 second exposure, then to use ISO 100 @ f/8 you would need to increase the shutter speed (Tv) seven stops to compensate for the ISO difference and another five stops for the narrower aperture. Increasing Tv 12 stops from 1 second yields a Tv of 4,096 seconds, or roughly 68 minutes.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
0
Generated from our catalog & community — verify before relying on it.
A practical method is to make a quick test exposure at very high ISO and the lens’s widest aperture, then convert that result to your desired settings by counting stops.
Example: if a correct test exposure is 1 second at ISO 12800 and f/1.4, and you want ISO 100 at f/8, reduce ISO and aperture to those settings and add the same total number of stops to shutter time. Each stop down in ISO or aperture requires doubling the exposure time.
This lets you find the exposure in seconds instead of waiting through multiple 10–15 minute trials.
Also, some cameras offer a live long-exposure preview/build-up feature. One example mentioned is the Olympus OM-D E-M5, which can display the cumulative light gathering during a long exposure so you can stop when the image looks right.
So the two main solutions are:
- Use a high-ISO, wide-aperture test shot and calculate the equivalent long exposure.
- If your camera supports it, use a live long-exposure preview feature.
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