How Long Should I Expose?
Much has been written on this subject. Many factors must be considered when determining the best exposure for a given object. In some cases there is no single best exposure, a target may have a wide brightness range which requires multiple exposure lengths to catch the bright and dim areas. Simply put, experimentation is required.
Here are some of the considerations for calculating exposures:
1. Target magnitude. The higher the number, the dimmer the object.
2. ISO setting. The higher the number, the shorter the exposure, but the more noise in your image.
3. F/number. The higher the number, the longer your exposure.
4. Sky brightness, which may limit your maximum exposure.
5. Tracking accuracy. How long can you track or guide accurately?Three targets were shot at increasing exposure durations:
The Orion Nebula, The Andromeda Galaxy, and the constellation Cygnus.See also General Notes concerning exposures.
M42, The Orion Nebula
Photographed at 500mm f/5 with broadband LPR filter.
Canon XTI unmodified, ISO 1600.
4 seconds.
7 seconds.
15 seconds.
30 seconds.
60 seconds.
3 minutes.
4 minutes.
6 minutes.
8 minutes.
10 minutes.
11 minutes.
40 seconds.
120 seconds.
240 seconds.
300 seconds.
360 seconds.
480 seconds.
600 seconds.
