How To Calculate Exposures Quickly at Night Time


Nighttime photography often demands extended exposure times, frequently spanning several minutes. To attain optimal image quality, it's crucial to maintain a low ISO setting to minimize noise, hot pixels, and potential banding artifacts.

Many photographers may resort to guessing their exposure times during nighttime shoots, leading to wasted time on "dud exposures" due to incorrect guesses. Here's a systematic approach to streamline the process.

Assume you intend to capture images at ISO 100 with an aperture of f/11 but are uncertain about the appropriate shutter speed. Rather than guessing between 2 minutes, 4 minutes, or 8 minutes, consider using the ISO advantage of modern DSLRs, which can reach up to ISO 6400.

Transitioning from ISO 100 to ISO 6400 corresponds to increasing sensor sensitivity by a factor of 64. Consequently, the exposure time (shutter speed) can be proportionally reduced by the same factor. Remarkably, 64 is very close to 60, the number of seconds in a minute. This insight leads to a simple equivalence:

  • 1-second exposure at ISO 6400 equates to a 1-minute exposure at ISO 100
  • 2-second exposure at ISO 6400 equates to a 2-minute exposure at ISO 100
  • 8-second exposure at ISO 6400 equates to an 8-minute exposure at ISO 100
  • 30-second exposure at ISO 6400 equates to a 30-minute exposure at ISO 100
  • and so on...

While using ISO 6400, experiment with different shutter speeds until you achieve the desired result (through trial and error). These trials involve only several-second exposures, preventing wastage of valuable time. Once you determine the ideal shutter speed in seconds, switch back to ISO 100, and apply the same exposure time in minutes instead of seconds.

This technique isn't exclusive to night photography; it's also valuable when employing dark filters like ND400 filters. It facilitates rapid assessments of long exposures and aids in composition when the scene is exceptionally dark, even rendering Live View ineffective (and when frequent filter adjustments are undesired).

For photographers with cameras limited to a maximum ISO of, for instance, ISO 1600, a similar approach can be adopted. In this case, a different translation table applies, where 4 seconds at ISO 1600 equals 1 minute at ISO 100, and 16 seconds at ISO 1600 equals 4 minutes at ISO 100. This compensates for the 4x sensitivity difference between ISO 6400 and ISO 1600.