How To Calculate Long Exposures
Your camera’s meter does a pretty decent job of giving you an accurate exposure for shutter speeds up to 30 seconds. But what if you want or need an exposure time of more than 30 seconds or perhaps several minutes? How do you calculate long exposures, the proper ISO and shutter speed combination, to avoid having the image be over or under exposed?
It’s easy. First, establish a base exposure by using 30 second as your shutter speed and ISO as the exposure variable for preliminary calculations. In Manual exposure mode, select 30 seconds for the shutter speed, an ISO of 100, and the appropriate f-stop for depth of field considerations (This is all done after you’ve focused and finalized your composition, obviously). During late twilight, early dawn, or if using a strong neutral density filter, the meter should show the image as being underexposed at this point.
Now, simply increase the ISO in full stops until the meter indicates that the scene is exposed correctly. Count the number of increased stops. If it was two stops, for example (ISO 100 to 400) then you just add those two stops to the shutter speed (30 seconds to 2 minutes) after resetting the ISO back to 100 and the exposure mode to Bulb. These are reciprocal exposures (30 seconds and 400 ISO equals 2 minutes and 100 ISO).
An important consideration when doing long exposure photography in the early morning and late evening is that the light is changing very quickly. If you are doing a 3-minute exposure, for example, the intensity of light at the begining of the exposure will be very different than when you finish. That means there’s a risk of overexposure during the fast-changing light of the mornings and underexposure in the evenings. This should be factored into the information offered above in order to get the correct exposure during these times of day.