Long exposure photography

For super long exposures at night, that is longer than the max. 30 seconds a digital camera will allow for automatic shutter operation, the use of bulb mode is necessary. The guide below shows how to calculate the length (in time) for a perfect exposure of the subject.

Exposure length

1. Set camera to aperture priority mode (Av) and the lowest aperture opening, say f/3.5
2. Set ISO high, 1600 or 3200, to make the sensor sensitive enough to pick up the available night light and report a shutter reading under the 30-seconds camera limit
3. Read the shutter speed indicated on camera, at night 10" (seconds) is normal, take a photo and review it and examen if it's well exposed
4. Divide the ISO used, 1600, by the desired ISO for the final photo, say 200, to get a factor of 8
5. Take the 10" shutter speed and multiply by 8 to get 80 seconds, this is the new exposure length for the final photo
6. Set the camera to manual mode and scroll the shutter parameter until it reads Bulb
7. Use stopwatch or timer to help track the time and close the shutter at the right time for perfect exposure
8. Press the shutter button or a external shutter release control (IR control, wire, servo)

References

Some site