How to take the guessing out of making a low ISO night-time exposure.
Shooting a night scene using a high ISO would allow the camera’s exposure system to set a fairly short shutter speed of a few seconds but the image would have increased noise due to the high ISO.
Capturing a night time scene using a low ISO to avoid noise, could run into minutes or even hours, depending how dark it is, so it would be beneficial to get the exposure correct at the first time of trying.
The steps below show how to capture a correctly exposed (long exposure) night-time scenes at ISO 100 to avoid lower quality images shot at a higher ISO.
Requirements: DSLR camera with a live view screen, a tripod and a means of triggering the shutter without causing camera shake - you could use a simple remote trigger device or just use the camera’s Self Timer mode.
Step by Step
1. Place the camera a tripod and set up the camera to ISO to 6400.
2. Set the desired aperture in Aperture Priority mode.
3. Set the focus to the desired distance and switch to manual focus if not already set.
4. Frame your shot using the Live View screen and press the shutter.
5. Take a note of the shutter speed and aperture used.
6. Switch the camera to Manual mode.
7. Set the ISO to 100 with the same aperture value as before.
8. The shutter speed required, will be derived by converting the shutter speed in seconds, used for the ISO 6400 shot, to the same number of minutes.
9. For example, a two-second shot at ISO 6400 will be a two-minute shot at ISO 100 and a 1/60 second shot at ISO 6400 becomes a becomes a 1 second shot at ISO 100.
General Rule: Multiply the time in seconds required at ISO 6400 by sixty to give the shutter speed required at ISO 100.
Note
Exposures longer than about 8 seconds can cause the sensor to heat up, which will also cause additional digital noise on the image. Long exposure noise can be reduced by using the camera's “Long Exposure Noise Reduction” setting. With this setting "on”, any exposures longer than 8 seconds, will result in a second “blank exposure” of the same duration being automatically taken. The second blank image is subtracted from the actual image and thus cancel any noise. This would double the time required to complete the shot as the camera has to take a second similar length “exposure” reading of the sensor with the shutter closed.