How to Use Long Exposure Noise Reduction on a Canon EOS 70D
Noise, the defect that gives your pictures a speckled look, can occur when you have a long exposure time. The Canon EOS 70D offers a noise-removal filter, to address this cause of noise, however, you can control whether and how they're applied only in the advanced exposure modes. In other modes, the camera makes the call for you.
Long Exposure Noise Reduction, found on Shooting Menu 4 and featured in the figure below, goes after the type of noise that's caused by a slow shutter speed. The settings work as follows:
Off: No noise reduction is applied. This setting is the default.
Auto: Noise reduction is applied when you use a shutter speed of 1 second or longer, but only if the camera detects the type of noise that's caused by long exposures.
On: Noise reduction is always applied at exposures of 1 second or longer. (Note: Canon suggests that this setting may result in more noise than either Off or Auto when the ISO setting is 1600 or higher.)
Long Exposure Noise Reduction can be fairly effective, but it has a significant downside because of the way it works. Say that you make a 30-second exposure. After the shutter closes at the end of the exposure, the camera takes a second 30-second exposure to measure the noise by itself, and then subtracts that noise from your real exposure. So your shot-to-shot wait time is twice what it would normally be. For some scenes, that may not be a problem, but for shots that feature action, such as fireworks, you definitely don't want that long wait time between shutter clicks.