Originally posted by DeadJohn ETTR +2 stops and your shutter speed changes to 1/25. The slower shutter will allow more total noise to creep in but you'll also have more signal. Then you dial exposure down 2 stops during processing and everything, signal+noise, returns closer to the level as if you perfectly exposed.
The idea is to not vary ISO, leave it as close to base as possible, and only lessen your shutter speed.
THe increase in noise due to these shutter speeds is really negligible and is much more influenced by ISO. It's only when you get into really long shutter speeds that it will really effect noise.