Low light shooting

In extreme low light when shooting people, you have no choice but to shoot wide open and to use a very high ISO.  No one likes to raise the ISO high due to the increase in digital noise, but the choice is between (1) not taking the picture, (2) getting a blurred picture with a low ISO, or (3) getting a sharp picture with a high ISO.  If you use a tripod, you can get away with a lower ISO if and  only if the subjects are perfectly still.  However, most people can't be perfectly still long enough for a slow shutter to give you a sharp picture.  In this photo of monks in Mandalay, Burma, I raised the ISO to 6400, used 1/50th of a second at f/4, and then lessened the noise using Nik Software's Dfine 2.0 in Photoshop.  I used a Canon 5D Mark III to make the picture.