It’s easy to get good exposures on a bright, sunny day. But when you want to shoot pictures inside a museum, church or any low-light situation in which tripods are not allowed, you can still get good exposures by using a high ISO setting.
I attended my nephew’s wedding several years ago, and wasn’t allowed to use flash inside the chapel (besides, my flash may have interfered with that of the official wedding photographer’s). Using a tripod would have also been inconvenient, so I raised my ISO speed to 1600 - the fastest setting on the Canon EOS 20D that I was using at the time - and utilized the existing light.
Because there was some nice illumination coming in through a bank of windows on one side of the room, in addition to incandescent lighting above, I was able to get good color balance with my Auto WB setting.
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