Need an ISO setting higher than the maximum provided by your camera? You may be able to cheat.
Shoot in RAW format. Set the camera to its highest ISO, then use the EV control to reduce the exposure
by –1 or –2 EV. That gives you the same result as shooting at double and quadruple the marked
ISO setting. When you convert your RAW files, you can try to resurrect your severely underexposed
photo using the exposure, contrast, brightness, and histogram/curves controls in your RAW converter.
Your final image may be very noisy and have poor tonal rendition, but what did you expect from ISO
6400 or ISO 12,800?
When shooting at higher ISO settings, make sure that your camera’s noise reduction feature is
sufficient to cut down on that noxious, multicolored hash that results from boosted ISOs.
You can also apply noise reduction to a lesser extent when converting RAW files to some other
format, using your favorite RAW converter, or an industrial-strength product like Noise Ninja
(www.picturecode.com) to wipe out noise after you’ve already taken the picture. Two things that I like most about Noise Ninja is that it can suppress noise without significantly degrading
your image, and it can be used with photos taken at relatively low ISO settings as well as your
“pushed” shots. It uses a sophisticated approach that can identify and recognize noise that presents itself
at different frequencies, parts of your image, and in varying color channels.
Reference: Mastering Digital SLR Photography By David Busch.