60 Better Available Light Digital Photography
under normal exposure duration retain their light sensitivity
better at long exposures. Most fi lm manufacturers publish data
on the necessary reciprocity corrections, but digital shooters can
ignore this information.Do a noise test
What is surprising to me is how often people are surprised by
the amount of noise in their images. They crank up their ISO
speed to 1600, shoot at 1/4 second, and are surprised by the
amount and size of the noise. If you like to shoot available light
images—and we have to assume that you do, or you wouldn’t
be reading this book—one of the fi rst things that you should do
with a new camera is make a noise test. Don’t freak out. It won’t
take long and the information will be invaluable.
Set up a situation that will produce longer-than-normal shutter
speeds and use high ISO speeds. Don’t use just one ISO—shoot
at 400, 800, 1600, and whatever else your digital SLR can
produce. Be sure to bracket exposures. Even though digital
capture is not subject to exposure shifts under slower shutter
speeds, bracketing exposures might tell you that slight over- or
underexposure in a given lighting condition might increase or
decrease the effect of digital noise. Be sure to include a known
color target such as a Macbeth ColorChecker (www.gretagmac-
beth.com). Color won’t usually shift, but it provides some smooth
patches that will show noise better than textured ones, and there
are cameras whose color fades as ISO increases.
Open the fi les and use your image-editing program’s magnifying
tool to look at the noise and determine the optimum settings to
produce the least amount of noise. Take your time and compare
over- and underexposed examples. Although this process won’t
eliminate noise, it will make you aware of what switching from
ISO 400 to 800 will cost you. You may be able to live with the
amount of noise, but if you can’t, you’ll now know that you’ll
need a tripod so you can use a lower ISO speed. Be aware that
longer shutter speeds and any underexposure will exacerbate
digital noise. By taking the time to make these tests, you will
have the information needed to make the right choice for the
lighting conditions you are working under.Built-in noise suppression
The way most camera manufacturers accomplish in-camera
noise suppression is a blend of marketing buzzwords and high
technology, and how noise is handled in-camera varies greatly
between camera manufacturers and is highly proprietary. For
those readers who are interested in how on-chip noise reduction
is accomplished, here’s a brief overview.