Canon Camera Hackers Manual page 48

Longer exposure times (section 4.3.1). How long depends on the camera
f
4
model , but the minimum is 64 seconds. Many cameras allow for up to
2147 seconds.
Dark Frame Subtraction (DFS) (section 4.5.2). DFS can be useful when
f
you're shooting with long exposure times at low ISO speeds. In such
conditions, it will effectively remove all hot pixels. When used with high
ISO values, however, it can make things worse (see below). The CHDK
allows you to switch DFS on or off, or to use the Auto setting when DFS
will only be used for exposure times less than or equal to 1.3 seconds.
ABOUT NOISE
Each image contains noise—dominantly sensor noise (mostly in the blue
channel) and amplifier noise. Small sensors cannot catch as much light as
large sensors and thus require more amplification, resulting in a lower signal-
to-noise ratio. The higher the amplification (and the ISO speed), the higher
the noise.
Noise is a random phenomenon. Therefore, subtracting two independent
noise sources from each other does not cancel out the noise. In this case,
subtraction actually has the same effect as adding both noise signals. This is
the reason why Dark Frame Subtraction does not cancel out stochastic noise
but rather increases it. Where DFS helps is with Hot Pixels because they are
not random.
Noise is reduced when averaging several images. Again, the reason is the
stochastic character of noise. Adding the signals from n images results in an
image signal of n-times the strength, but the strength of the noise signal
multiplies only by the square-root of n. So, the image-to-noise ratio improves
by the square-root of n. For example, when shooting a series of 16 shots and
averaging them, you end up with an image where the signal-to-noise ratio is
improved by a factor of √16 = 4.
4 See http://chdk.wikia.com/wiki/CameraFeatures.
39
4.3 Exposure