There are actually a lot of factors that come into play here, but looking at
it in the most simple terms:
If you have a long exposure you are letting less light in (either because
there is less light available or you have incresed the f number).
If you have less light coming in the signal detected in dark areas of the
image will be closer to the noise floor in the detector (known as dark count
or dark current). This gives more noise in dark areas or otherwize you lose
the dynamic range in the image.
With longer exposures a greater signal builds up from the dark current this
limits the dynamic range available for the image signal so any errors
(noise) on the pixels represents a greater part of the of the dynamic range
available for the image.
There is more to it but these are the basic problems.
So in short you are right you do tend to get more noise with longer
Many cameras will employ noise reduction above a certain exposure time. This
is like taking an exposure without opening the shutter, it allows the camera
to better determine where the black level should be but it doesn't get round
the fact that you are left with less dynamic range available in the detector
to store the image in.
"Neil Bell" <att.net> wrote in message