On Sat, 14 Jul 2007 06:48:17 -0700, "
wrote:

I understand the concept of "ISO" within the context of camera film
what I don't understandis the meaning in the context of a digital
camera. What occurs when the ISO setting is changed (increased to
higher speeds) assuming that one is in Manual Mode?

Each pixel of the sensor collects photons and converts them to
electrons. During readout, the charge in each pixel (basically, the
number of electrons) is converted to a voltage, and that is then
amplified and converted to a digital value. The ISO setting determines
the amount of amplification applied.

For the most part, changing the gain (ISO setting) has very little
effect on the image data. That's because in most cameras the gain stage
is after the readout stage, and noise gets amplified to the same degree
as signal. In other words, the S/N remains the same. The actual
sensitivity of a digital camera is determined by the choice of sensor,
and not by the choice of ISO setting.

The ISO setting is useful for terrestrial imaging to help ensure that
the limited dynamic range of a JPEG file is utilized efficiently. For
most astronomical imaging (normally, dim targets), ISO settings are not
useful- especially if you have a raw mode file output. You will get the
same results using a high or low ISO setting. Actually, there's
generally one particular setting that gives the best S/N- usually one of
the lower settings, 100 or 200.

Astronomical cameras usually have their gain set a fixed value designed
to just produce a saturated output signal when a pixel is holding its
maximum number of electrons. Since that's usually about one count for
every two electrons, some cameras have the ability to change their gain
to achieve additional sensitivity when the overall source is
particularly dim. There's no point, however, to having the gain exceed
one count per electron.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com