Canon Digital Rebel Questions

One thing that still bothers me about these digital cameras is how they
assign film speed to them. The way I understand it a ccd does not have
variable sensitivity. How can you state that the film speed is say ISO400
and you can also select ISO1600? How is this accomplished, by binning, or
some other way? With film, fast films are coarse grained and slow films are
fine grained. What is the base film speed for the 300Dm Canon D20 etc? I
see they use ISO100 for flash. Is this the base speed for a digital?

On Fri, 28 Jan 2005 18:19:11 GMT, Ed Majden wrote:

One thing that still bothers me about these digital cameras is how they
assign film speed to them. The way I understand it a ccd does not have
variable sensitivity. How can you state that the film speed is say ISO400
and you can also select ISO1600? How is this accomplished, by binning, or
some other way? With film, fast films are coarse grained and slow films are
fine grained. What is the base film speed for the 300Dm Canon D20 etc? I
see they use ISO100 for flash. Is this the base speed for a digital?

The ISO equivalents are assigned by empirical comparison to film; that
is, a digital camera set to ISO 100 will deliver an image similar to a
film camera using ISO 100 film, given equivalent exposure settings. In
the case of a digital camera, the adjustable ISO setting is accomplished
by changing the gain of the output amplifier (that is, the amplifier
that boosts the signal as it is read from the sensor). There is no such
thing as a "base speed". with a CCD or CMOS sensor.

When you increase the gain, you are also amplifying noise. In the case
of the 300D, the optimal ISO setting appears to be 200. At that setting
you get the best S/N ratio. So for astronomical imaging, it is better to
work with a low ISO setting and a longer exposure if your tracking can
support it.

_________________________________________________

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

in article , Chris L Peterson at
wrote on 1/28/05 10:47:

When you increase the gain, you are also amplifying noise. In the case
of the 300D, the optimal ISO setting appears to be 200. At that setting
you get the best S/N ratio. So for astronomical imaging, it is better to
work with a low ISO setting and a longer exposure if your tracking can
support it.

_________________________________________________

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

Hello Chris:

What you say is correct for deep sky imaging. I want to try one for
meteor spectroscopy where the duration of a meteor track is rather short.
Normally I use TriX Pan and this will record a spectrum of a meteor that is
~ -2.0 magnitude or brighter. I wonder if the digital will respond the same
as there is no time for image build up! The narrow spectral range is also
troublesome but it should be okay centered on the visual region on the
spectrum. I just bought a Canon D20 which has better noise characteristics
than the Rebel 300D and D10. One could remove the IR blocking filter but I
don't want to void warranty just yet! The blue end response is also
limited.

Ed Majden
AMS Meteor Spectroscopy

On Fri, 28 Jan 2005 19:12:04 GMT, Ed Majden wrote:

Hello Chris:

What you say is correct for deep sky imaging. I want to try one for
meteor spectroscopy where the duration of a meteor track is rather short.
Normally I use TriX Pan and this will record a spectrum of a meteor that is
~ -2.0 magnitude or brighter. I wonder if the digital will respond the same
as there is no time for image build up! The narrow spectral range is also
troublesome but it should be okay centered on the visual region on the
spectrum. I just bought a Canon D20 which has better noise characteristics
than the Rebel 300D and D10. One could remove the IR blocking filter but I
don't want to void warranty just yet! The blue end response is also
limited.

The ISO values should be equivalent for short exposures. Where the ISO
equivalents fall apart is for long exposures, where the digital camera
will continue to deliver the same sensitivity, but the film's
sensitivity will drop because of reciprocity failure.

The sensor in the camera has at least ten times the quantum efficiency
of film. You do have some sensitivity loss because of the color sensor
in the Canon, however. The little filters used over the pixels aren't
the greatest in the world. It's a shame you can't get the Canon cameras
with B&W detectors- that would make them very attractive for
astronomical imaging. Still, I think you can expect that your 20D will
prove at least as sensitive as the Tri-X for your meteor spectroscopy.
I've found that my 300D does a better job catching meteors than
Ektachrome, both at ISO 200.

_________________________________________________

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