My take is this: I've used the boost on my 20D to great success, and
gotten great (usable) shots using it that I'd not have had if I stayed
at 1600.

That said, this boost is a small reason to pick the 20D and 30D over
the XT (and now XTi). The build quality and ergonomics far outweigh the
gain in ISO (or resolution in the case of the XTi). The 20D and 30D are
solid pieces of kit. If small size isn't the most important thing
you've got, you'd do well to grab the 30D. Its ergonomics alone make it
worthwhile. The build quality seals it. 3200 ISO is just a bonus (but
it does work and is great to have- hardly "fools gold").

That's me, but I love low light stuff...

Will

If you take an image shot at 800 ISO and one shot at 3200, and equalize
the lighting, you'll find that the 800 ISO image can retain as much
detail with less noise but the 3200 preserves contrast and colour
better in darkened areas.  However, I'm not sure that 3200 boosting is
the same as having a "real" 3200 ISO, as the image illumination
(compared to 1600 ISO) does not seem to suggest a full stop increase.
Though technically, everything beyond 100 or 200 ISO is "boosted" from
what I've read.  Cameras with sensors with large pixels probably do
better here.

That's a bit harsh, I think.  It is true that you can shoot at a lower
ISO and then push it in PP, and get results that will probably be as
good as the 3200 that may be provided on camera - at least, if you shoot
RAW.  But it can be useful to being able to see the image at the
"correct" exposure on the LCD - it's much easier to judge whether the
shot came off well enough in terms of sharpness etc.  That said, I
seldom use ISO 3200, because it's still way too noisy on my Pentax
*istDS (which doesn't *call* it a "boost", but as someone else says, in
some sense, anything above the native ISO of the sensor is just a boost
of one sort or another).

---------------
Marc Sabatella
marc@outsideshore.com

Music, art,  & educational materials
Featuring "A Jazz Improvisation Primer"
http://www.outsideshore.com/

I recall reading somewhere that if you look into a ISO3200 RAW file from a
Canon camera that you will find only even numbers suggesting the values are
simply doubled from a ISO 1600 exposure, 1 stop underexposed.  If this is
truly the case, then you can get pretty much identical results from your
Rebel XT by sliding the exposure compensation down and stop and pushing by
one stop in your RAW converter.

This works well for me with my XT when I am already at ISO1600 and
desperately want more shutter speed.  It's especially helpful when you are
in low light with a telephoto hand-held and weren't prepared.  You get the
added benefit of more headroom for highlights too that you can compress in.
If you use the noise reduction in the new Canon Digital Photo Professional
or Adobe Camera RAW, you can get a quite nice picture as long as you don't
expect to blow it up huge and way better than high-speed 35mm film in the
same circumstance.

I don't think ISO3200 is a bunch of pyrite but just a convenience that Canon
through in for the 10/20/30D cameras.

I'd guess there's a slight improvement over boosting RAW files, easy
enough to test, mostly it's just more convenient when you need shutter
speed for low light, motion or DOF for macros.

In early DSLRs it was reportedly a mathematical boost, but it newer
cameras, it is not.

There is a simple upper limit beyond which you won't gain
anything.  Each camera's electronics had a gain stage before
feeding the A/D, and the gain is expressed in electrons/DN
where DN is "data number,"   There is no point in having
gain above 1 electron.  Most DSLRs have that gain point
between the iso 800 and iso 1600 levels.  For example,
see Table 1, column E at:

 Procedures for Evaluating Digital Camera
 Sensor Noise, Dynamic Range, and Full Well Capacities;
 Canon 1D Mark II Analysis
 http://www.clarkvision.com/imagedetail/evaluation-1d2

Other sensors can be found at:
  http://www.clarkvision.com/imagedetail
and scrolling down to sensor analysis.  I'll be adding
several other cameras in the next few weeks.
For example, preliminary analysis of the Canon S70
7-megapixel camera shows its gain at minimum is about
2 electrons/DN at ISO 50, so the gain =1 at iso 100.

Minimum noise in cameras is around 4 electrons (called the
read noise), so at a gain =1, read noise is well digitized.
There is no need to go higher in a camera, as you can
get the same result in post processing and have a higher
dynamic range.

I've tabulated gain data of mine and from the net here:
http://www.clarkvision.com/imagedetail/digital.signal.to.noise
in Table 1.  The 30D has the same sensor as the 20D, and
probably the same gains.  The 20D gain is 1.5 at ISO 800, and
0.8 at iso 1600.  There is no advantage going above iso 1600
unless you want to not post-process images.

Amateur astronomers using these cameras have independently
found that iso 3200 has no advantage.  Most use ISO
1600 or 800.

Roger