Know your camera character first.
Some ppl consider their camera's ISO 400 is ok , some not. .
But to grab a fast shutter speed for freezing moment ppl tend to go to that
ISO speed outdoor [or even ISO 800

]
If you are shooting night scenery ... get the lowest ISO you can ... my S2
Pro ISO 100 is just amazing ... almost 0 noise during 32" exposure.
Again , if the source of lighting you have is the ambient .. crank it up to
the highest acceptable ISO ..
I've seen 350D ISO 800 produced a very ugly grain on skin .. I don't know
its the image compression
or the photographer just started to play with his new toy.
Digital brings a totally new experience in photography. Before your variable
are only 2: Shutter speed and lens aperture only.
Now .. shutter speed, lens aperture and sensor sensitivity
[well . .you can argue you can swap ISO 50 film for scenery, then roll it
and change it to ISO 800 when you shoot sport ]
They just get more interesting by the day
Btw .. your IS lens is good for shooting at low speed (1/30" or lower

)
and you should still get quite a sharp shot.
=bob=
"Celcius" <> wrote in message
news:sr6dnVHCCqvKLyvfRVn-...
> Hi all!
> I finally got my Canon 350Dwith the 17-85mm IS USM lens.
> I guess I know that with poor light condition and the impossibility of
> using
> flash, such as in a museum, I can up the ISO to 800 and even 1600 with
> some
> noise. However, I don't understand why and when I should up the ISO when
> photographing outside by a clear day. I saw in different posts people
> shooting at 400 ISO.... Could someone please enlighten me on this matter?
> Thanks,
> Marcel
>
>