You can reduce noise without lowering ISO by slightly overexposing your picture, especially if you shoot RAW.
From the Expose (to the) Right article at Luminous Landscape:
A 12 bit image is capable of recording
4,096 (2^12) discrete tonal values.
One would think that therefore each
F/Stop of the 5 stop range would be
able to record some 850 (4096 / 5) of
these steps. But, alas, this is not
the case. The way that it really works
is that the first (brightest) stop's
worth of data contains 2048 of these
steps — fully half of those available.
Why? Because CCD and CMOS chips are
linear devices. And, of course, each
F/Stop records half of the light of
the previous one, and therefore half
the remaining data space available.
This little table tells the tale.
Tone Level | Levels dedicated | Stops of difference
Brightest Tones | 2048 levels | Within first stop
Bright Tones | 1024 levels | Within second stop
Mid Tones | 512 levels | Within third stop
Dark Tones | 256 levels | Within fourth stop
Darkest Tones | 128 levels | Within fifth stop
The simple lesson to be learned from
this is to bias your exposures so that
the histogram is snugged up to the
right, but not to the point that the
highlights are blown. This can usually
be seen by the flashing alert on most
camera review screens. Just back off
so that the flashing stops.
Now of course when you look at the RAW
file in your favourite RAW processing
software, like Camera RAW, the image
will likely appear to be too light.
That's OK. Just use the available
sliders to change the brightness level
and contrast so that the data is
spread out appropriately and the image