I know that high ISOs tend to produce more noise, and some cameras' software can handle that noise better than others, but are there any other settings or conditions tha affect visible noise?
I'm using a micro-four-thirds camera (E-PL1) if it matters.
Keep the camera as cool as possible! High temperature increases the thermal noise in your images. That's why certain astrophotographers actively cool their camera!
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 looks "right".
A few options for reducing the noise other than lowering the iso or increasing the light:
If you have to work with available light and you are already at a high ISO then the only way is either to expose for a longer period at a lower ISO or use noise reduction software in post-production.
Bottom line, higher ISO will produce more noise and the only way to reduce the ISO is to have more light.
An expensive solution: buy a cam with better ISO performance (usually containing a bigger sensor).
When you are a lot in situations with little light and moving picture content, the investment might be worth it. I bough my D700 mainly for stage fotografie, since the light onstage often is quite dark.
The best way for most photographs is to simply get rid of the noise in post-production using software. In the end, it will be easier & cheaper than most of the other options described here.
Pretty much the only way to alter the noise is to lower the ISO. If you are afraid of having darker photos, you can look into getting a faster lens (lower f-stop), or take longer exposures (at the risk of having blurry photos).
Image stabilization (or vibration reduction), or a tripod/monopod, along with longer exposures will lower noise. Assuming your subject is sitting still, that is.
