Most people set their cameras to about 400 ISO (formerly called 400 ASA) in low light conditions. At that setting, your camera will be sensitive enough to capture subtle light anomalies.
Settings lower than 400 ISO don’t detect much in low light conditions.
Some ghost photographers recommend a setting of at least 1000 ISO to detect the maximum number of anomalies. I disagree, but it depends on the camera.
In low-light conditions (like at night), anything higher than 400 ISO can produce images so grainy, it’s difficult to figure out what I’m looking at.
Don’t take my word for it. Test ISO levels for each camera you intend to use. See what works best for your research.
In addition, I generally set my camera so the flash will always be triggered. If you’re taking photos of ghostly anomalies, you’ll usually get better results with a flash.
Unfortunately, the light from the flash can reflect your own breath. This is especially risky when the weather is chilly or the dew point is high.
Experiment so you know what your own breath looks like in photographs.
I believe that breath is the number one cause of false anomalies in ghost photos.
Be sure you know what it looks like with each of your cameras. Test that in a variety of weather and light conditions.
This is important: Your breath can show up in photos on warm summer evenings as well as on icy, bone-numbing winter nights.
Once you’re comfortable with ghost photography, experiment with other camera settings. I’ve seen interesting results in low light conditions when I’ve left the lens open for an extended period of time. I’m sure there are other settings you can test to see if they improve your results.