Adrian Price-Whelan and I figured out today the dependence of the RAW-file median levels and noise on exposure time and ISO for the Canon Digital Rebel XSi cameras we have been testing. We are trying to build a noise models for the pixels—on average and individually—as a function of illumination and camera settings. I have always wondered if changing the ISO setting on a digital camera changes the bias or anything else about the readout electronics, or whether it just changes the way the camera converts the RAW file into a human-viewable JPEG. We started to get some evidence today that it actually changes the bias on the CCD, but we haven't built a noise model yet. That's the project for the coming week.