At base ISO and up to ISO400 the Sigma delivers some outstanding resolution. Even at ISO400 though, the Quattro starts to show signs of weakness. I am fairly certain that most buyers aren’t looking at the DP1Q for its ability to handle noise at high ISO. The Sigma shines at ISO100 and ISO200 and that’s where most will use it, but there are going to be times when you may need to stretch it up to 800.
It is not a complete loss if you need to move up to 1600, but I would probably reserve 1600 for black and white developing. The images below were converted from raw X3F files with the default settings in Sigma Photo Pro 6.2.
In the end, I’m only going to shoot the Quattro at 100, 200 and 400 ISO and will save ISO800 for emergencies.