The baseline sensitivity setting of ISO 160 delivers sharp, noise-free images
Moving up to ISO 200 there is no visible difference in quiality to ISO 160
At ISO 400 there is the merest hint of softening, although this is only visible when images are viewed at 100% or more
At ISO 800 this softening becomes more pronounced, although again only when the image is viewed at 100% or more
ISO 1600 is a touch softer than ISO 800, although overall image quality still holds up very well
IS0 3200 is where noise begind to become an issue with noise visible even when the image is viewed at less than actual size
ISO 6400 isn't the greatest, with very visible image degradation
The top setting of ISO 12,800 is really for emergencies only
Stepping outside the studio for a moment, it's important not to lose sight of how well the GX1 performs in a real-life situations. The image above was shot at ISO 800 and has been resized to fit a 15inch desktop monitor (click on the image to enlarge it). Viewed at this size the image remains perfectly sharp with good levels of detail and accurate colour reproduction. In fact, far from being problematic noise really isn't an issue at all.
The image of the bi-plane above was shot at ISO 1600 and, again, noise is far from being a problem. Indeed, overall image quality is very good.