The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with ISO 12232:2006, the standard used by camera manufacturers.
By our tests, the G1 X's measured sensitivities are about 1/3 stop lower than indicated across the ISO range (i.e. images are fractionally darker than expected for any given set of exposure values). A discrepancy this small has little practical impact in real world use.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
At its default 'Standard' noise reduction setting, across the ISO range, the Canon G1 X captures JPEG images that show a remarkably good mix of noise reduction and detail retention. Up to ISO 1600 you pretty much don't need to worry about noise, with only some smearing of fine low-contrast details but very good overall and high-contrast detail.
At ISO 3200 and higher the loss of fine detail is starting to become more obvious, albeit initially only at a 100% magnification. Chroma noise is remarkably well under control but makes a first visible appearance at ISO 6400, when you can spot purple traces in plain-colored areas. That said, these traces pretty much disappear if you reduce the viewing size to the dimensions of a 24 inch screen, so there is no need to worry if you don't print or otherwise output very large formats.
Chroma noise gets more intrusive at the very highest ISO setting, occasionally in a banding pattern, but again even these highest ISO images retain good saturation and high-contrast detail and are therefore perfectly usable at smaller output sizes or for web display.
All in all the PowerShot G1 X yields better high-ISO results, with more detail and more pleasant rendering of noise, than the current line of APS-C Canon DSLRs, such as the EOS 7D or 600D. The fact that the raw output of these cameras is near identical this can be attributed to improvements in the JPEG engine.
The G1 X gives you some control over high-ISO noise reduction - you switch from 'Standard' to either "Low' or 'High' - but unfortunately you loose this control as soon as you switch to raw or raw+JPEG shooting. In those modes noise reduction is fixed at the 'Standard' setting.
Raw noise (ACR 6.7 Beta noise reduction set to zero)
As we've seen above the G1 X's JPEG output is very good in terms of both measured noise and retention of detail. That is only partly due to the Digic 5 processor's JPEG engine doing a very good job. In addition the camera performs very well at the sensor and raw capture level. Both chroma and luminance noise levels are low for this class of camera and roughly in line with the enthusiast DSLR Canon EOS 7D, the sensor of which is closely-related to the G1 X's.