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 G15'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. The Coolpix P7700's measured sensitivity was the same as indicated.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
At lower ISOs, the PowerShot G15 and Coolpix P7700 perform similarly, with very little noise to speak of. At ISO 400, noise reduction is just beginning to affect detail on the P7700. Things get quite a bit worse for the Nikon at ISO 1600, and the Canon maintains its advantage through ISO 3200. The 'Queen' has almost no detail left at ISO 6400 on the Coolpix P7700, and not a ton more on the PowerShot G15. Through all this, there's nearly no chroma noise to be found.
Raw noise (ACR 7.4 RC noise reduction set to zero)
There's very little luminance or chroma noise on either camera at lower ISOs. At ISO 400 and 800, chroma noise becomes quite visible on both cameras, with very similar-looking crops. Noise starts to take out detail at ISO 1600, especially on the Coolpix P7700. When we reach ISO 3200, there's so much chroma noise that the 'Queen' is starting to disappear, and things only get worse at ISO 6400. Keep in mind that these results can be improved on - quite significantly in some cases - by adjusting noise reduction in your favorite Raw editor.