The PowerShot G15 may have a different sensor than the G12, but the lens has more of an impact.
Areas of focus look pretty good in JPEGs through ISO 200; then color noise and noise-reduction artifacts kick in. You can see how much detail you lose on the fur as well as the softness of the details on the tape measure. I wouldn't recommend going above ISO 1600.
This detail comes from an out-of-focus area of the previous image. You can see how the sharpening in the JPEG produces some contouring and graininess in the out-of-focus areas, but the raw trade-off is softness in the sharper areas.
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