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 NEX-6's measured sensitivities are about 1/3 stop higher than indicated (i.e. images are fractionally brighter than expected for any given set of exposure values), which is within the tolerance allowed by the ISO specification.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The NEX-6 has two noise reduction settings: low and normal. As you can see from the graph, measured noise levels stay on the same course through ISO 400, after which they diverge. Visible differences don't start to show up until around ISO 3200, at which point details become much more 'blotchy' at the low NR setting.
Raw noise (ACR 7.3 noise reduction set to zero)
Here we look at the raw files processed through Adobe Camera Raw (in this case version 7.3). Images are brightness matched and processed with all noise reduction options set to zero. Adobe does a degree of noise reduction even when the user-controlled NR is turned off.
The amount of NR applied 'under the hood' is not high, but it does vary by camera (Adobe is attempting to normalize output across different sensors), so inevitably we are still looking at a balance of noise and noise reduction, rather than pure noise levels. However, the use of the most popular third-party raw converter is intended to give a photographically relevant result, rather than simply comparing sensor performance in an abstract manner.
At lower ISOs, the NEX-6 shows very little chroma noise. This noise becomes much more noticeable at ISO 1600, though detail remains intact until ISO 6400. At this point you'll see that the NEX-6 is again slightly better than the Panasonic GX1, though not nearly as clean as the Fuji X-E1.