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-F3'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
Here, we're comparing the NEX-F3 against three of its rivals, the Olympus PEN E-PM1, the Panasonic Lumix DMC-GF3 and the Nikon V1. You can override these comparisons if you prefer.
In JPEG mode, the the F3 stands up very well against its competition at low ISO settings. Detail is high and it's only at ISO 800 that noise begins to be visible in areas of pain tone, and even then, only just. There's a noticeable drop in resolution at ISO 6400, but the F3 still gives more satisfying image quality at this setting than the other cameras in this comparison. The CX-format Nikon V1, especially, really struggles, and at default NR settings, the Panasonic GF3 gives extremely mushy output beyond ISO 1600. Even with its noise reduction system turned to 'low' the F3 still gives impressively clean output compared to the other cameras in this comparison.
Raw noise (ACR 7.2 noise reduction set to zero)
Here we look at the raw files processed through Adobe Camera Raw (in this case version 7.2). 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.
In Raw mode, with noise reduction turned off in Adobe Camera Raw, the gap between these four models (remember you can select your own comparison cameras if you wish) narrows at low ISOs, to the point where at a pixel level, there's not much to choose between them, visually, until ISO 1600. A look at the graph view shows us that measured noise from these four cameras is indeed very similar until ISO 3200. Beyond this point, the Olympus E-PM1 and Panasonic GF3 get visibly noisier, whereas the F3's measured noise stays more or less the same from ISO 3200 to 6400. If you pull up the Sony NEX-5N in the comparison window, you'll see that it has a measured noise curve that continues upwards, which strongly suggests that some in-camera noise reduction is applied to the F3's Raw files.
At ISO 3200 and above, the F3 delivers more satisfying image quality than its nearest mirrorless competitors, and as we'd expect from a camera with such a large sensor, it compares very well with some of the best low-end and mid-range DSLRs, as well.