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). We found that measured ISO from the Sony NEX-7 is roughly 1/3 stop higher than indicated across the ISO range - so ISO 100 indicated = ISO 125 measured.
Noise and Noise Reduction (JPEG)The NEX-7's high pixel count inevitably means more noise at the pixel level, and Sony has countered this in its JPEG processing by applying fairly aggressive noise reduction, especially at higher ISOs. This can be seen in our graphs, which give almost the same pixel-level noise for the NEX-7 and NEX-5N. The numbers don't tell you anything about detail retention, though, but looking at the crops we can see that the overall image detail recorded at high ISOs is roughly the same too.
The NEX-7 produces clean, highly detailed output at low ISOs (100-400), with low contrast detail progressively degrading as ISO in increased further, although 3200 is still very usable. ISO 6400 shows a sudden drop in mid-contrast detail, but the image is still recognizable. As usual the very top ISO settings - 12800 and 16000 - really aren't pretty, with almost all detail obliterated, but to be fair no other APS-C camera is likely to do much better here.
Compared to the Micro Four Thirds flagships - the Panasonic Lumix DMC-GH2 and the Olympus PEN E-PL3 - the NEX-7 performs visibly better at the very highest ISOs, showing the advantage of its larger sensor. This becomes very apparent at ISO 3200 and higher, at which point the E-P3 and GH2 are both struggling to suppress noise while retaining colour saturation and low-contrast detail.
RAW noise (ACR 6.6, noise reduction set to zero)
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 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.
This comparison, with NR turned to zero, is intended to give an impression of the underlying image data, rather than represent typical output. The NEX-7 gives noticeably gritty output as low as ISO 400, and noise starts to become obviously destructive of image detail by ISO 1600. Above this the image degrades rapidly, indicating just how hard the camera's NR system has to work to deliver recognizable output at the top settings.
The graphs confirm that the NEX-7's pixel-level noise is higher than the NEX-5N's, and indeed about the same as the GH2's. They also show the advantage the NEX-7 has over the SLT-A65 and SLT-A77, with measurably lower noise at all ISOs; this is due to fixed semi-transparent mirror in the SLTs that deflects light to the AF system, meaning that the NEX-7's sensor receives more light at any given ISO.
The NEX-7's graphs also show a clear downwards kink at ISO 6400, suggesting perhaps a degree of noise suppression in the RAW files themselves at the very highest ISOs.