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 SLT-A99 is within +/- 1/6 EV of indicated ISO across its entire ISO range.
Noise and Noise Reduction (JPEG)At its default 'Normal' setting the A99's JPEG output delivers very impressive noise performance throughout its ISO range. You can see from the graph that among its full frame rivals, the A99 meets or exceeds their suppression of chroma and luminance noise. And a look at the image samples here reveals an even more impressive story, as the A99 shows a more natural, less 'processed' look, avoiding the more obvious image artifacts seen in the Canon EOS 5D Mark III. Fine detail is retained impressively well up until ISO 6400, where image smearing starts to become rather noticeable. Yet it is only really at the highest ISOs of 12,800 and above that noise suppression significantly degrades image quality. And even at these sensitivities the files are still eminently usable for medium-sized print output.
RAW noise (ACR 7.3 beta, noise reduction set to zero)
The A99's overall raw file performance is in line with that of its full frame peers. Up through ISO 6400, there is little meaningful distinction between them in luminance noise. The A99 displays slightly less chroma noise than some of its peers at ISO 6400 and higher. Above ISO 6400, noise patterns become significantly more prominent. The A99 still manages, however, to maintain a degree of image detail that ranks it among the top performers in the full frame DSLR market.