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). Measured ISO from the SLT A55 is within +/- 1/6 EV of indicated ISO across its entire ISO range.
Noise and Noise Reduction (JPEG)
As you can see from these images, it is only at ISO 800 that visible noise starts to creep into the A55's JPEG output, but not to the extent that it is problematic in normal-sized prints. Even at ISO 3200 the A55 is capable of producing very good image quality which is at least on a par with the best of the other cameras in its class. By ISO 12,800 image quality has dropped significantly but high-contrast detail remains impressively well-defined, making this setting genuinely useful, rather than for 'emergency use only'.
We've also included an example of the A55's 'Multi-shot NR' ISO 25,600 mode, and as you can see, image quality is slightly higher using this function than it is at ISO 12,800. Of its competitors, the A55 offers the best performance in our measured noise tests, and it is interesting to see how much higher measured noise levels are from the Panasonic Lumix DMC-G2, which uses a smaller Four Thirds sensor.
With noise reduction set to 'auto' (default) the Alpha 55 matches the NEX-5 noise performance until ISO 3200, beyond which it manages to produce slightly less chroma and gray noise, ultimately managing almost a 1EV improvement over the NEX's 14MP CMOS sensor. When both cameras are set to NR 'weak' the gap is narrower. The difference between the visual output of these two settings is subtle, to say the least, until the ISO is increased to 3200 and higher, at which point 'auto' gives smoother but slightly less detailed results.
RAW noise (ACR 6.2 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 normalise 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.
All the cameras show signs of noise even at the lowest sensitivity settings (remember these samples have noise reduction turned off) but up to ISO 400 the difference between them is pretty marginal. At higher ISOs the SLT-A55 is again pretty much on the same level as its APS-C competitors but visibly better than the Micro Four Thirds Panasonic G2.