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 DSLR-A580 is roughly 1/3 stop higher than indicated across the ISO range - so ISO 100 indicated = ISO 125 measured, etc. This is not unusual but interesting in so far that the SLT-A55, which we tested in August 2010, reported its ISO very accurately, despite using, as far as we are aware, an imaging pipeline that is identical to the DSLR-A580's. As you can read on this page of our A55 review the camera only allows approximately 70% of the light that strikes it straight through to the sensor. The remaining proportion is reflected upwards, by a fixed, translucent mirror, to the AF sensors. This equates to a roughly 1/3 EV reduction in light transmission for live view/image capture but means the AF sensor is always receiving light.
It can therefore be concluded that the 'light loss' caused by the translucent mirror is responsible for the slightly darker exposures (at identical settings) on the A55. In (semi-) automatic exposure modes the camera's metering compensates for this by typically selecting a 1/3 of a stop faster shutter speed or larger aperture, resulting in exposures that are equal in brightness to the A580. In practical terms this means that the DSLR-A580 has approximately 1/3 of a stop 'noise advantage' over the A55, something you'll struggle to detect on anything but the largest magnifications.
Noise and Noise Reduction (JPEG)
Both the sample images and the graphs show that the Sony DSLR-A580's performance is extremely close to its stablemate the SLT-A55. There are very slight differences in the measured noise results (the A580 measures slightly lower black noise levels but slightly higher chroma and gray noise levels at ISO 6400 and 12800) but it's nearly impossible to say if those differences stem from modified image processing on the A55 (due to the fact that light has to pass through a translucent mirror before it hits the sensor) or simply sample variation. It's safe to say though that in your day-to-day photography you'd struggle to find any differences between the output of the two cameras.
When shooting with the A580 noise is really not much of an issue before you get to ISO 1600 and even then it would not be problematic in normal-sized prints. Even at ISO 3200 the A580 is capable of producing very good image quality which is on par with the best cameras in its class. At the very highest settings, ISO 6400 and 12800, both noise and low-contrast detail blurring caused by noise reduction become much more intrusive but high-contrast detail remains well-defined. As a result the highest ISO settings remain useful for at least the family album or web-use.
RAW noise (ACR 6.4 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.
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 1600 the difference between them is practically not visible. At the very highest settings the two Sonys appear marginally noisier than the Canon and Nikon and the graphs confirm this. Again, in practice these difference will be pretty much irrelevant though.