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-A35 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 samples show that it is only at ISO 800 that noise starts to become visible in the A35's JPEG output. That said the camera appears to be using higher levels of noise reduction than some of the rivals, resulting in more loss of detail at higher sensitivities. To spot these differences you would have to view these images at a fairly large magnification though. At ISO 3200 the image is still very clean but fine detail is clearly being lost. At the two highest ISO settings image quality is dropping further but while you'd probably not want to make any large prints from ISO 6400 and 12800 output it is still usable for smaller viewing sizes.
The A35's measured noise levels are in line with the competition but the 'cleanliness' is partly paid for with a loss of fine detail at higher sensitivities. The camera comes with only two noise reduction settings. If you switch from 'Auto' (default) to 'Weak' the difference is measurable from ISO 400 but only visible once get past ISO 1600. However, even the 'Weak' setting still applies comparatively strong levels of noise reduction. So, if you would prefer some more control over your noise reduction your best bet is to shoot raw and apply your own NR recipe in the conversion process.
We've also noticed that, despite the in theory virtually identical sensors, the A35's measured noise levels are a touch higher than its sister model SLT-A55's. Sony told us that, due to the minor redesign of the sensor, this was to be expected. That said, in real life the difference will be negligeable for most photographers.
RAW noise (ACR 6.5 beta 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.
Converting the A35 images in Adobe Camera RAW 6.5 beta confirms the suspicion that we had when examining the camera's JPEG output. The Sony sensor is at all sensitivity settings producing slightly higher noise levels than the competition. At base ISO the difference is not noticeable (only measurable) but as you go up the sensitivity scale the gap widens. This explains why the A35's JPEG output requires slightly higher doses of noise reduction in order to keep noise on levels that are comparable to the competition.