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-A57 is within +/- 1/6 EV of indicated ISO across its entire ISO range.
Noise and Noise Reduction (JPEG)
As with Sony's current Alpha models, the A57 has three levels of noise reduction (high, normal and low), as opposed to the more limited choice between 'Auto' and 'Weak' that was found on the SLT-A55. Looking at the crops, all three settings give visually the same results up to ISO 1600.
Default noise performance of the A57 is nearly identical to that of the Sony NEX-5N from base ISO to ISO 3200. The A57 does a slightly better job of balancing noise suppression and image detail compared to its predecessor, the A55. As you can see in both the samples and the graph, the A57 performs very well in comparison to its peers, providing usable detail up to ISO 6400, and in a pinch, at ISO 12800.
ISO 25600 is only available as a multiple shot NR setting, in which the camera merges six separate exposures, averaging out the noise. This feature that requires you to be in JPEG-only mode. As you can see in the samples above, choosing this option minimizes noise without losing much detail compared to ISO 16000. For a look at this multi shot NR feature in a real-world example see our Sony SLT-A55 review.
RAW noise (ACR 7.2 Beta, noise reduction set to zero)
Looking at the raw file samples above, highlights just how much noise reduction is applied during JPEG processing, as chroma noise is visible even at base ISO. It is still well-controlled, however, and does not begin to obscure fine detail until ISO 1600. As you can see from the graph, the A57's noise performance is virtually identical to that of the A55 up through ISO 3200. This performance places it among the best of the A57's APS-C and Micro Four Thirds rivals.
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