Sony DSLR-A580 Review

Sony A580 Imatest Dynamic Range Analysis

We routinely use Norman Koren's excellent "Imatest" analysis program for quantitative, thoroughly objective analysis of digicam test images. I highly recommend it to our technically-oriented readers, as it's far and away the best, most comprehensive analysis program I've found to date.

My comments below are just brief observations of what we see in the Imatest Dynamic Range results. A full discussion of all the data Imatest produces is really beyond the scope of this review: Visit the Imatest web site for a full discussion of what the program measures, how it performs its computations, and how to interpret its output.

A key parameter in a digital camera is its Dynamic Range, the range of brightness that can be faithfully recorded. At the upper end of the tonal scale, dynamic range is dictated by the point at which the RGB data "saturates" at values of 255, 255, 255. At the lower end of the tonal scale, dynamic range is determined by the point at which there ceases to be any useful difference between adjacent tonal steps. Note the use of the qualifier "useful" in there: While it's tempting to evaluate dynamic range as the maximum number of tonal steps that can be discerned at all, that measure of dynamic range has very little relevance to real-world photography. What we care about as photographers is how much detail we can pull out of the shadows before image noise becomes too objectionable. This, of course, is a very subjective matter, and will vary with the application and even the subject matter in question. (Noise will be much more visible in subjects with large areas of flat tints and subtle shading than it would in subjects with strong, highly contrasting surface texture.)

What makes most sense then, is to specify useful dynamic range in terms of the point at which image noise reaches some agreed-upon threshold. To this end, Imatest computes a number of different dynamic range measurements, based on a variety of image noise thresholds. The noise thresholds are specified in terms of f-stops of equivalent luminance variation in the final image file, and dynamic range is computed for noise thresholds of 1.0 (low image quality), 0.5 (medium image quality), 0.25 (medium-high image quality) and 0.1 (high image quality). For most photographers and most applications, the noise thresholds of 0.5 and 0.25 f-stops are probably the most relevant to the production of acceptable-quality finished images, but many noise-sensitive shooters will insist on the 0.1 f-stop limit for their most critical work.

The image below shows the test results from Imatest for an in-camera JPEG file from the Sony A580 with a nominally-exposed density step target (Stouffer 4110), at default contrast and with its DRO and HDR settings turned off.

We got the best dynamic range scores from the Sony A580 at ISO 100. Here, we can see that the tone curve maintains excellent gradation in the highlights while the shadow end also trails-off quite nicely.  Total dynamic range was 10.6 f-stops, and at the highest quality level, dynamic range was 8.51 f-stops, the best High Quality Imatest score for an in-camera JPEG we have tested to date.

Processing the Sony A580's RAW (.ARW) files through Adobe Camera Raw (ACR) version 6.4 increased dynamic range by about 1.25 f-stops at the highest quality level (9.76 f-stops), compared to the in-camera JPEG (8.51 f-stops) while total dynamic range reported increased from 10.6 to 12.2 f-stops. These results were obtained by using ACR's automatic settings; slightly better results may be possible by manually tweaking from there, but we weren't able to improve on them. It's worth noting here that ACR's default noise reduction settings reduced overall noise (see the plot in the lower left-hand corner) relative to the levels in the in-camera JPEG, which would tend to boost the dynamic range numbers for the High Quality threshold. Also, the extreme highlight recovery being performed by ACR here would likely produce color errors in strong highlights of natural subjects.

Dynamic Range, the bottom line:

The net result was that the Sony A580's JPEGs showed excellent dynamic range scores, some of the best we've tested to date. RAW dynamic range performance with Adobe Camera RAW was also outstanding, producing a score that put it within the top three cameras. That's no surprise, since the top three happen to use the same or very similar 16-megapixel sensor.

To get some perspective, here's a summary of the Sony A580's dynamic range performance, and how it compares to other digital SLRs that we also have Imatest dynamic range data for. (Results are arranged in order of decreasing dynamic range at the "High" quality level.):

Note that this test is repeatable to within 1/3 EV according to the Imatest website, so differences of less than 0.33 can be ignored.

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