Sony A55 Imatest Results

Sony A55 Imatest Results

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 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.

Here's some of the results produced by Imatest for the Sony A55:

In typical Sony fashion, the A55 showed generally good color accuracy, with only minor oversaturation of bright reds and deep blues, and to a lesser extent, some oranges, greens, purples and browns. Hue accuracy was also generally good, with the typical (typical meaning significant) cyan shift we see in most cameras we test, and also a little shift in other colors such as aqua, red, orange and yellow. Average saturation for the Sony A55 was 111.2% (oversaturated by 11.2%). Average "delta-C" color error was 4.59 after correction for saturation, while not quite as good as the Canon T2i and Nikon D90, still places the A55 in the top tier. (Delta-C is the same as the more commonly referred to delta-E, but delta-C takes into account only color differences, ignoring luminance variation.) All in all, a very good color response for this class of camera. Mouse over the links below the illustration above to compare results with the others in this group.

Most SLRs/SLDs dramatically oversaturate colors when shooting in Adobe RGB mode, and the Sony A55 follows suit. Average saturation was 116.8% which is significantly higher than the sRGB result, and average saturation-corrected hue error was 4.96 "delta-C" units, just slightly less accurate than sRGB. A typical result. Again, mouse over the links below the illustration above to compare results with the same group.

This image shows how the Sony A55 actually rendered the colors of the MacBeth chart, compared to a numerically ideal treatment. In each color swatch, the outer perimeter shows the color as actually captured by the camera, the inner square shows the numerically correct color after correcting for the luminance of the photographed chart (as determined by a second-order curve fit to the values of the gray swatches), and the small rectangle inside the inner square shows the numerically correct color, without the luminance correction. This image shows the generally very good hue and saturation accuracy. Most colors are rendered reasonably close to the correct brightness, the most obvious exceptions being the red and cyan swatches, which are noticeably brighter than the luminance-corrected versions. Overall, though, the Sony A55's color is quite good.

The bottom row of the chart shows exaggerated white balance errors, revealing that the camera produced a slightly cool color balance when using Custom white balance, with a WhiBal card as the reference.
 

As always, there's more in this particular graph than we really have room to go into here. (Also note that this set of plots has also changed a few revisions back in Imatest. Some of the plots that were once shown here are now shown in other Imatest output. Since we largely focus on the Noise Spectrum plot, we only show the graphic above, which includes that plot.)

In comparing these graphs with those from competing cameras, we've found that the Noise Spectrum graph at lower right is often the most important. Cameras that manage to shift their noise spectrum to higher frequencies have much finer-grained noise structures, making their noise less visually objectionable. In the graph above, this would show up as a noise spectrum curve that remained higher on the right side, representing higher noise frequencies.

Above, we see the results at ISO 100. The luminance curve is reasonably flat, but the red, blue and green channels exhibit much higher noise values at lower frequencies. While overall noise levels are very low, the dominance of low frequencies in the color channels points to some low-level blotchiness. Some chroma noise blotches can be seen in darker midtone and shadows of the Sony A55's ISO 100 images, but only with extreme pixel-peeping.

Above is the same set of noise data at ISO 1,600. Here, the overall Noise Spectrum graph is shifted a fair bit toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns. There's much less difference between the color channels in this plot, with green and blue channels tracking luminance noise very closely, and only the red-channel noise being a bit elevated at the lowest frequencies, no doubt due to noise reduction processing. This translates to ISO 1,600 images with pretty low levels of chroma noise, while luminance noise is lower or competitive to other models in its class.

Here's the same set of noise data at ISO 3,200. Here again, the Noise Spectrum graph is shifted even more toward the left-hand side, further coarsening the "grain" of the image noise. Overall noise levels are again excellent, though.

This chart compares the Sony A55's noise performance over a range of ISOs against that of a few other SLR and SLD cameras. While we continue to show noise plots of this sort because readers ask for them, we each time point out that the noise magnitude is only a small part of the story, the grain pattern being much more important, not to mention what the camera does to achieve a given noise level. (Some cameras obliterate subject detail along with noise, so the camera with the lowest noise levels on MacBeth chart swatches may not necessarily produce the most appealing images.)

Here, the Sony A55's luminance noise is equal to, or lower than the rest of this group, across the entire range of ISOs. However, close visual examination of key test images reveals that the A55 appears to be applying slightly stronger noise processing then some of the others in the group. This leads to a slight softness in its high ISO images, but not nearly to the extend we saw with previous generations of Alphas. Sony's recent advances in noise reduction techniques continue to work well to maintain very good levels of detail in A55 high ISO files while keeping noise in check. However, compared to the NEX-5 and A560, the A55 is at a distinct disadvantage. This is because the sensor's gain has likely been boosted to compensate for the loss of light caused by the translucent mirror, as well as the reduced photosite size as a result of the higher pixel count. Noise reduction has to work a little harder in the A55 to achieve similar noise levels. Still, all things considered, the Sony A55's high ISO performance is quite good, though we wish Sony would have provided more flexibility in noise reduction settings (there are only two: "Weak" and "Auto", with the above results taken at the default "Auto" setting).

 

Sony A55 Dynamic Range Analysis

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 A55 with a nominally-exposed density step target (Stouffer 4110), and its settings such as DRO and HDR turned off.

Here, we can see that the tone curve maintains excellent gradation in the highlights though the shadow end doesn't trail-off quite as nicely.  Still, these results are very much on par with those from the best SLRs with APS-C sensors, with an Imatest score at the highest quality level in the top 10 cameras we have tested.

Processing the Sony A55's RAW (.ARW) files through Adobe Camera Raw (ACR) version 6.2 increased dynamic range by about 1.4 f-stops at the highest quality level (9.16 f-stops), compared to the in-camera JPEG (7.74 f-stops) while total dynamic range reported increased from 9.8 to 11.1 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 A55 showed excellent dynamic range, performing at least as well as the NEX-5 (within the margin of error). What's more, it's really in the upper percentile ranking of all the cameras we've tested with APS-C sensors. A very impressive performance.

To get some perspective, here's a summary of the Sony A55'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.

Sony A55 Resolution Chart Test Results

The chart above shows consolidated results from spatial frequency response measurements in both the horizontal and vertical axes. The "MTF 50" numbers tend to correlate best with visual perceptions of sharpness, so those are what we focus on here. The uncorrected resolution figures are 2,280 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 2,059 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 2,169 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius decreased the horizontal number slightly, resulting in an average of 2,139 LW/PH. The very similar standardized versus uncorrected results indicate the Sony A55 is applying an appropriate amount of in-camera sharpening to its JPEGs at default settings.

To see what's going on, refer to the plots below, which show the actual edge profiles for both horizontal and vertical edges, in both their original and corrected forms. Here, you can see that an almost ideal level of in-camera sharpening is applied in both the horizontal and vertical directions. Imatest reports a modest 1.56% of oversharpening in the horizontal direction, and only 0.03% oversharpening in the vertical direction. As usual, though, if you turn the camera's sharpening down a little, you'll get better results when sharpening in-camera JPEGs post-exposure in Adobe Photoshop or other image editing software, but the A55's in-camera sharpening is better controlled than we see on many cameras.

Note: We don't feature SFR-based LW/PH resolution numbers more prominently in our reviews (eg, outside the Imatest pages) because we've found that they're *very* sensitive to minor differences in in-camera image processing. Relatively small changes in the amount of in-camera sharpening can have a large effect on the resulting resolution numbers. Imatest attempts to compensate for this by adjusting to a "standard" sharpening, but this approach can't completely undo what happens inside the cameras, and so often gives inconsistent results. Sometimes the "standardized" sharpening happens to just match the shape of the edge profile with the in-camera sharpening applied, and you'll get wildly high results. At other times, it will tend to correct in the opposite direction. Unfortunately, ignoring the in-camera sharpening entirely can result in even greater discrepancies, particularly between models from different manufacturers. Turning off sharpening in the camera may or may not fully eliminate the sharpening, so simply turning off sharpening in the camera JPEGs isn't a reliable solution. It also wouldn't be the way most people shoot the cameras. We could process RAW files with no sharpening (as shown below), but then that'd only suit the people working primarily or exclusively from RAW, and would open another can of worms as to what RAW converter was used, etc, etc.

The bottom line is that numbers for resolution only take you so far. Detail handling and edge acuity are very complex issues; ones that don't easily boil down to a single number. The best approach is to simply look look at the broad array of standardized test shots we take with each camera, to the point of downloading and printing them with whatever processing you'd use if you owned the camera and shot with it. See how the differences stack up for you visually, and make your decision on that, rather than on abstract resolution numbers.