Sony DSLR-A350 Imatest Results

Sony A350 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 A350:

The Sony A350 showed very good color accuracy, as well as fairly accurate saturation levels. Hue accuracy was quite good, with most of the hue shift occurring in the cyans, blues and reds. Average saturation was 103.8% (3.8% oversaturated) and average "delta-C" color error was only 5.2 after correction for saturation, which is also very good. (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.) Overall, excellent color accuracy for an SLR, especially a consumer oriented model. (Most consumer-oriented SLRs produce more oversaturated color. Many consumers prefer this to the more accurate color of the A350. If you like less-"punchy" color, the Sony A350 would be a good choice.) Mouse over the links below the illustration above to compare results with other recent models.

As is true of most SLRs, when using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Sony A350 delivers more highly saturated color, with an average saturation of 108.5% and average saturation-corrected hue error of 5.38 "delta-C" units. Again, mouse over the links below the illustration above to compare results with other recent SLRs.

This image shows how the Sony A350 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 ideal 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 ideal color, without any luminance correction. This image shows the excellent hue accuracy, as well as a contrast curve that results in slight overexposure at the ends of the tonal scale. Overall though, the colors are impressively close to their "correct" values.

There's a lot in this particular graph, as always, a lot more than we have room to go into here. (This set of plots has also changed a little in the more recent versions of Imatest. Some of the plots that were shown here previously are now shown in other Imatest output. Since we largely focus on the Noise Spectrum plot, we'll only show the graphic above, which includes that plot.)

In comparing these graphs with those from competing cameras, I've found that the Noise Spectrum graph at lower right is 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. The champion at this was the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs.

The Sony A350 does a good job of keeping a fair of the luminance noise energy at high frequencies; what little low-ISO image noise that's there is more fine-grained as a result. The red and blue channels are a little noisier than the others, but that's not at all unusual , and not visible even when closely inspecting low ISO shots. (Unless you look at the individual color channels, where you can see some blotches in the red channel in flat tints of certain colors.)

What's a little odd here though, are the spikes that occur in the Noise Spectrum plot every 0.125 or 1/8 cycles per pixel. The source of the noise spikes appears to be the lighter pixels along the edges of the 8x8 pixel blocks used in the A350's JPEG encoding. We first saw this phenomenon with the Sony A200 and it appears the A350 is similar, though the spikes are not quite as pronounced. To illustrate, below are modified crops from the Sony A350 (left) and Nikon D60 (right). After cranking up the contrast and magnifying by 200% (overall, a pretty significant manipulation, these artifacts aren't nearly as visible in the unaltered image), the abrupt edges of the 8x8 blocks in the A350 crop can be easily seen. Block edges cannot be seen in the Nikon D60 crop, nor can they been seen in converted A350 RAW files. Interestingly, "normal" JPEG compression reduces the size of the spikes, compared to "fine," rather than increasing them. This might be an issue, but the overall noise levels are so low that the noise spikes aren't really visible at ISO 100 under normal conditions. Bottom line, the Sony A350 noise levels are low at low ISO.

Here's the same set of noise data at ISO 3,200. Here we can see there is a shift towards the left of the graph, indicating a much courser grain structure to the noise. And indeed, when ISO 3,200 images from the A350 are closely examined, this effect can be easily seen. The red channel also has significantly more noise at lower frequencies than the other channels, though at higher frequencies the noise in both the red and green channels are much closer to the others, compared to the results at low ISOs. Visually, red-channel noise has a very distinct, prominent grain structure, more so than that in the blue and green channels. As a result, you'll tend to see red blotches more so than other colors in shadow areas at 3,200 ISO.

This chart compares the Sony A350's noise performance over a range of ISOs against that of other cameras. While I continue to show noise plots of this sort because readers ask for them, I each time point out that the noise magnitude is only a small part of the story, the grain pattern being much more important. Here, we can see that the Sony A350's luminance noise starts out quite low, comparable to most of the competition, then rises at roughly the same rate as the others to ISO 400. It continues to rise to ISO 800, where some of the competition's high ISO NR has already kicked in. (Decreasing noise levels, but at the expense of some fine detail.) Then we see A350 noise levels drop dramatically at ISO 1600, where its more aggressive high ISO NR kicks in. It then rises again, ending up at level much lower than the D60, the only other camera in this group that has an ISO 3,200 setting. (Note however that the Nikon D60's High ISO NR option defaults to "Off", which explains why it appears to do worse than the D40x model it replaces, which defaulted to "On".) Do keep in mind these measurements are at default settings, so the shape or position of the curve could be influenced by the settings you choose to use. The Pentax K200D's plot is a good example of this. Its noise magnitude is higher than the others, but that's partly because the K200D's default contrast and sharpness settings are a bit on the high side.

Sony A350 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 A350 with a nominally-exposed density step target (Stouffer 4110), and the A350's contrast and DRO settings at their default positions.

These are pretty good numbers for a current-model DSLR, in the mid- to upper range of cameras we've tested. More significant than the absolute dynamic range (7.19 EV) is the somewhat steep slope and/or relative abruptness with which the density curve (the curve in the top left of the figure above) ends in the highlight area, and particularly so in the shadows. The ideal shape for a tone curve is more of an "S" shape, with a longer tail in both the highlights and shadows. A long tail at both ends of the curve means that the camera approaches pure black and pure white gradually as the subject passes outside its tonal range. While this doesn't necessarily preserve more data in an absolute sense, it will provide smoother gradations in highlights and shadows, and in particular will preserve more shape in objects having highly saturated colors. Bottom line? Not quite as good as the A200 (which has larger pixels), but still a nice job.

Processing the A350's RAW (ARW) files through Adobe Camera Raw (ACR) version 4.4 improved dynamic range by about 2/3 stop at the highest quality level. (It bears noting though, that extreme manipulation of RAW files to extend dynamic range can introduce severe color shifts in the resulting files: This dramatically increased dynamic range may only be really usable when converting the images to black and white. - As is the case with most cameras.) These results were obtained by using ACR's automatic settings; slightly better results may be possible by adjusting the sliders manually, but I found little improvement in my own twiddling. Worth noting here is that ACR's default noise reduction settings reduced the red- and blue- channel noise (see the plot in the lower left-hand corner) relative to the levels in the in-camera JPEG. ACR also shifted even more of the noise energy into high spatial frequencies, making its residual noise even less objectionable than that found in the camera JPEGs.

Dynamic Range, the bottom line:

The net result was that the A350 exhibited very good dynamic range, its in-camera JPEGs perhaps helped a little by the A350's default "Standard DRO" (Dynamic Range Optimizer) setting. It should be noted though, that the DRO feature does nothing to increase the fundamental dynamic range of the camera: That's purely a factor of the sensor and associated electronics. We saw almost no difference in DR values (within 0.02 EV) with the three DRO settings (Off, Standard and Advanced DRO+), which isn't really a surprise: What DRO does is optimize use of the available dynamic range, pulling some of the extreme tonal values into the visible range. It doesn't change the actual underlying dynamic range of the sensor.

To get some perspective, here's a summary of the Sony A350'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.):

The results shown in the table are interesting. One of the first things that struck me back when I initially looked at test data for a wide range of d-SLRs, was that here again, purely analytical measurements don't necessarily correlate all that well with actual photographic experience. There's no question that the Fuji S3 Pro deserves its place atop the list, as its unique "SR" technology does indeed deliver a very obvious improvement in tonal range in the highlight portion of the tonal scale. I was surprised to see the analytical results place the Olympus EVOLT as highly as they did, given that our sense of that camera's images was that they were in fact noisier than those of many other d-SLRs that we looked at. In the other direction, I was quite surprised to see the Nikon D2x place as low on the listings as it did, given that we found that camera's shadow detail to be little short of amazing.

One thing that's going on here though, is that we tested each camera at its lowest ISO setting, which should produce best-case noise levels. This is in fact what many photographers will be most interested in, but it does perhaps place some of the Nikons (like the D40) at a disadvantage, as their lowest ISO setting is 200, as compared to the ISO 100 settings available on most other models.

Sony A350 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 I focus on here. The uncorrected resolution figures are 1,902 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,630 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,766 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution slightly, resulting in an average of 2,249 LW/PH. Corrected results are good, but not quite as high as one might expect for a 14-megapixel SLR.

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 the edges (first plots below) have a very subtle in-camera sharpening applied, resulting in very clean edges with little overshoot. (Once again, we're happy to see that the Sony A350 doesn't sharpen its images too much, so it doesn't obscure fine detail as much as some cameras do.)