Sony DSLR-A330 Imatest Results

Sony A330 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 A330:

The Sony A330 showed very good color accuracy, as well as fairly accurate saturation levels. Hue accuracy was quite good, with small hue shifts occurring in the cyans, blues, reds, oranges and yellows. Average saturation was 104.4% (4.4% oversaturated). Average "delta-C" color error was only 4.59 after correction for saturation, which one of the best scores we've seen for a recent, non-pro model. (Only the Canon XS beats it in this group.) (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, a very good color response for an SLR, especially a consumer oriented model. With very similar color to the other A-series models announced at the same time (the A230 and A380), the Sony A330 pushes strong reds, dark blues and some greens just a little, but actually undersaturates bright yellows, light greens, and cyan tones slightly: Overall saturation is closer to real life than that of most consumer SLRs we test. Mouse over the links below the illustration above to compare results with competing models.

Unlike most cameras, when using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Sony A330 delivers somewhat muted colors (most cameras produce more highly saturated colors when using Adobe RGB color space). Average saturation was only 90.77%, and average saturation-corrected hue error was 5.33 "delta-C" units. Again, mouse over the links below the illustration above to compare results with other consumer SLRs.

This image shows how the Sony A330 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 very good hue accuracy of the Sony A330, and also a tendency to boost the brightness (luminance) of highly saturated colors. We we've observed this brightness boost in many cameras we've tested; it seems to be a way to get brighter-looking color without completely overdoing the saturation. In the neutrals, the A330 holds back the highlights slightly, but increases contrast by pushing the shadows a bit deeper than the numerically exact numbers. As noted above, the overall color accuracy of the Sony A330 is noticeably better than that of most cameras we test.

There's a lot in this particular graph, a lot more than we have room to go into here. (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 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 Sony A330 does a pretty good job of keeping plenty of the noise energy at high frequencies; what little low-ISO image noise that's there is very fine-grained as a result. The color channels are much noisier than the luminance, though, especially at lower frequencies, but this is quite common these days. This manifests itself as small, colored blotches in monochromatic midtones and shadows, and is only visible at ISO 100 when pixel-peeping to a pretty extreme extent. Overall noise levels (the graph at the lower left of the graphic above) are roughly on par with the Sony A330's closest competitors, the Canon Rebel XS and Nikon D3000.

As we saw with previous Sony SLRs, there are 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 sometimes slightly lighter pixels along the edges of the 8x8 pixel blocks used in the A330's JPEG encoding. Overall noise levels are so low however that the noise spikes aren't really visible at ISO 100 under normal conditions.

Here's the same set of noise data at ISO 1,600. Here, the Noise Spectrum graph is shifted towards the left-hand, lower-frequency side than it was at ISO 100, though it's hard to tell because of the change in scale. (Interestingly, the red and blue channel's low-frequency noise spectrum is much higher at low frequencies than it was at ISO 100.) This indicates a coarsening of the "grain" of the image noise patterns by quite a bit, and indeed that's what we see when inspecting the A330's ISO 1,600 images.

Here's the same set of noise data at ISO 3,200. The Noise Spectrum graph is similar to what we saw at ISO 1,600, though the A330's noise reduction has held the chroma noise to follow luminance noise a bit more closely. Notice however that the overall noise levels are much higher. (Particularly evident in the pixel noise plot in the upper right corner of the illustration, and in the midtones and below in the graph in the lower left hand corner.)

This chart compares the Sony A330's noise performance over a range of ISOs against that of other current consumer models. 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. Here, we can see that the Sony A330's luminance noise magnitude starts out a pretty low relative to its "competition", and stays fairly low compared to the others up to 800, where it's about average compared to most of the competition. At ISO 1,600, the graph takes a deep dip, down to below the ISO 400 level, indicating strong noise reduction has kicked in (taking its inevitable toll on fine detail). At ISO 3,200, the A330's graph is still much lower than the competition. This may seem like good performance, but examining high ISO images from the A330 shows that the decrease in noise has been at the expense of fine subject detail, that's been blurred away by noise reduction. Do keep in mind these measurements are at default settings, so the shape or position of the curve is influenced by the settings you choose to use. Turning off the high ISO reduction (the Sony A330 has only Off and On options) would have certainly raised the values in the graph above, but would also result in more detail retention. Note that the Sony A330 forces high ISO noise reduction off when in continuous mode, and forces it on in some exposure modes, such as Auto or Scene Selection.

 

Sony A330 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 A330 with a nominally-exposed density step target (Stouffer 4110), and the A330's contrast and other exposure-related settings at their default values. Note that the A330 selects different levels of DRO depending on the exposure mode, so we've included plots from all three settings (Off, Standard and Advanced or Plus) here.

Here, we can see that the tone curves show pretty good gradation in the highlights and the shadows trail off nicely for all three settings. (The order above is Off, Standard and Advanced/Plus). As is usually the case with dynamic-range enhancement modes on consumer cameras, the total dynamic range actually remains constant (the camera just flattens the highlight end of the tone curve a little) and is pretty good (at 10.5 f-stops), while the score (7.28 to 7.31) at the highest quality threshold is about average among current consumer SLRs with APS-C size image sensors.

Processing the Sony A330's RAW (.ARW) files through Adobe Camera Raw (ACR) version 5.4 increased dynamic range by more than a 1.5 f-stops at the highest quality level (8.86), compared to the in-camera JPEG (7.31) while total dynamic range increased from 10.5 to 11.1 f-stops. These results were obtained by using ACR's automatic settings; slightly better results may be possible with manual adjustment. It's worth noting here that ACR's default noise reduction settings reduced overall noise somewhat (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 A330's JPEGs scored about average in Imatest's dynamic range analysis when compared against other current APS-C sensor models. Sony A330 RAW files converted with ACR did better, scoring in the top 10 cameras we've tested to date.

To get some perspective, here's a summary of the Sony A330'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.):

Comparing the Sony A330 to the rest of the field, its dynamic range in camera-produced JPEGs is about in line with competing models. Adobe Camera Raw managed to boost the DR pretty significantly, but as noted above, the extreme processing it's applying in the highlights would almost certainly result in color errors in strong highlights in highly-colored natural subjects; the actual dynamic range of the sensor is likely much closer to what we see in the JPEGs.

Sony A330 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 1,815 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,783 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,799 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius only increased the resolution score by only a tiny bit, for an average of 1,802 LW/PH. The corrected numbers put the A330 pretty much on par with its 10-megapixel competitors.

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 what Imatest considers an ideal amount of in-camera sharpening is applied in the horizontal direction (undersharpened by only 0.03 %), as well as in the vertical direction (0.14 % undersharpened), explaining why standardized sharpening didn't increase the MTF 50 numbers. There's still a noticeable bump in the intensity profiles of abrupt edges, though, meaning that you'll want to dial down the A330's in-camera sharpening if you want to sharpen post-capture in Photoshop or other photo-editing applications.

These are very good results. Still, (as is almost always the case), you'll extract the most detail from the Sony A330's images by careful processing of its ARW RAW files.