Sony DSLR-A380 Imatest Results

Sony A380 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 A380:

With very similar color to the other A-series models announced at the same time (the A230 and A330), the Sony A380 pushes strong reds, dark blues and some greens just a little, but actually undersaturates bright yellows, light greens, and cyan tones slightly, but overall saturation is closer to real life than that of most consumer SLRs we test. Average saturation was 107.9% (7.9% oversaturated). Average "delta-C" color error was only 5.15 after correction for saturation, which is 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, a very good color response for an SLR, especially a consumer oriented model. Mouse over the links below the illustration above to compare results with competing models.

Like most other cameras, the Sony A380 delivers more highly saturated colors when using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed). Average saturation was 109.8%, just slightly higher than the sRGB results. Average saturation-corrected hue error was 4.86 "delta-C" units, a little more accurate than the sRGB results. Again, mouse over the links below the illustration above to compare results with other consumer SLRs.

This image shows how the Sony A380 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 A380, 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 A380 holds back the highlights slightly, but increases contrast by pushing the shadows a bit deeper than the numerically exact numbers. Overall, the colors are impressively close to their "correct" values.

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 A380 does a fair job of keeping much of the luminance noise energy at high frequencies, so what little low-ISO image noise that's there is pretty 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 other 14- to 15-megapixel cameras we've tested (a little higher than those of the Canon T1i, a little lower than those of the Pentax K-7, both of which are more expensive cameras).

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 A380'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. (Interestingly, the red and blue channels' noise spectrum is still much higher at low frequencies, something most noise reduction systems tend to minimize at higher ISOs.) 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 A380's ISO 1,600 images. Noise levels at ISO 1,600 are a little higher than other 14- to 15-megapixel cameras we've tested.

Here's the same set of noise data at ISO 3,200. The Noise Spectrum graph is similar, though the A380's noise reduction has further reduced the chroma noise to more closely follow luminance noise, but it's still higher, especially at low frequencies. Notice also that the total 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.) Chroma noise levels (especially red-channel noise) are quite a bit higher than what we've seen in other 14- to 15-megapixel cameras at ISO 3,200.

This chart compares the Sony A380's luminance noise performance at midtone grey density over a range of ISOs against that of other current, similarly priced 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 A380's luminance noise magnitude follows that of its predecessor, the A350, extremely closely (the values are almost identical). The Sony A380's noise starts out average relative to its "competition", and remains about average to slightly above average compared to the others up to ISO 800. Like most other Sony SLRs, the A380's graph takes a deep dip at ISO 1,600, down to below the ISO 400 level, indicating strong noise reduction has kicked in (and having the effect you'd expect on fine subject detail). At ISO 3,200, the A380's graph is still much lower than most of the competition (the Nikon D5000 being the exception). This may seem like good performance, but examining high ISO images from the A380 reveals that a substantial price has been paid, as much fine detail has 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 noise reduction (the Sony A380 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 A380 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 A380 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 graphic below shows the test results from Imatest for an in-camera JPEG file from the Sony A380 with a nominally-exposed density step target (Stouffer 4110), and the A380's contrast and other exposure-related settings at their default values. Note that the A380 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 but the shadows trail off more abruptly for all three settings. (The order above, from top to bottom 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, remaining in a fairly tight range between 10.1 and 10.4 f-stops. The scores at the highest level are all similar (7.29 to 7.36 f-stops), and about average for an SLR with an APS-C size sensor.

Processing the Sony A380's RAW (.ARW) files through Adobe Camera Raw (ACR) version 5.5 increased dynamic range by more than 1.25 f-stops at the highest quality level (8.62 f-stops), compared to the best in-camera JPEG (7.36 f-stops) while total dynamic range reported increased from 10.4 to 12.5 f-stops. These results were obtained by using ACR's automatic settings; then manually tweaking from there. 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 A380's JPEGs scored about average in Imatest's dynamic range analysis when compared against other current APS-C sensor models. Sony A380 RAW files converted with ACR did much better, scoring near the top 10 cameras we've tested to date.

To get some perspective, here's a summary of the Sony A380'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 A380 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 A380 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 1,735 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 2,037 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,886 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius only changed the resolution score by quite a bit, for an average of 2,253 LW/PH. The corrected numbers are quite good for a 14-megapixel APS-C 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 a conservative amount of in-camera sharpening is applied in the horizontal direction (undersharpened by 13.9 %), while in the vertical direction, Imatest reports undersharpening of 7.58%. This is why standardized sharpening improved the MTF 50 numbers by so much. The conservative in-camera sharpening means that camera-produced JPEGs will take unsharp masking in Photoshop or other image-editing software better than most.

These are very good results. Still, (as is almost always the case), you'll extract the most detail from the Sony A380's images by careful processing of its ARW RAW files.