![]() These are good results for in-camera JPEGs, near the results of other recent Sony SLRs we've tested, but not as good as recent results from other manufacturers. The image below shows the test results from Imatest for an in-camera JPEG file from the Sony A900 with a nominally-exposed density step target (Stouffer 4110), and the A900's settings at their default positions, where Dynamic Range Optimization is set to Off. 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 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). To this end, Imatest computes a number of different dynamic range measurements, based on a variety of image noise thresholds. 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. (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.) This, of course, is a very subjective matter, and will vary with the application and even the subject matter in question. What we care about as photographers is how much detail we can pull out of the shadows before image noise becomes too objectionable. ![]() 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. 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. 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. Keep in mind these are at default settings, so the shape and positions of the curves will be influenced by your settings.Īs evidence of the difference between what these graphs show and the appearance of actual images, check out the Sony A900 High-ISO NR page of this review for direct comparisons against the Canon EOS-1Ds Mark III.Ī key parameter in a digital camera is its Dynamic Range, the range of brightness that can be faithfully recorded. There's quite a large difference at ISO 6,400, but still a pretty good performance for a camera with 24-megapixels. This brings the curve back down to the A900's ISO?800 level, and in-line with the Canons, but higher than the Nikons, which are "only" 12-megapixels. In the case of the Sony A900, the magnitude of the image noise starts out slightly above most of the competition at ISO 100, and remains higher at all ISOs until the A900's high ISO?noise reduction kicks in aggressively at ISO 1,600. Also, this is a plot of luminance magnitudes, where the A900 does relatively well: A plot of chrominance noise would tell a different story. 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. This chart compares the Sony A900's noise performance over a range of ISOs against that of other current, full frame cameras, using default settings. This graph reinforces our own visual observations of the A900's images, in which we saw that the luminance noise was relatively low, but that the chroma noise was much higher. ![]() We also see the now familiar spikes (though to a much lesser degree), which are a result of how Sony's JPEG engine processes and assembles each block of the image (see the Sony A200 or A350 review for details). The luminance curve is quite flat (meaning luminance noise is very fine-grained), however the color channels exhibit higher noise values, especially at lower frequencies. Here, we see the results at ISO 100, which is as a low extension of the normal ISO range for the A900. The champion at this was (and still is) the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs. In the graph above, this would show up as a noise spectrum curve that remained higher on the right side, representing higher noise frequencies. Cameras that manage to shift their noise spectrum to higher frequencies have much finer-grained noise structures, making their noise less visually objectionable. In comparing these graphs with those from competing cameras, I've found that the Noise Spectrum graph at lower right is the most important.
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