Nikon D200 Imatest Results

Nikon D200 Imatest Results

We routinely Norman Koren's excellent "Imatest" analysis program for quantitative, thoroughly objective analysis of digicam test images. I highly commend 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 Nikon D200:

The Nikon D200 showed good color accuracy overall, but got a little carried away with strong reds. This is a fairly common response with this target (the MacBeth ColorChecker), but the D200's response is more exuberant than most professional SLRs. Hue accuracy is excellent, with most of the hue shift occurring in yellow-greens and yellows, which are shifted clockwise on the L*a*b* color error plot above. Average saturation was 107.2% (oversaturated by 7.2%, largely in the reds, but somewhat in the blues as well), average "delta-E" color error was 6.69 after correction for saturation. All in all, a good color response for a pro SLR, but not quite the best we've seen.

The plot above shows the color error in Adobe RGB mode. Overall color mapping is very similar, but hue and saturation accuracy are both improved somewhat. Hue accuracy approaches that of the best cameras on the market.

This image shows how the Nikon D200 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 color after correcting for the luminance of the photographed chart (as determined by a 2nd-order curve fit to the values of the gray swatches), and the small rectangle inside the inner square shows what the color should actually be, based on perfect rendering in the sRGB color space. This image shows the excellent hue accuracy, as well as a gamma curve that results in an overexposure of highly saturated swatches.

There's a lot in this particular graph, a lot more than we have room to go into here. Bottom line, the Nikon D200's noise levels are low at low ISO, but the frequency spectrum of its noise leads to a coarser noise pattern than some of its competition. In comparing these graphs with those from other 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. While good, the D200's noise spectrum is much more conventional, with more weight on the left side of the noise spectrum plot than the 1Ds managed.

Here's the same set of noise data at ISO 1600. Here, the Noise Spectrum graph is shifted a bit more toward the left-hand, lower-frequency side than it was at ISO 100, further coarsening the "grain" of the image noise patterns. Once again, not bad, but not a standout among competing dSLR models.

This chart compares the Nikon D200'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. In the case of the Nikon D200, while the magnitude of its image noise is lower than essentially all of its competition, the dominance of low spatial frequencies gives it a coarser and therefore much more visible grain than that of cameras like the Canon EOS-1DsMkII. Prints from ISO 1600 shots as large as 13 x 19 inches would still be acceptable for many purposes, but this isn't a camera you'll buy ahead of its rivals, primarily for its high-ISO noise characteristics.

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 Nikon D200 with a nominally-exposed density step target (Stouffer 4110), with the D200's contrast setting at its default position.

These are decent if not exceptional numbers for an in-camera JPEG, but the dynamic range number at "high" quality itself (7.11) isn't the most important thing to look at here. A dynamic range of 7.11 f-stops at high image quality is within a half a stop of most of the D200's competition, not enough difference to be a significant issue. (Yes, a half a stop more dynamic range would be nice, but that's not the main thing that affects what a photographer will perceive as the D200's dynamic range here.)

More significant than the absolute dynamic range 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. - More on this a bit further down.

That's the story with JPEG files created in the camera, how about in an external RAW converter, such as Adobe Camera Raw (ACR)?

Using its automatic settings (which in my experience has generally worked best for dynamic range tests like this), ACR managed to extract about a half-stop more dynamic range from the NEF file at the highest quality level, but also produced a tone curve with a much nicer-looking tail in the shadow range. Also, while it doesn't show in the tone curve above, ACR gained most of its half-stop advantage in increased highlight retention. All in all, a worthwhile result: I particularly like the improved tone curve shape.

While I've generally used ACR's auto settings in the past, I found with the D200 that dropping the contrast to its minimum and using a linear tone curve did gain an additional 0.2 EV of dynamic range, as you can see above. For the sake of consistency with past results though, I'll use the 7.61 EV dynamic range rating for the purposes of comparing to other cameras.

What about Nikon's own RAW processor, Nikon Capture though? I tried playing with the D200's NEF files using Capture version 4.4.1, with somewhat mixed results. Let's take a look, starting with its default exposure settings:

Say what? We actually lose a bit less than a half an f-stop of dynamic range, and the shadow end of the tone curve is just flat-out missing! These results were so bad that I actually re-ran Capture 4 three times to verify that it was really doing what it seemed to be. There's one bright spot here, namely that the noise spectrum is shifted quite a bit toward the high frequency end of the spectrum, but that doesn't begin to make up for the terrible shadow curve.

After a few unsuccessful passes, some pixel peeping and a little deep thought, I wondered if some of the horrible results in the deep shadows might not be due to JPEG compression throwing away useful detail. To explore that, I saved a copy of the same file using the same settings, but this time as a 16-bit TIFF. The result was the tone curve seen above. It still has a bit of an unorthodox shape, but at least there are valid data points in the deep shadows, and the dynamic range according to Imatest is almost back to what the camera delivered in its JPEGs directly. (Note though, that some of the issue here isn't necessarily the fault of the JPEG, but rather the fact that the JPEG is limited to 8 bits of data, and Capture 4's tonal compression just isn't bringing the deepest shadows up into a visible range. Note that the vertical scale on this latest tone plot now extends to -4. That corresponds to a brightness value 1/10,000 that of the brightest point, a value far too small to be expressed in the 8-bit range of JPEGs.)

That's what I got with Capture 4's default settings, what could I get by fiddling with the exposure and tone curve adjustments?

Yeesh, same screwy tone curve, but at least the dynamic range is up to 7.71, actually a bit better than ACR managed. Here again though, this is from a JPEG output from Capture 4, which we know from above doesn't pull the deepest shadows up into the range that can be represented in a JPEG. What's a TIFF look like?

Ahh, that's more like it: Still a strange-looking tone curve, but now we've got a dynamic range of 8.22 stops at High quality, a very respectable result.

Dynamic Range, the bottom line:

So what does all this fiddling with file conversion prove? I think two things: First, that the D200's internal JPEG output shows an odd-shaped tone curve that's somewhat contrasty on both ends, and that has relatively limited dynamic range. The second conclusion though, is that there's quite a bit of additional tonal data available in the D200's NEF (RAW) files, that the right RAW converter software should be able to extract. At least with the tools I played with though, it's a lot of work to go in and extract that detail. Why can't the D200 use a more reasonable tone curve itself, with a nice long tail on both the highlight and shadow ends like the Canon 5D does? I don't know, but it's a shame that more of the D200's true capabilities aren't made available in its JPEGs. (Or made more easily available in Nikon Capture 4.)

To get some perspective, here's a summary of the Nikon D200'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 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 at a disadvantage, as their lowest ISO setting is 200, as compared to the ISO 100 settings available on most other models.

Regardless of the positions of the other cameras though, the Nikon D200 does appear to have fairly reasonable dynamic range, hampered somewhat by the camera's odd tone curve that produces higher contrast at both the shadow and highlight ends than that seen in many of its competitors.

As I always say though, at the end of the day, I think you have to take the figures here with a grain of salt, and look at actual images with your own eyes to see what you make of each camera's tonal range and noise levels. We'll continue performing these dynamic range tests on the digital SLRs that we review, but (just as with the laboratory resolution target results), we suggest that you not rely on them exclusively for making your purchase decisions.

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 1050 line widths per picture height in both the horizontal direction (corresponding to the vertically-oriented edge), and along the vertical axis (corresponding to the horizontally-oriented edge). Correcting to a "standardized" sharpening with a one-pixel radius increases this number dramatically, to an average of 2918 LW/PH. This characteristic is very similar to what we saw with the Nikon D2x, which also showed low uncorrected numbers and very high corrected ones. Both cameras use very conservative in-camera processing at their default settings, producing soft-looking edges but more importantly, ones with no overshoot or other sharpening artifacts. This means that images look very soft straight from the camera, but take subsequent sharpening very well. It appears that the "standard" 1-pixel radius sharpening that Imatest applies almost exactly compensates for the soft edge profile of the D200, with the result that the edges become almost arbitrarily sharp.

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. Bottom line, the D200 captures excellent detail, really making the most of its 10.2 megapixel sensor. This was borne out by our own playing about with the D200's images in the lab: Careful sharpening in Photoshop produced images that held up remarkably well, even at sizes as large as 20x30, showing plenty of detail and virtually zero artifacts.