Nikon D3X Imatest Results

Nikon D3X 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 Nikon D3X:

The Nikon D3X showed very good color accuracy overall. Hue accuracy was quite good, with only small shifts relative to mathematically correct colors Average saturation was 108.1% (oversaturated by 8.1%, mostly in the deep blues and reds, with just a little in some greens). Average "delta-C" color error was only 4.8 after correction for saturation, which is low, though not quite as good as the Canons in the group, but better than the D3's score. All in all, a very good color response for an SLR. Mouse over the links below the illustration above to compare results with competing models.

Using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Nikon D3X delivers more highly saturated color, with an average saturation of 113.7% and average saturation-corrected hue error of 5.31 "delta-C" units. These are good results, but not quite as good as sRGB. (We very commonly see some over-saturation in SLRs when shooting in the Adobe RGB space.) Interestingly, though, the Canon models that showed better color accuracy than the D3x in sRGB mode do somewhat worse in the Adobe RGB color space. - And the Sony A900, which did worse in sRGB is better in Adobe RGB. Again, mouse over the links below the illustration above to compare results with competing models.

This image shows how the Nikon D3X 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 D3X's very good hue accuracy, as well as a gamma curve that results in an overexposure of some colors, notably cyans, reds, blues and some greens. (We've seen this in a wide range of cameras, increasing the "punch" of images by brightening saturated colors a little, while actually keeping both the hue and saturation level very close to technically accurate 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 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, 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 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.

Here, we see the results at ISO 50, which is a low-side extension of the normal ISO range for the D3X. The luminance curve is exceptionally flat (meaning luminance noise is very fine-grained), however the blue color channel exhibits much higher noise values. (This is quite unusual, we more commonly see the green channel noise being the lowest, and the red noise curve tracking that of the blue channel more closely.) Because the overall noise levels are slow low, we normally can't see noise shown on this graph at low ISOs, but in the case of the Nikon D3x operating below its official ISO range, we do see some blotchiness in the blue channel on some of the dark grey patches of the MacBeth chart. (Do note, though, that ISO 50 is below the official range for the camera.)

Here are the results at the Nikon D3X's base ISO of 100. Again, the luminance curve is quite flat and the D3X does a good job of keeping plenty of the noise energy at high frequencies. All three RGB channels are higher than the luminance, this time the red channel being the highest. Still, the Nikon D3X noise levels are very low at low ISO (as you can see in the lower left graph, which shows noise levels vs target brightness), with a frequency spectrum that has more energy on the higher frequency (right-hand) side of the curve than do most cameras. As with the ISO 50 results, though, we do see some blotchiness in the red, green, and blue channels dark grey patches of the MacBeth chart, so minor points off for that.

Above is the same set of noise data at ISO 3,200. Here, the luminance channel of the Noise Spectrum graph has a dramatic shift toward the left-hand, lower-frequency side than at ISO 100, coarsening the "grain" of the image noise patterns quite a bit relative to the results at lower ISOs, but the grain size still appears impressively small to the eye. The color channels follow the luminance much more closely, except at the very low end of the frequency spectrum, no doubt due to the default noise reduction taking place. Overall noise is still quite low; surprisingly so for such a high-resolution sensor. (Compared to the Canon 5D Mark II at ISO 3,200, the D3x's overall noise levels are a little higher, but its images are also noticeably more crisp, revealing more detail.)

Here's the same set of noise data at ISO 6,400. Very similar to ISO 3,200, with a larger shift to the left indicating a coarser "grain" caused by the D3X's noise reduction attempts to blur out the noise. Note however the peak RMS noise (lower left chart) and Pixel noise (upper right) have jumped from about about 0.17 to 0.24 f-stops and about 2.5 to 3.5 % respectively. Comparing again against the Canon EOS-5D Mark II, there's less difference in overall noise level, but the difference in sharpness persists. (We suspect that comparisons done with similar levels of in-camera sharpening would reveal that the two cameras are in fact very close to each other in noise performance.)

This chart compares the Nikon D3X's luminance noise performance over a range of ISOs against that of other current, full frame cameras, using default settings. 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. In the case of the Nikon D3X, the magnitude of the image noise starts out about the same as the competition at ISO 50, but increases at ISO 100, remaining slightly higher than all but the A900 at all ISOs. This is a good example of how misleading these charts can be as at ISOs 3200 and 6400, luminance noise levels are virtually the same as the Sony A900, but when comparing images from the two cameras, it's clear than the D3X has at least a 1-stop advantage over the A900 in terms of noise. Keep in mind these are at default settings, so the shape and positions of the curves will be influenced by your settings.

The graph above shows the luminance noise magnitude using the four levels of high-ISO noise reduction available on the Nikon D3X. Since high ISO noise reduction is not performed under ISO800, results at ISO50 through 400 are identical.

All in all, the Nikon D3X delivered some pretty amazing noise levels, particularly considering its smaller pixel dimensions.

Nikon D3X 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 D3X with a nominally-exposed density step target (Stouffer 4110), and the D3X's Active D-Lighting function set to Off.

These are very good results for in-camera JPEGs, near the top of the results from other recent SLRs we've tested. The tone curve shows excellent gradation in highlights, but the shadow end trails off much more abruptly.

Enabling Active D-Lighting at its lowest setting improves the score to 7.91 EV at the highest quality level, but overall dynamic range has dropped a bit from 8.4 EV to 8.29 EV.

Increasing Active D-Lighting a notch to its Normal setting reduces the score to 7.52 EV at the highest quality, but overall dynamic range has increased to 8.72 EV.

Increasing Active D-Lighting to the High setting has little effect, actually reducing the highest quality score a bit, back to 7.51 EV.

Increasing Active D-Lighting to the maximum Extra High setting reduced the score at the highest quality level to 7.23 EV, but overall dynamic range increased to 9.01 EV.

Overall, given the relatively minor effect on the tone curve here, we suspect that the D3X's Advanced D-Lighting function isn't being triggered by the particular (and unusual) distribution of tonality in the density step wedge used for this test. Note, too, though, that tweaking the tone curve can't actually increase the dynamic range, it can only shift how much emphasis is placed on highlights vs shadows.

Processing the D3X's RAW (NEF) files through Adobe Camera Raw (ACR) version 5.3 beta improved dynamic range dramatically to 9.64 stops at the best quality level, and by more than 3 stops total. ACR's automatic settings were used, then manually tweaked for slightly better results. Worth noting here is that ACR's default noise reduction settings reduced the average noise relative to the levels in the in-camera JPEG. ACR also shifted slightly more of the noise energy into lower spatial frequencies.

Dynamic Range, the bottom line:

The net result was that the D3X JPEGs with Advanced D-Lighting Off came in just above the middle of the pack, but near the top with it was turned to Low. Dynamic range from ACR processed RAW file was outstanding, at the very top of the heap.

To get some perspective, here's a summary of the Nikon D3X'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 (at the time) deserved 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 E-300 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 (base) 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.

With the preceding as caveats, what we can say about the Nikon D3X's dynamic range is that it's quite good, but perhaps a third of a stop or so below the current best of the field. This roughly matches what we felt we were seeing in its images - It does a very good job, particularly with highlights, but plugs the very darkest shadows somewhat. - But keep in mind that we're splitting hairs, and talking about very deep shadows indeed.

Nikon D3X 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 2,807 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 2,728 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 2,767 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution, resulting in an average of 3,212 LW/PH, which is very good, though not quite as high as we found with the Sony A900. (It could be that the D3x's oversharpening of its default JPEGs is what's causing this result: Its uncorrected resolution numbers are higher than those of the A900, while its corrected numbers are lower.)

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 there is fairly conservative in-camera sharpening applied (almost ideal bumps at the top ends, but no noticeable bumps at the bottom ends of the black edge profile curves). Imatest reports that the horizontal direction (vertical edge) is "undersharpened" by 9.69% while the vertical direction (horizontal edge) is undersharpened by 10.6%. This strikes us as a bit of a misnomer, since the D3x's edges are clearly oversharpened to begin with. Lower sharpening would likely produce higher corrected resolution numbers, but Imatest is just looking at how much the slope of the intensity profile across an edge increases in response to the "standardized" sharpening. (As usual, for the ultimate in fine detail from the Nikon D3x, begin with a RAW file and a good RAW converter, rather than a JPEG.)