Nikon D7000 Imatest Results

Nikon D7000 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 D7000:

The Nikon D7000 showed pretty good color accuracy, with mild to moderate oversaturation of deep blues, bright reds, and dark greens, and to a lesser extent, some oranges, greens, purples and browns. Some colors such as yellow, light green, aqua and cyan are slightly understaturated. Hue accuracy was very good, with the typical (typical meaning significant) cyan shift we see in most cameras we test, and also a little shift in other colors such as aqua, red, orange and yellow. Average saturation for the Nikon D7000 was 107.3% (oversaturated by 7.3%) which is fairly typical. Average "delta-C" color error was 5.23 after correction for saturation, a little less accurate than the D90 and D300S, and the least accurate of the group above except for the Pentax K-5. (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.) All in all, a pretty good color response for an SLR. Mouse over the links below the illustration above to compare results with the others in this group.

Most SLRs oversaturate colors when shooting in Adobe RGB mode, and the Nikon D7000 is no exception. Average saturation was 109% which is higher than the sRGB result, but not by as much as we often see. Average saturation-corrected hue error was 4.92 "delta-C" units, slightly better accuracy than sRGB. Again, a pretty good result overall. Mouse over the links below the illustration above to compare results with the same group.

This image shows how the Nikon D7000 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 what the ideal color would look like if the brightness were adjusted to match the camera's tone curve, while the small rectangle shows the numerically correct color with no adjustment. This image shows the good hue accuracy of the D7000, as well as a commonly-employed trick of overexposing some of the highest-saturation swatches, to give an impression of "brighter" color, without actually oversaturating.

The bottom row of the chart shows exaggerated white balance errors, revealing that the camera produced a slightly cool color balance when using Custom white balance, with a WhiBal card as the reference, though mid-one white balance looks quite accurate. (Note that the examples above deliberately exaggerate the white balance error.)

As always, there's more in this particular graph than we really have room to go into here. (Also note that 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 often 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.

Above, we see the results at ISO 100. The luminance curve (in black) is fairly flat, with blue and red channels only slightly elevated lot lower frequencies, however the red channel exhibits much higher noise values at lower frequencies. The dominance of low frequencies in the color channels points to some low-level blotchiness, but noise levels are so low in the red channel (bottom left plot), that subtle chroma blotches are only seen with extreme pixel-peeping in midtone and shadows.

Above is the same set of noise data from the Nikon D7000 at ISO 3,200. Here, the overall Noise Spectrum graph is shifted a fair bit toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns. There's much less difference between the color channels in this plot, with the green channel tracking luminance noise very closely, and only the red and blue channel noise being a bit elevated at the lowest frequencies, no doubt due to noise reduction processing. This translates to ISO 3,200 images with pretty low levels of noise, slightly lower than the D300S but slightly higher than the D90. An impressive performance considering the smaller pixel size of the D7000.

Here's the same set of noise data at ISO 6,400. Here again, the Noise Spectrum graph is shifted even more toward the left-hand side, further coarsening the "grain" of the image noise patterns further. Overall noise levels are higher than ISO 3,200 as you'd expect, but very good compared to most other APS-C SLRs.

This above chart is a little crowded, so below is a version with ISO range 100-6,400.

This chart compares the Nikon D7000's noise performance over a range of ISOs against that of a few other SLR cameras in a similar price range or resolution. 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, not to mention what the camera does to achieve a given noise level. (Some cameras obliterate subject detail along with noise, so the camera with the lowest noise levels on MacBeth chart swatches may not necessarily produce the most appealing images.)

As you can see, the Nikon D7000's luminance noise levels are competitive with and often better than most of the cameras in this group, with a nice, fairly linear progression of noise levels versus ISO sensitivity. What's not shown here, though, is how good the Nikon D7000's JPEG processing is at holding on to fine detail while keeping noise in check, especially in difficult subjects with subtle tone-on-tone detail. The Sony A580 and Pentax K-5 score better at very high ISOs (above 6,400) with cleaner looking images, but they don't show quite as much detail. All in all, a very good performance from the Nikon D7000. Keep in mind, though, that these results are at default noise reduction settings, so the shape and positions of the curves will be influenced by your settings.

 

Nikon D7000 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 D7000 with a nominally-exposed density step target (Stouffer 4110), with its settings such as contrast and Active D-Lighting set to the defaults.

Here, we can see that the tone curve maintains pretty good gradation in the highlights but the shadow end doesn't trail-off quite as nicely, though shadow noise is quite low. Total dynamic range is very good at 10 f-stops, almost a full stop better than the D90 and D300S, while the score at the highest quality is also very good, at 7.97 f-stops versus the D90's 7.77 and D300S' 7.49.  (But keep in mind the base ISO of the D90 and D300S is ISO 200, and higher ISOs tend to reduce Imatest scores.) We didn't shoot with Active D-Lighting enabled for this test, as the increased noise levels resulting from the boost in shadows usually results in a lower Imatest score at the highest quality levels. The high quality score at default settings is excellent for an in-camera JPEG from an APS-C sensor; among the very top performers.

The illustration above shows the results from an Adobe Camera Raw 6.3 converted NEF file, using the Auto setting. Slightly better results are possible by manually tweaking, but we weren't able to improve the scores. As can be seen, the score at the highest quality level was much better than the in-camera JPEG, at 10.1 f-stops versus 7.97 f-stops, which is one of the highest scores to date of any camera we've tested, aided by the D7000's low shadow noise. Total dynamic range also improved considerably, from 10 to 12.1 f-stops. It's worth noting here that ACR's noise reduction 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 Nikon D7000's JPEGs showed excellent dynamic range, performing almost as well as the best cameras we've tested, and that's with Active D-Lighting set to Off (default). RAW dynamic range performance with Adobe Camera RAW was outstanding, producing the second best score we've tested to date, besting even full-frame models.

To get some perspective, here's a summary of the Nikon D7000'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.):

Note that this test is repeatable to within 1/3 EV according to the Imatest website, so differences of less than 0.33 can be ignored.

Nikon D7000 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 1,961 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,995 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,978 LW/PH. That's somewhat lower than you'd expect from a 16-megapixel camera. Correcting to a "standardized" sharpening with a one-pixel radius improved the numbers significantly, however, resulting in an average of 2,321 LW/PH. The much improved standardized versus uncorrected results indicate the Nikon D7000 is applying a conservative amount of in-camera sharpening to its JPEGs at default settings.

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 fairly modest level of in-camera sharpening is applied in both the horizontal and vertical directions. Imatest reports 11.2% undersharpening in the horizontal direction, and 10.9 % undersharpening in the vertical direction. This means that the Nikon D7000's JPEG images should respond well to some additional post-exposure sharpening in Adobe Photoshop or other image editing software, without too many visible sharpening artifacts. (Of course for best results, shoot RAW and apply all sharpening while post-processing.)

Note: We don't feature SFR-based LW/PH resolution numbers more prominently in our reviews (eg, outside the Imatest pages) because we've found that they're *very* sensitive to minor differences in in-camera image processing. Relatively small changes in the amount of in-camera sharpening can have a large effect on the resulting resolution numbers. Imatest attempts to compensate for this by adjusting to a "standard" sharpening, but this approach can't completely undo what happens inside the cameras, and so often gives inconsistent results. Sometimes the "standardized" sharpening happens to just match the shape of the edge profile with the in-camera sharpening applied, and you'll get wildly high results. At other times, it will tend to correct in the opposite direction. Unfortunately, ignoring the in-camera sharpening entirely can result in even greater discrepancies, particularly between models from different manufacturers. Turning off sharpening in the camera may or may not fully eliminate the sharpening, so simply turning off sharpening in the camera JPEGs isn't a reliable solution. It also wouldn't be the way most people shoot the cameras. We could process RAW files with no sharpening (as shown below), but then that'd only suit the people working primarily or exclusively from RAW, and would open another can of worms as to what RAW converter was used, etc, etc.

The bottom line is that numbers for resolution only take you so far. Detail handling and edge acuity are very complex issues; ones that don't easily boil down to a single number. The best approach is to simply look look at the broad array of standardized test shots we take with each camera, to the point of downloading and printing them with whatever processing you'd use if you owned the camera and shot with it. See how the differences stack up for you visually, and make your decision on that, rather than on abstract resolution numbers.