Nikon D700 Imatest Results

Nikon D700 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 D700:

The Nikon D700 showed good color accuracy, though like the D3, it had saturation levels slightly higher than we're accustomed to finding in professional SLRs, although most of the saturation boost was in blues, purples and reds. Other parts of the spectrum had essentially neutral saturation. Hue accuracy was quite good, with most of the hue shift occurring in the cyans and sky blues, with a little in the reds and oranges. Average saturation was 109% (oversaturated by 9%, as just mentioned, mostly in the blues, reds and a bit in the greens). Average "delta-C" color error was only 5.19 after correction for saturation, which is quite low, albeit very slightly higher than most of the competition. 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 D700 delivers more highly saturated color, with an average saturation of 114.5% and average saturation-corrected hue error of 5.34 "delta-C" units, which in this case is a smidgen more accurate than most of the competition. Again, mouse over the links below the illustration above to compare results with competing models.

This image shows how the Nikon D700 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 D700's excellent hue accuracy, as well as a gamma curve that results in an overexposure of some colors, notably blues, reds, and some greens. (We've seen this in most cameras, increasing the "punch" of images by lightening bright 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, 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 (and still is) the Canon EOS-1Ds Mark II, which produced remarkably fine-grained image noise, even at very high ISOs.

Here, we can see why ISO 100 is treated as a "LO" extension of the normal ISO range for the D700, as noise is a bit higher than at ISO 200, particularly in the blue channel. Below is the plot for ISO 200, the D700's normal base sensitivity.

The D700's noise performance at ISO 200 doesn't quite rise to the level of the 1Ds Mark II, but it nonetheless does a good job of keeping plenty of the noise energy at high frequencies, at least in the blue channel and in the luminance values. The green and especially red channel oddly show a lot of low-frequency energy, which would translate into blotchiness in the final images. The good news though, is that overall noise levels at ISO 200 are quite low, so the blotchiness isn't visible without considerable tone-curve tweaking. Bottom line, the Nikon D700 noise levels are low at low ISO, with a frequency spectrum for the luminance and blue channels that has more energy on the higher frequency (right-hand) side of the curve than do most cameras in the blue and luminance signals, but less so in green, and particularly in red.

Here's the same set of noise data at ISO 3,200. Here, the Noise Spectrum graph is shifted a bit more toward the left-hand, lower-frequency side than it was at ISO 200, coarsening the "grain" of the image noise patterns somewhat, but the spike in the color channels has been reduced somewhat. (And it's now the blue and red channels that are blotchy, with green behaving itself better. Odd/interesting...) Also, while there's less energy at the highest spatial frequencies (far right hand side of the plot), the mid-frequency range (middle of the plot) is quite a bit higher than we're accustomed to seeing at very high ISOs. This is reflected in the D700's grain structure at ISO 3,200, which isn't as fine as some, but which also lacks the very prominent, large blotches that are common in some SLRs. Once again, very good performance.

Here's the same set of noise data at ISO 6,400. A surprisingly flat curve, indicating noise is well controlled and fine grained even at this high ISO, but again with more low-frequency energy present in the red and blue channels.

Here's the same set of noise data at ISO 12,800. It a little hard to tell because of the change in scale, but there is a significant shift towards the left, lower-frequency region.

The trend of increasing the coarsening the "grain" of the image noise patterns continues at ISO 25,600, once again with a big spike in the red channel at the lowest frequencies.

This chart compares the Nikon D700's noise performance over a range of ISOs against that of other 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 D700, which follows the D3 very closely, the magnitude of the image noise starts out slightly below most of the competition at ISO 100, and remains lower for all ISOs in common. A really excellent performance, and a major benefit of Nikon not succumbing to the temptation of cramming more pixels into the full-frame sensor of the D700. Keep in mind these are at default settings, so the shape and positions of the curves will be influenced the settings you choose to use.

Here's a plot comparing results using the D700's four High ISO noise reduction settings. Note that Hi ISO NR does not apply under ISO 2,000, so the graph starts at ISO 1,600, where the results of the different settings are the same. Also note that above ISO 6400, the "Off" setting still applies "minimal" noise reduction, explaining why the "Off" and "Low" curves are so close to each other at the highest ISOs.

Nikon D700 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 D700 with a nominally-exposed density step target (Stouffer 4110), and the D700's Active D-Lighting and contrast settings at their default positions.

These are excellent results for in-camera JPEGs, near the top of the SLRs we've tested. The tone curve shows excellent gradation in highlights, but the shadow end trails off more abruptly. A sharp tail-off at the shadow end usually signals plugged shadows and lost detail in images, but the D700 really didn't seem to show this. There's clearly more gradation at the highlight end of the tonal scale, but examining deep shadows on the D700's test images showed excellent detail, in all but the very darkest shadows. - In the darkest shadows though, there was indeed some plugging, rather than a smooth trailing-off. (We're not sure why Imatest sometimes doesn't report DR for the lower quality thresholds like this. We've called it to the attention of the program's author; will try to come back and re-do these graphs if/when he develops a fix.)

Processing the D700's RAW (NEF) files through Adobe Camera Raw (ACR) version 4.5 improved dynamic range by over 1-3/4 stops at the best quality level, and by more than 3 stops total. ACR's automatic settings were used. Tweaking the sliders manually may result in slightly better results. Worth noting here is that ACR's default noise reduction settings reduced the pixel noise quite a bit overall, but had more trouble with the red channel. ACR also shifted slightly more of the noise energy into higher spatial frequencies.

Active D-Lighting. The charts below show the effect of the Nikon D700's three Active D-Lighting settings: Low, Normal and High.

These plots show that most of the D700's Active D-Lighting impact occurs at the highlight end of the tone curve, at least when confronted with an artificial image like the greyscale step-wedge used for our dynamic range measurements. What's a little hard to see with the plots arranged vertically like this though, is that the shadow end of the curve is lifted somewhat as well: Imatest adjusts the vertical axis to show the most detail in the curves overall, but the result is that the vertical scale can change somewhat between plots. Looking closely, you'll see that the bottom end of the Density Response curve is a fair bit higher on the "High" plot than on the "Low" one. The major impact though, is the way Active D-Lighting flattens-off the very top of the tone curve. This translates into excellent preservation of detail in strong highlights, something the D700 does a better job of than most cameras we've tested.

One thing you soon learn when playing with tone curves though: There's no free lunch. Ultimately, there's only so much dynamic range to work with, you just get to choose whether you "spend" it on the highlights or shadows. Because Active D-Lighting (with the step-chart image, at least) is choosing to pay a lot of attention to the highlights, it should come as no surprise that the deepest shadows suffer as a result. In fact, because the camera is compressing the tone curve the way it is, the noise levels in the shadows limit the calculated dynamic range a fair bit, relative to the camera's performance using its default settings. Still, for images with very strong highlight detail, Dynamic D-Lighting really works very well. Equally important, the resulting images look very natural, not appearing as though they'd had their tone curves massaged to any great extent.

Dynamic Range, the bottom line:

The net result was that the D700 JPEGs came in ahead most other DSLR models we've tested, but slightly below the D3. When it comes to images processed from RAW files, it was at the top of the list.

To get some perspective, here's a summary of the Nikon D700'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 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.

Nikon D700 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 1,637 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,696 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,666 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased both vertical and horizontal resolution rather significantly, resulting in an average of 2,420 LW/PH, almost identical to the D3's corrected results.

(Note: We shot these with a Sigma 70mm f/2.8 macro lenses, one of the sharpest lenses we've tested to date over on SLRgear.com. With the D3 though, the Sigma 70mm produced significant exposure errors, so we reverted to the Nikon 105mm f/2.8 macro (the original, not the newer VR model, which is softer), also an exceptionally sharp lens.) This may explain the better uncorrected results with the D700, though Nikon may have tweaked the D700's default sharpening as well. Stopped down to f/8, though, there should be very little difference in the sharpness of the tow lenses in question.)

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 very conservative in-camera sharpening applied (no noticeable bump at the top and bottom ends of the black edge profile curves). Imatest reports that the horizontal direction (vertical edge) is "undersharpened" by 22.1% while the vertical direction (horizontal edge) is undersharpened by 18.5%. Professionals and serious amateurs much prefer this to oversharpening, and the D700's images respond very well to the use of strong/tight sharpening post-exposure in Photoshop or some other image editor. (That said, you can still extract more fine detail if you begin with a RAW file, rather than a JPEG.)
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