Nikon D40 Imatest Results

Nikon D40 Imatest Results

We routinely 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 D40:

The Nikon D40 showed pretty good color accuracy, not quite as good as the D80, but still very good among the cameras we tested. It got a little carried away with strong reds and blues (a fairly common response with this target, the MacBeth ColorChecker), and was slightly undersaturated on the strong yellow and yellow-green swatches Hue accuracy is very good to excellent, with most of the hue shift occurring in the cyans and blues. Average saturation was 113.4% (oversaturated by 13.4%, mostly in the reds and blues as well), average "delta-E" color error was 6.57 after correction for saturation. All in all, a very good color response for an SLR, just a little off the best we've seen.

This image shows how the Nikon D40 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 second-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 D40's noise levels are low at low ISO, with a frequency spectrum that has a bit more energy on the higher frequency (right-hand) side of the curve than do some cameras. 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. The D40's noise spectrum a bit more conventional, with more weight on the left side of the noise spectrum plot than the 1Ds managed, but still with a good noise structure overall.

Here's the same set of noise data at ISO 1,600. Here, the Noise Spectrum graph is shifted a bit more toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns somewhat. In the D40, while the frequency distribution of the noise shifts more to the lower range, its overall noise amplitude remains pretty low.

This chart compares the Nikon D40'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 D40, we were consistently impressed when we subjected the D40's images to the real-world test of printing them out at 8x10 inches on the Canon i9900 printer in our studio. Time after time, ISO 1,600 shots printed up clean and beautiful looking. There was definitely some noise present, but our assessment was that 95% of the D40's customer base would find the results entirely acceptable. (Even ISO 3,200 shots printed surprisingly well.)

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 D40 with a nominally-exposed density step target (Stouffer 4110), with the D40's contrast setting at its default position.

These are very good numbers for an in-camera JPEG, but the dynamic range number at "high" quality itself (7.28) isn't the most important thing to look at here. A dynamic range of 7.28 f-stops at high image quality is within a hair of the best DSLRs we've tested (apart from the Fuji S3 Pro), but equally important is the shape of the tone curve.

More significant than the absolute dynamic range is the fact that the curve tails off nicely at both the highlight and shadow ends, providing a smooth transition from quarter-tones to both highlight and shadow, making for smooth-looking images even after considerable tone-curve adjustment on the computer.

We normally process the dynamic range test files through Adobe Camera Raw (ACR), but as of this writing in early December, 2006, Adobe hadn't yet released support for the Nikon D40. We'll try to come back and update these results once a version of ACR is available that supports the D40.

Dynamic Range, the bottom line:

While we don't have the results of a RAW conversion available to judge by at this point, the JPEG images straight from the D40 show really excellent dynamic range. Previously, the champion for dynamic range from Camera JPEGs (leaving out the Fujifilm S3 Pro, with its dual-pixel sensor technology) was the Nikon D50, with the Canon EOS-20D coming in close behind, exceeding the D50 at some noise thresholds, falling behind very slightly at others. Looking at the table below, the D40 appears to run neck and neck with its predecessor, with about 7.3 f-stops of dynamic range at the highest quality level (restricting images noise to 0.1 f-stop), and 8.9 stops with a noise threshold of 1/4 stop.

To get some perspective, here's a summary of the Nikon D40'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 (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.

Regardless of the positions of the other cameras though, the Nikon D40 does appear to offer surprisingly good dynamic range, particularly for an entry-level consumer camera. While it doesn't show in the tests above, the bulk of the D40's dynamic range comes in the shadows: With its default contrast setting, we found it was disconcertingly easy to lose highlight detail, but a lot of detail could be found far into the shadows. (For critical shots, expose for the highlights, and pull the shadows up post-capture on the computer.)

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 1,570 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,207 lines along the vertical axis (corresponding to the horizontally-oriented edge). Correcting to a "standardized" sharpening with a one-pixel radius brought the vertical resolution close to the horizontal, resulting in an average of 1,615 LW/PH. This is a good performance for a 6-megapixel camera.

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 (for whatever reason), the vertical edges (first plots below) have a very subtle in-camera sharpening applied, resulting in very clean edges with almost no overshoot. In contrast, the horizontal edges show more evidence of sharpening, with the unaltered JPEG showing moderate overshoot. (This results in slight halos around vertical edges.) As discussed in the main part of this review, the D40's unadjusted JPEG images make good-looking (if very slightly soft) prints as large as 13 x19 inches. For the ultimate in sharpness though, dial down the camera's internal sharpening and process the resulting images after the fact in Photoshop or other image-processing application to bring out the maximum detail without introducing sharpening artifacts.