Nikon D50By: Dave Etchells and Shawn BarnettNikon develops an "entry-level" SLR loaded with features for less than $750. (Body only) <<Reference: Datasheet :(Previous) | (Next): D50 Sample Images>> D50 Imatest ResultsReview First Posted: 05/20/2005, Updated: 08/10/2005 |
Detailed analysis of the Nikon D50 images, from Imatest(tm)
I've recently begun using 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 I 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 D50:
Color Accuracy
The Nikon D50 showed good but not exceptional color accuracy in its default "Mode III" color setting. It tended to overemphasize strong reds quite a bit, while slightly undersaturating greens, yellow-greens, and yellows. Strong blues were also oversaturated. Other colors were rendered very accurately however. Average saturation was 115.9% (oversaturated by 15.9%, mostly in the reds and blues), average "delta-E" color error was 6.33. (Hue error, after correction for saturation.) Compared to the D70S, the D50's default color space is quite a bit more highly saturated, most likely in an attempt to appeal to more "consumer" photographers.
In color "Mode I", the oversaturation in reds and blues is reigned in quite a bit, without further dulling the rendering of greens and yellows. Very bright reds are still quiet oversaturated, but the overall color is much more accurate, with an average saturation of 107.9% (oversaturated by 7.9%, mostly in the reds), and an average "delta-E" color error of 5.5. (Hue error, after correction for saturation.) The overall "look" of this color space is quite a bit duller, but will probably be preferred by many more advanced shooters who dislike highly-saturated "consumer color."
Color Mode II (Adobe RGB) produces the most hue-accurate color, with an average delta-E hue error of only 4.07. Overall saturation is slightly higher than in Mode I, but the saturation boost is more evenly distributed across the spectrum, with the very hot reds much better behaved, and other colors receiving a relatively gentle saturation bump.
Color Analysis
These images show the color behavior of the D50 in its Mode III default directly. 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 to the sRGB color space.
Gray Patch Tone and Noise Analysis
There's a lot in this particular graph, a lot more than I have room to go into here. Bottom line, the D50's noise levels are very low, although the noise spectrum is such that the noise has a coarser-grained character to it.
Here's the same set of noise data at ISO 1600. What's most interesting here is that the Noise Spectrum plot actually shows more high-frequency content (the plot stays higher longer as you move to higher cycles/pixel), making the D50's noise finer-grained at high ISO than at low ISO. This is just the opposite of what usually happens, and helps explain the pleasing appearance of the D50's images at very high ISOs.
This chart compares the Nikon D50's noise performance over a range of ISOs against that of other cameras. As you can see, the D50's noise levels are lower than essentially all of its competition, matched only by the Konica Minolta Maxxum 7D. What isn't shown here though, is the "grain" structure of the camera nor the extent to which it trades away fine detail as the ISO is increased. The D50's noise is a little coarser-grained than the best of its competition, but it does manage to strike a very good balance between trading away subtle subject detail or allowing noise too free a reign. In the chart above, the Konica Minolta Maxxum 7D posts very low noise numbers, but its high-ISO images are very soft as a result. By contrast, the D50's images maintain very low noise levels up to ISO 800, yet with much less loss of fine detail in regions of subtle contrast. Very impressive.
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 the Nikon D50.
With that as background, here's how the Nikon D50 performed, 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.):
| Dynamic Range (in f-stops) vs Image Quality (At camera's minimum ISO) |
||||
| Model | 1.0 (Low) |
0.5 (Medium) |
0.25 (Med-High) |
0.1 (High) |
| Fujifilm S3 Pro | -- | 10.3 | 9.02 | 7.9 |
| Nikon D50 | 10.7 | 9.93 | 8.70 | 7.36 |
| Canon EOS 20D | 10.3 | 9.66 | 8.85 | 7.29 |
| Canon Digital Rebel XT | 10.3 | 9.51 | 8.61 | 7.11 |
| Olympus EVOLT | 10.8 | 9.26 | 8.48 | 7.07 |
| Canon Digital Rebel | 10.1 | 9.11 | 8.47 | 6.97 |
| Pentax *istDs | 10.2 | 10 | 8.87 | 6.9 |
| Nikon D2x | -- | 8.93 | 7.75 | 6.43 |
| Nikon D70S | 9.84 | 8.69 | 7.46 | 5.85 |
| Nikon D70 | 9.81 | 8.76 | 7.58 | 5.84 |
The results shown in the table are interesting. One of the first things that struck me when I initially looked at all the data, 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 dramatic 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 the Nikons at a disadvantage, as their lowest ISO setting is 200, as compared to the ISO 100 settings available on most other models.
I saw these numeric noise results for the D50 before I'd actually inspected its images, and was initially surprised to see it placing so highly. While I at first thought this had to be a fluke, close examination of its images revealed that it does indeed deliver very low noise levels and impressive shadow detail. While I do feel that the D70 and D70S deserve better positioning based on subjective evaluations of their performance, it's clear that the D50 does very well in this area, both objectively and subjectively. An impressive performance for a "value-priced" camera.
Bottom line though, 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.
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 1553 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1299 along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1426 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increases this number a lot, to an average of 2888 LW/PH. The uncorrected numbers are right where they should be, on the good side of average for a 6-megapixel d-SLR. The corrected numbers are wildly high though, and really shouldn't be taken as corresponding to any physical reality - See the discussion below for further explanation.
For the real techno-geeks, the two plots below show the actual edge response of the Nikon D50, for horizontal and vertical edges. Here, we can see that the D50's default sharpening is quite understated, resulting a very smooth luminance profile across the edges, with absolutely no sharpening artifacts at all, but producing a rather soft edge in the process. Curiously, the standardized 1-pixel sharpening is apparently almost perfectly matched to the task of optimizing apparent edge sharpness from the rounded profile produced by the D50, with the result that it produces an almost instantaneous transition from dark to light tones, resulting in unrealistic post-correction MTF 50 numbers. As noted above, this is really just an artifact of the standard sharpening operator, and doesn't really indicate any unusual sharpness in the D50's images.
In the main review, I commented on a slight coarseness in the D50's images. With the default sharpening as shown here, we see that the issue isn't so much coarseness, as a rather rolled-off edge response that's actually a desirable characteristic. The advantage here is that the smooth transition shown below preserves edge detail by not introducing any artifacts at all, at the expense of slightly soft-looking images when taken straight from the camera. As seen by the response of the standard sharpening operator below, the resulting images take unsharp masking very well indeed, revealing lots of fine detail. Where the real coarseness comes in with the D50 though, is when you boost its in-camera sharpening, which seems to have a larger than average pixel radius over which it works. This does cause some loss of information, and a "bulking up" of fine detail in its images. Bottom line, my recommendation is to shoot with the D50 at its default sharpening setting, and then sharpen your most critical shots post-exposure in Photoshop or other imaging software does a very nice job in the vertical direction, producing almost no "overshoot" that would tend to produce halos around objects. Along the horizontal axis, there's more overshoot, but it's still fairly well-controlled. The fact that there is.
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