Panasonic GH1 Imatest Results

Panasonic GH1 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 Panasonic GH1:

The Panasonic DMC-GH1 showed good color accuracy, with only minor oversaturation of bright reds, deep blues and some purples. The GH1 actually undersaturates some yellows and greens. Hue accuracy was also generally good, with the typical cyan shift we see in most cameras we test, but a larger than average shift in the range of spectrum from orange through yellow. The shift of oranges toward yellow and yellows toward green was evident in some of our test images, notably those of the Still Life scene. The earlier G1 showed some of these tendencies, but to a lesser extent, so its color was a slightly more accurate overall. (The biggest difference in the color-error maps of the two cameras is that the GH1 saturated the deep purple color swatch a good bit more than did the G1.) Average saturation was 110.9% (oversaturated by 10.9%, mostly in the reds, blues, and purples). Average "delta-C" color error was 6.59 after correction for saturation, which is pretty good, just a step down from the best cameras we've tested. (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 very good color response for this class of camera. Mouse over the links below the illustration above to compare results with recent consumer SLRs with movie modes, as well as the Panasonic G1.

As is true of most SLRs, when using the Adobe RGB color space (which provides a much wider gamut, or range of colors that can be expressed), the Panasonic GH1 delivers more highly saturated color, though the increase compared to sRGB is not as stark as with most cameras. Average saturation was 112% which is only slightly greater, and average saturation-corrected hue error was 5.9 "delta-C" units, actually slightly more accurate than the sRGB result. Again, mouse over the links below the illustration above to compare results with recent entry-level SLRs.

This image shows how the Panasonic GH1 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 correct 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 correct color, without the luminance correction. This image shows the generally good hue and saturation accuracy, although the greenish tinge in the bright yellow and orange swatches is quite evident. The as-captured colors are a little bright for many blues, and a little dark for some warm tones, notably yellows and oranges. We did notice some yellows looked a little dull and greenish in some of our test images (see the yellow embroidery thread in our Still Life target), but felt that the overall visual impact of the GH1's color was quite positive. If Panasonic could just clean up their handling of the range of spectrum from orange through yellow-green, the GH1's color would be truly impressive. (The same comment we made for the G1 before it.)

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 Panasonic GH1 does a very good job of keeping a lot of the luminance noise energy (indicated by the black line) up at high frequencies. What little low-ISO luminance image noise that's there is quite fine-grained as a result. Chroma noise at low spatial frequencies, though, is much higher, especially in the blue and red channels, showing the greatest low-frequency noise bias we've seen in the DSLR category. When inspecting low ISO GH1 images very closely, you can see blotches of chroma noise in neutral colors, such as the darker gray patches in the MacBeth chart (see the exaggerated example at right - NOTE that this is zoomed 400% over the original file data, and the tone curve has been stretched quite a bit to reveal the noise). While the Panasonic GH1 did quite well at high ISO shooting, we did see quite a bit of chroma noise in shadow areas. Cranking the noise reduction setting up to +2 alleviated a lot of this, but we'd have been happier if it wasn't there in the first place.

Above is the same set of noise data at ISO 1,600. Here, the Noise Spectrum graph is shifted quite a bit toward the left-hand, lower-frequency side than it was at ISO 100, coarsening the "grain" of the image noise patterns quite a bit. The red, blue and green channels still exhibit a bit more noise at the low end of the frequency spectrum, but, interestingly, are much lower relative to the luminance noise than at lower ISOs, tracking it closely at all but the lowest frequencies. We were a little surprised to see this; that the differential between chroma and luminance noise was actually lower at high ISOs than at low ones. - Particularly given that our perception was consistently that chroma noise was the biggest issue at high ISO settings. Perhaps this is because the luminance noise tended to be so fine-grained, while the red and blue-channel noise tended to be rather blotchy. (Even though the luminance curve in the figure above is shifted quite a bit toward the left, low-frequency side relative to the curve at ISO 100 we saw earlier, the GH1 still spreads much more of its noise energy to higher frequencies than is common.)

Here's the same set of noise data at ISO 3,200. Here again, the Noise Spectrum graph is shifted even more toward the left-hand side, coarsening the "grain" of the image noise patterns further.

This chart compares the Panasonic GH1's noise performance over a range of ISOs against that of other SLR-style cameras capable of video recording. 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. Here, we can see that the Panasonic GH1's luminance noise magnitude starts out about average relative to its competition, but stays at or below the levels of the rest up to ISO 800. At ISOs 1,600 and 3,200, luminance noise levels are higher than its APS-C competition, but the GH1's noise levels are significantly lower than the Panasonic G1. This improvement in high ISO noise can be clearly seen when comparing GH1 to G1 shots. The GH1's shots are cleaner, and yet don't suffer in a loss of detail, so it's not due to more aggressive noise reduction. Great results! Do keep in mind these measurements are taken with each camera set at default settings, so the shape or position of the curve could be influenced by the settings you choose to use. Like the G1, the Panasonic GH1 offers five levels of noise reduction, with the above data collected using the default, middle level.

Panasonic GH1 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 Panasonic GH1 with a nominally-exposed density step target (Stouffer 4110), and the GH1's settings such as Contrast (0) and iExposure (Off) at their default positions.

Here, we can see that he tone curve shows pretty good gradation in highlights, but the shadow end trails off rather abruptly. These are very good numbers for a Four-Thirds sensor, but lag behind results from the larger APS-C sensors the competition uses.

Above are the results with iExposure adjusted to the Low setting. It produced almost no difference in the Imatest scores. (This is consistent with what we've seen with other approaches to extending "dynamic range" in photos. What we're measuring here is the overall, true dynamic range, as revealed in the camera's files. What features like iExposure do is to change how the camera uses its available dynamic range, playing with the tone curve to open up shadows and/or tone down highlights. This produces images that show more image detail in the highlights and shows to a human observer, but don't actually increase the underlying dynamic range at all.)

Above are the results with iExposure turned at the Standard setting. This time, there was a slight increase at the highest quality. A very subtle effect.

Above are the results with iExposure turned at the High setting. This time, there was a slight drop at the highest quality level. Again, a very subtle effect.

The illustration above shows the results from Adobe Camera Raw 5.4 beta, with Auto settings (slightly better results are likely possible with manually tweaking, but we weren't able to do much better, due to what appeared to be some truncation of the individual color channel tone curves at the deep shadow end of the tonal range). As can be seen, the score at the highest quality level wasn't any better than camera JPEG when using the "Standard" iExposure setting at 7.76 f-stops, though total dynamic range increased to 9.88 f-stops.

Dynamic Range, the bottom line:

The net result was that the Panasonic GH1 showed very good dynamic range when compared against other current Four-Thirds models, but lags behind most DSLRs with APS-C size sensors. Lumix GH1 RAW files processed in Adobe Camera Raw didn't improve the Imatest score at the highest quality setting, but the total dynamic range did improve. Note that the version of Adobe Camera Raw we used was a release candidate, so slightly better results may occur with the final release of that software tool.

To get some perspective, here's a summary of the Panasonic GH1'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 DSLRs, 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 deserved its original place atop the list, as its unique "SR" technology did indeed deliver a very obvious improvement in tonal range in the highlight portion of the tonal scale relative to competing models of its day. (Amazing that it's now surpassed by even consumer-level models using today's technology.) I was also surprised to see the analytical results place the original 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 DSLRs 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 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.

Panasonic GH1 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 2,094 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1,843 lines along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1,969 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increased vertical resolution by quite a bit, but decreased the horizontal resolution slightly, resulting in a slightly lower average of 1,943 LW/PH.

The Panasonic GH1 delivers excellent resolution, very competitive with 12-megapixel DSLRs currently on the market. Its images straight from the camera show a lot of detail, and even more can be extracted with careful RAW processing.

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 moderate amount of in-camera sharpening is applied in the horizontal direction (oversharpened by 5.13 %, explaining why standardized sharpening wasn't able to improve on the MTF 50 numbers much), while in-camera sharpening is not as aggressive in the vertical direction (3.87 % undersharpened). You should thus turn the camera's sharpening down a little for optimal results when sharpening in-camera JPEGs post-exposure in Adobe Photoshop or other image editing software.