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| Click photo to see our Sigma SD14 Image Quality Gallery. |
All pixels are not created equal. That's what Foveon and Sigma say, and we agree -- though not completely.
According to these companies, the pixels used in Foveon's X3 sensor for the new Sigma SD14 DSLR are superior, resulting in the "only image sensor that captures full color at every point in the captured image." The sensor "efficiently reproduces color more accurately, and offers sharper resolution, pixel for pixel, than any conventional image sensor." This means that the SD14 "can out-perform other DSLR cameras in being able to accurately reproduce finer details in color images."
Foveon also says that its X3 sensor produces very low noise levels at high ISOs, requires less postprocessing of RAW data, and eliminates the moiré patterns that plague finely detailed areas in images from traditional CCD and CMOS sensors.
Truth or hype? See how the Sigma SD14 performed in the Pop Photo Lab Test. As for the X3 sensor...
Traditional CCD and CMOS sensors use a single layer of pixels that are set in a grid, relying on a pattern of microscopic red, green, and blue filters to separate the components of light on each pixel. Interpolation and postprocessing merge the data into the color image we see on screen.
The most widely used sensor grid is the Kodak-developed Bayer pattern, with two green pixels for each red and each blue pixel. This aims to take advantage of the human eye's higher sensitivity to detail in green objects. For improved black-and-white detail, to which the eye is even more sensitive, luminance information from all pixels is combined.
Foveon takes a different approach, designing its X3 sensor in three layers to mimic film. Instead of using dyes and other materials to filter the R, G, and B components of light, as film does, it relies on the spectral absorption properties of silicon. In the X3, high-energy blue light is absorbed closer to the surface, green light further down, and lower-energy red light at the deepest level. Precise alignment of the pixels in each of the three layers lets the sensor capture "full color" at each photosite. There are 4.7 million photosites on the SD14's X3 sensor; Foveon multiplies that number by three to get its 14.1MP sensor rating.
But black-and-white test targets for measuring resolution don't show as much detail as Foveon's 14.1MP count implies. Analysis of the IT-10 black-and-white resolution target we use in the Pop Photo Lab finds the Sigma SD14 on par with a good 8-9MP camera (in RAW mode), but not in the same class as 10MP models such as the Nikon D80.
Using a color spiral target that Foveon sent us for additional tests, we found the SD14 captures slightly more detail in red-and-blue areas than the Nikon D80 -- but not in the target's black-and-white or dual-color areas that contain green. So, in real-world shooting -- where it's hard to find finely detailed red-and-blue subjects or scenes that have little green or b&w detail -- the D80 has the sharpness advantage and outperforms the SD14 at high ISOs.
Foveon and Sigma are right in saying that color moiré in very fine detail was a problem in early digital cameras using Bayer pattern sensors. However, most manufacturers now use low-pass filters and image processing to minimize or eliminate moiré. The loss in resolution goes unnoticed because of higher pixel counts.
RAW data from the X3 sensor still seems to require intensive (not minor) postprocessing to convert color information into the final image. And more processing translates into a slower burst rate on the SD14 compared with other DSLRs. In-camera JPEG images from the Sigma SD14 also show less color accuracy and detail than RAW files processed by a computer.
The final proof is in the pictures that the SD14 produces, which look pretty good. And that's a fact. Foveon and Sigma might just try dialing down the hype a notch.
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