In the hierarchy of resolution, after the D800/D800E in the DSLR megapixel scale, you have the 20- to 24-megapixel full-frame cameras: the Nikon D600 and Sony SLT-A99 (24.3 megapixels), Canon EOS 5D Mark III (22.1 megapixels) and Canon EOS 6D (20.2 megapixels)—all capable of excellent image quality, but not providing the resolving power of the D800/D800E. And APS-C DSLRs have reached the 24-megapixel plateau, with Nikon's D3200, D5200 and new D7100 joining Sony's SLT-A77 and SLT-A65 at that level. These cameras have the pixel counts to produce huge enlargements, but their smaller sensors mean they will have more photonic noise than full-frame DSLRs, and thus somewhat lower image quality and resolution, although still outstanding.
Sigma SD1 Merrill And The Foveon Sensor
Sigma's SD1 Merrill DSLR (and DP1, DP2 and DP3 Merrill fixed-lens compact cameras) features the unique Foveon X3 image sensor, which operates on a different concept than the sensors in other DSLRs and medium-format digital cameras.
The photodiodes ("pixels") in an image sensor can't detect color; they just detect how much light strikes them. To get color information, conventional sensors employ a grid of red, green and blue filters called a Bayer array (named after the Kodak scientist who developed it), so that each pixel receives red, green or blue light.
The missing colors are derived from data from neighboring pixels using complex proprietary algorithms in a process known as demosaicing. This process produces moiré and other artifacts. To minimize those, manufacturers place an optical low-pass filter over the sensor, which slightly blurs the image at the pixel level.
Rather than use the Bayer filter grid, the Foveon sensor takes advantage of the fact that light penetrates silicon to different depths, depending on wavelength. The Foveon sensor stacks three layers of pixels, the top layer recording mainly blue light, the middle layer, green, and the bottom layer, red. Thus, every pixel site receives all three primary colors of light, so there's demosaicing, no moiré and no need for the blurring low-pass filter. This gives the Foveon sensor higher resolution than conventional Bayer sensors of equal horizontal-by-vertical pixel count. (The Foveon sensor requires processing of its images, of course, but doesn't produce the Bayer-sensor artifacts.)
Bottom line: The APS-C-format Foveon X3 sensor delivers images measuring 4704x3136 pixels (14.8 megapixels), but they provide much higher resolution than conventional sensors of that horizontal-by-vertical pixel count, and images rivaling those of 20-plus-megapixel Bayer-sensor DSLRs.
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