In the next few days, Sigma will release the SD1, its first foray into the realm of professional digital SLR photography. With an ostentatious street price of around $7,000 (and an MSRP of $9,700!), the SD1 is priced above both Canon and Nikon’s flagship 1D and D3 cameras, yet feature-wise it is similarly matched against the 5D and D300, which retail for only $2000. Buyers of the SD1 won’t be new photographers, either: they’ll be professionals with a bag full of lenses, none of which will fit the Sigma body. So why is Sigma charging almost $10,000 for a new and unproven camera? How is the SD1 different — or better — than a Canon or Nikon that can be had for a quarter of the price? It’s all about the sensor.

Beneath the SD1’s rather mundane exterior lies a brand new 46-megapixel Foveon X3 direct image sensor. Unlike the standard CMOS image sensor found in every other DSLR camera, the Foveon X3 has (you guessed it) three layers of photon-sensitive silicon. In conventional CMOS sensors, there is just one layer of silicon that is divided up, using a Bayer filter, into red, green, and blue receptors. Light passes through these filters, the luminance is recorded, and then the camera’s software turns the data into an RGB image. With the Foveon sensor, there are three layers of photo-sensitive silicon, each made up of 15 million receptors, and each one only receptive to one color. For more details, or if you’re having a tough time visualizing how it works, check out the demo that compares X3 to standard single-layer CCDs.

This unique way of measuring visible light — taking photos — utilizes a well-known phenomenon: different wavelengths of light penetrate silicon differently. Blue light, which has a short wavelength, doesn’t penetrate very far, while green penetrates a bit further, and red penetrates the furthest. Foveon’s sensor sandwiches three layers of silicon together, with different colors of light passing through each layer to reach the next. Blue light is measured the first layer, green on the second, and red on the last. The luminance values from all three are then combined to create an RGB pixel value.



It’s this final point that — in theory — makes all the difference. In conventional CMOS sensors, while there might be 20 to 30 million pixels (photodetectors), only one half of those pixels are measuring red and blue light, while the other half measure green. The RGB values of pixels are interpolated from this mix of data to create a color image. As you can imagine, such a system is far from accurate: if a photon of green light hits a red-filtered receptor, it is simply ignored. With the Foveon sensor, every last scrap of color data is received by the three layers of silicon. No interpolation is required, and the final image is much more “true to life” — it creates images with a quality that looks a lot like film, in fact. This is no surprise when you learn that color photography film also uses three distinct layers to capture red, green, and blue light.

The question, of course, is whether this new sensor is worth the massive premium that Sigma seems to be charging for the SD1. The sample images are definitely impressive, with both blues and reds being both sharper and more vibrant. Without interpolation, the images should be a lot sharper, too. For now, we only have sample images from Sigma itself, though, and it would be foolish to draw any conclusions until the sensor has been reviewed independently. There is also the matter of megapixels: is the SD1 a 15-megapixel camera, or a 45-megapixel camera? While there’s no doubt that the Foveon X3 captures more color data than a standard single-layer CCD the concept of “megapixels” is founded on sensors that use the Bayer filter method. The X3 is an entirely new beast that could revolutionize megapixels in the same way that gigahertz have grown out of fashion when comparing processors.

Beyond the sensor itself, there is also the question of lenses. Sigma could have the greatest sensor ever made, but it’s meaningless without the glass to back it up. With the SD1 comes the SA mount, which will part photographers from their trusted Canon, Nikon, and Zeiss glass. Furthermore, at between $7,000 and $10,000 for a new SD1, Sigma is edging into medium-format territory, and there are scant few photographers who would dare compare Sigma’s finest pieces of glass to the illustrious lenses found on the front of Leica and Hasselblad cameras.

Read more about the Foveon X3 sensor and Sigma SD1