From the original Sigma DP2 (non-Merrill non-Quattro) catalog:
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| Foveon X3 versus Bayer filter |
The DP2's the Foveon X3® direct image sensor utilizes the special properties of silicon, which is penetrated to different depths by different wavelengths of light, to successfully achieve full-color capture for the first time ever in a single-chip configuration. No color filter is required.
Since the Foveon X3® can capture all the color information in its three layers, the aforementioned late-stage color interpolation necessitated by the Bayer filter image-sensor is not required, and exquisite, nuanced color expression can be created in pixel-location units. Compared with the color-fudging Bayer filter image sensor, the Foveon X3® takes color resolution to a new and truly amazing level.
Since it does not need a color filter, the direct image sensor does not generate the color artifacts that color filters, by their very nature, tend to produce. This, of course, means that no optical low-pass filter is needed either. This full-color capture system can cope with all kinds of high-frequency areas, and capture the full complement of colors. That's why the results are both absolutely natural and truly exquisite.
The Foveon sensor is what makes Sigma cameras special. Each pixel has full colour information from the 3 colour channels rather than performing interpolation. This provides a very sharp image at the pixel level and the full colour information leads to very smooth tones. Although Sigma advertises the camera as being a 14 MP camera, in reality the true spatial resolution is 4.6 MP. The images can be upsampled to be comparable to a 10-12 MP Bayer filter camera. More technical information about the sensor can also be found on Wikipedia.
A clever use of the Foveon sensor is by taking only the top blue channel for black and white. This can help to take relatively less noisy images at higher ISO. The colour adjustment is easily performed in Sigma Photo Pro.
The Sigma Merrill took this another step with a 15.3 MP sensor with 3 colour channels. The Sigma Quattro series regressed by having a 19.6 MP higher resolution blue channel but 4.9 MP resolution green and red channels.
Sigma cameras will always be niche. As great as Foveon sounds in theory, the market is still dominated by Bayer filter cameras. Some cons of Foveon to note:
- Only offered by one manufacturer
- Greater manufacturing complexity leading to higher costs
- Increased processing power necessary leading to reduced battery life
- At high ISO settings, images are noisy with colour deterioration as light needs to travel through 3 sensor layers rather than one
- Colour casts are not uncommon in photos with the Sigma DP2 often showing a green tint
- Occasional banding issues
- Sensor flaring was noted on the Sigma Merrill series
- Many new cameras offer sensor shifting to provide the full colour information at the pixel level but with higher resolution