The image sensor is the most important part of a digital SLR camera. What, then, should a photographer choosing a new camera look for in an image sensor?
Most camera makers today emphasize megapixels, which are of course a factor in determining image resolution. Sigma believes, however, that a theory of image quality that begins and ends with the megapixels is grossly incomplete, since it is the structure of the image sensor that has the greatest impact on finished image quality.
From the first generation onward, Sigma digital cameras have featured the Foveon X3® direct image sensor. This sensor leverages the special qualities of silicon, which is penetrated to different depths by different wavelengths of light. It was the world’s first and remains the world’s only image sensor to capture fully the three basic colors of light—red, green, and blue—in each pixel location without relying on color filters.
With the exception of Sigma’s products, almost all digital cameras available today contain monochrome image sensors. Unable to distinguish between colors, these sensors receive light through a filter that has an even distribution of red, green, and blue filtering units. As a result, the sensor records color not vertically, but horizontally.
Since each light-sensing photodiode has above it a tiny filter unit that lets in only one color, each pixel captures only one color, and data for the other two colors of light is never collected at all. A color interpolation process known as demosaicing guesses at the missing colors based on the colors of neighboring pixels, adding them back in.
Developed and refined over an extended period, this method of processing images has matured, and the interpolation of colors is now fairly accurate. Inevitably, however, interpolating colors based on neighboring pixels results in the loss of subtle color details found in the original subject.
In fact, the Foveon X3® direct image sensor receives red, green, and blue wavelengths of light vertically—just like modern color film. Considering it far superior to the much more common Bayer filter sensors available, most experts agree that the Foveon X3® direct image sensor has a unique structure that makes it the image sensor of the day—and of the future.
Due to the demosaicing process, conventional digital cameras using color filter arrays also generate color artefacts—colors not found in the original subject at all. The problem arises when areas of the subject have a higher frequency of color detail than can be adequately covered by the grid of red, green, and blue units in the Bayer filter.
A conventional Bayer filter digital camera has an additional optical low-pass filter between the lens and the sensor. This filter acts on the high-resolution image from the imaging lens, eliminating high-frequency, detailed elements likely to generate color artefacts before they can reach the image sensor. Although it effectively suppresses color artefacts, the optical low-pass filter also systematically reduces the resolution of the image.
Sigma digital SLR cameras produce truly emotional images, thanks to the Foveon X3® direct image sensor. Completely different in structure from conventional image sensors, the Foveon X3® direct image sensor offers truly distinctive image quality characterized by fine detail—detail essential to capturing the emotional qualities of the subject. Sigma digital SLR cameras are designed to reproduce what the photographer shoots, right down to the feeling in the air—a result that is only possible with a vertical color-capture system that does not require color interpolation, and an image-processing system that does not require an optical low-pass filter.
In contrast, a conventional image sensor performs guesswork on colors and even cuts out high-frequency areas completely. To a degree, colors can be adjusted in post-processing. But the subject’s details and nuances, once lost, can never be recovered. To compensate, conventional cameras utilize sharpness processing to give the appearance of a higher resolution. As a result, these cameras produce images that have artificially sharp edges and give an uncanny and unnatural impression.