The Foveon CMOS sensor, as Sigma users know, is unique among camera imaging devices in abandoning the almost universally used Bayer mask approach in favour of a much more elegant three layer architecture which holds the promise of accurate colour rendition at each pixel site. Here, Julian Ashbourn gives his take on the subject.
In a Bayer mask design, adjacent pixels are filtered to each be sensitive to one of the three primary colours and then grouped together in order to extrapolate an appropriate colour as averaged within the group of pixels.
This works quite well, but has an inherent disadvantage in that the colour assigned to the pixel group will always be an approximation to the original colour rather than an absolutely accurate rendition. In addition, as it typically takes four pixel sites to render a single ‘colour’ site, this may be considered a somewhat wasteful use of the pixels deployed across the sensor surface, no matter how abundant they are. The Foveon sensor in some respects works in reverse, in that you effectively have three times the area pixel count, courtesy of the three layers and, in addition, each pixel site is a full ‘colour’ site capable of rendering accurate colour without the need for complicated extrapolations.
This is achieved by virtue of each of the three layers in a Foveon sensor being sensitive to one of the primary colours. In this respect, the sensor design concept is more akin to traditional film emulsion.
In adopting the Foveon sensor for their DSLR camera designs (and now the compact camera as well) Sigma have created a range of unique imaging devices which exhibit very distinct qualities.
In the SD14 in particular, the Foveon sensor is matched to a particularly well considered camera which will enable both enthusiasts and professionals to fully exploit the inherent characteristics of the sensor. Sigma are to be congratulated in this respect, as they have effectively raised the potential quality of digital photography by taking an unusual, but uniquely qualified, approach to camera design. In combination with a comprehensive range of quality optics, the enthusiastic photographer is consequently provided with a unique toolset for digital imaging.
The theory is interesting enough, but how does the Foveon sensor perform in practice? What beneficial characteristics will be apparent in the final images produced by a Foveon equipped camera to justify its choice against more conventional designs? Such questions will be particularly appropriate for professional photographers who may be considering whether to commit to the Foveon and Sigma philosophy. In this article, we shall therefore consider the more subjective qualities as apparent in realised images, rather than technical theory. After all, it is final image quality which is paramount for both professionals and enthusiastic amateurs alike. For a more technical appraisal, the reader may wish to consult the Foveon website which may be found at www.foveon.com
Perhaps the most striking and immediately noticeable artefact of Foveon produced images is the colour rendition. To the casual observer, this might at first appear to be in danger of over saturation. However, closer inspection reveals an extraordinary alignment with the original, natural colour. Landscapes and natural world images in particular are a revelation, with images accurately portraying the vibrancy and depth of colour inherent in mother nature’s creations which traditional sensors struggle to capture.
Furthermore, the subtleties inherent in these colours as minor transitions of hue occur across area, all seem to be captured faithfully. Grasses and flowers come alive: Textures within trees and rocks take on an uncanny realism: Water flows naturally again: Skies and clouds are more expansive. Similar qualities may be noted in portraits with subtle skin tones being faithfully reproduced and the colours of fabric designs accurate and natural. This vibrancy of natural c!
olours alone surely justifies the Foveon design concept. However, there is more.
In addition to the outstanding colour rendition, there are other qualities which will be immediately apparent to the experienced eye. Among these are a crispness of definition which brings a perceived ‘solidity’ to images unrivalled by other sensor designs.
This is no doubt a consequence of the precise colour alignment, allowing colour transitions and edges to be particularly well defined. In certain types of shots, this characteristic also promotes an almost three dimensional quality to the image which is most appealing.
Another quality lays in the absence of negative artefacts such as unwanted moiré patterns and random fringing which sometimes creeps into digital images. Furthermore, there is an unusual dynamic range which manifests itself as a more natural range of tones within complex scenes. This is especially useful for those who occasionally translate their images into monochrome, as well as more general photography. The combination of these properties results in images which appear almost as if an obscuring layer has been removed, opening a window onto the natural world and revealing all the subtleties that we readily perceive with the eye, but are rarely captured by the camera.
The unique qualities of the Foveon sensor are well suited to most areas of photography. Those interested in landscapes and nature photography in particular will relish the vibrant and accurate colour rendition. Those for whom portraits are more important will equally appreciate the natural skin tones and inherent fine detail. Architectural photographers will benefit from an uncommon clarity of line. Fashion and advertising practitioners will enjoy an extra vibrancy and life within their images. Certainly, the Foveon sensor design has unique qualities which are clearly apparent in resultant images, to the extent that, once familiar with such imagery, experienced photographers are unlikely to be satisfied with other approaches. In addition, the Foveon sensor will allow high quality optics to perform at their best, capturing all the delicacy of hue and fine detail which distinguishes them from simpler designs
Of course, the sensor cannot produce such qualities in isolation. The final image quality is a product of how well the sensor is integrated into the overall camera design. In this respect, Sigma deserve credit both for the choice of the Foveon sensor in the first place, and the integration of it into the cameras.
Any transducer typically requires operational software in order to interpret its output and reproduce it in a common format. In this context, the Sigma in-camera processing does a fine job in producing either RAW or JPEG files from the images captured by the sensor. Beyond the camera, the Sigma Photo Pro software additionally provides for some excellent processing capabilities, especially when translating from RAW to JPEG format.
In addition, it exploits the special characteristics of the Foveon sensor with features such as ‘fill light’ which allow for an increased flexibility when editing RAW images taken in challenging lighting conditions. Photo Pro may be considered as something of a digital darkroom, enabling experienced photographers to practice their traditional skills within the digital domain.
In conclusion, the combination of the unique Foveon sensor and its integration into the equally special Sigma camera designs, provides for an interesting alternative approach to digital imaging. It also represents a step change in the accuracy and vibrancy of colour rendition, overall definition and hence realised image quality available from consumer level designs, irrespective of their target application. Indeed, it is difficult to conceive of a branch of photography which would not benefit from this approach. The Foveon equipped Sigma SD14 thus deserves to find a wider audience in the field photographic art.
Update: As of 2008, Foveon is wholly owned by Sigma.
You can read some of the history behind the development of the Foveon sensor, and the company, in “The Silicon Eye” by George Gilder.