SD1 color cast with Art lens.

George_pix wrote:

Dear all, I was evaluating a Sigma camera with the foveon sensor and came across some threads like this one. I have some experience in optics and these strange color casts made me suspicious.

Apparently the multilayer pixel structure of the Foveon has a color filtering behavior that depends and the angle of incidence of the light rays. Maybe microlenses are used and they mitigate the phenomenon. Anyway the incidence angle of the rays on the sensor depends on the design of the lens, aperture, focusing distance, etc. The incidence angles are different for every pixel, and for every pixel the range of possible incidence angles may differ so the calculations are non simple. That is why non Sigma adapted lenses might not be fully color cast corrected by SPP, in fact a very detailed pixel by pixel map of color cast correction seems to be needed for each specific lens and it should be a function of aperture, focusing distance and focal length (for zooms).

You may imagine that if the aperture is 16 the light rays may impinge on a given pixel with a small range of angles and the filtering behavior of the multilayer will be well defined. On the contrary if we set aperture to 1.4 the range of incidence angles on each single pixel will be much larger and the color filtering behavior will be a superposition of different curves, that need to be analyzed and compensated for.

Bayer sensors have similar problems, but they do not cause color casts, they do cause vignetting that can be minimized by well designed microlenses.

This possibility, if true, might explain why only SPP can slowly convert RAWs and why the Foveon system is not open. On the other hand Sigma has obviously solved all the problems, but some kind of special lens mapping is used.

Please consider this a possibility for open discussion. I never used a Foveon camera and obviously never measured one. I might be totally wrong.

From a paper re: the first commercial Foveon sensor:

"5.1 Optical considerations

Although the stacked photodiode structure eliminates artifacts generated by the offsets of the color receptors in color filter array sensors, some subtle effects remain that should be considered. First, there is a potential with stacked photodiodes to have the sensitivity vary with f-number because lower-angle rays might escape detection by escaping from the side of the diode. However, the diode geometry is designed to minimize this and the effect has not been demonstrated. Next, the presence of low-angle rays might shift the color response due to the variation in ray path length through the silicon. This is possible, but because of the high index of refraction of silicon, incoming rays are bent strongly toward the normal. For most practical optics, the path length variation would be 20% at most. This effect could probably be demonstrated but has not yet been rigorously examined. Finally, the variation in absorption depth in silicon with temperature could result in a significant color shift in cryogenically-cooled cameras. This effect will need to be studied in detail and may require application of a set of temperature-dependent color matrix values. Similar effects are seen in two color single photodiodes.Although the sensor optical effects are generally minor, the quality of the optics used with the stacked-photodiode sensors can seriously impact image quality. Most significant is chromatic aberration, which is quite visible in monochrome images taken with the F7. Generally, color filter arrays mask chromatic aberration because geometrically the effects are on the order of one pixel. However, F7 images clearly show radially-symmetric color fringing resulting from lenses that suffer from an excess of this problem. Similarly, unsharpness in focus, astigmatism, and other lens defects can have clearly-observable negative effects on images made with stacked-photodiode sensors. Users of the SD-9 camera have often noted how important it is to use the best optics in obtaining the highest image quality from these sensors."

http://kronometric.org/phot/sensor/fov/SD9%20sensor%20in%20depth.pdf