Foveon Sensor Fun


Ian collects guitars. I seem to collect cameras. I’m playing with a second hand Sigma DP2s that I picked up as a kind of birthday present to myself. This is a neat looking compact that uses a Foveon sensor. This sensor works in a different way to the ones in most cameras which use a “Bayer” design. With this each sensor is covered by a coloured filter that lets the sensor read only that colour. So to get the colour of a particular pixel you need three sensors, for Red, Green and Blue. Actually most cameras use two Green sensors, since that is colour that has the greatest effect on brightness as far as the human eye is concerned.  So that makes four sensors in total.

But anyhoo, this means that to get the colour of a pixel the camera must read the brightness of a bunch of pixels and then combine them using some clever maths. Then the camera does something even cleverer and works out the colour it would be at each of the Red, Green and Blue positions, based on the readings from those around it. This is a bit sneaky, in that it means that your 16 megapixel camera isn’t actually reading 16 million coloured dots. It is actually working a lot fewer colour values and then spreading these back over 16 million points. This can lead to pictuers with artefacts (funny shapes that appear as a result of the averaging process) and noise (because the maths is really only a form of guessing).

The Foveon sensor doesn’t do any of this. It uses a cunning silicon sensor that reads all the colour values at each pixel position. The bad news with a Foveon sensor is that it doesn’t have quite many pixels as most “Bayer” designs, so the ultimate resolution is a bit lower than what you get from other cameras. However, all the dots are “real” dots, and I think this counts for quite a lot in terms of picture quality, as you can see above.

If you want a camera that is quite a challenge to use (the sensor and lenses on the Sigma camera are very good but the electronics and user interface are a bit of a pain when compared to more popular offerings) then you should take a look at the DP1 and DP2 devices. They are also a good way to learn how photography really works, in that to get the best from them you have to take things like exposure and focus very seriously.

You can pick them up second hand from ebay at reasonable prices, and they are great fun to use.