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.

Comments

[Andrew]

The best way I know of to learn about photography is to buy a fixed focus lens, 50mm prime lenses are usually very affordable, and read http://norvig.com/dance-photography.html. Don't be put off by the fact that Peter Norvig uses dance photography as his subject, he has written the best and yet concise description of how exposure and focus work in cameras that I've ever read.

The reason to use a fixed focus lens when learning to use a SLR is so that (a) you have access to wide apertures (< f/3.5) which are always missing on the zoom lenses that come with most SLRs and (b) it takes your brain away from zooming and onto the controls of the camera so you learn to get exactly the exposure and focus you really want. As a side benefit the prime lens will also show you the type of image quality you can get by spending the big money on high end zoom lenses.

[Rob]

I know exactly what you mean. Modern cameras are great "instruments of record" but with small sensors and large F numbers on their zoom lenses it is difficult to be creative because every picture comes out looking sharp all over no matter what you do. One of my favourite lenses for playing around with is the F1.7 50mm Canon prime lens that you can get for a very good price. This takes astonishingly sharp pictures but when you open it up you can really play around with the shallow depth of field that it has.

The DP1 and DP2 both have fixed focus lenses. The DP2 opens up to F2.8 which, with a large sensor, means that you can control which parts of a scene are in focus.