What’s The Big Deal With Foveon Sensors?

13 11 2007

One of the big selling points of the Sigma DP1 is its Foveon sensor. Why, you may ask, is that such a big deal? What follows is the short answer. (If you want the long answer, start Googling.)

Foveon is a company that makes sensors for digital cameras. The Sigma DP1 uses the Foveon X3 sensor, which is also found in larger cameras such as the highly anticipated and well received new Sigma SD-14 DSLR. Before we get into the specifics of the Foveon X3, what follows is a bit of background information.

There are basically two types of digital camera sensor; CMOS (complementary metal oxide semiconductor) and CCD (charge-coupled device). Each type has its advantages and disadvantages, which can be summarized as follows:

  • CCD sensors typically produce less “noise;”
  • CCD sensors typically are more light-sensitive;
  • CMOS sensors use far less power (up to 100 times less);
  • CMOS sensors cost less to produce.

Both types of sensor are currently in use, but CMOS seems to be gaining in popularity, due largely to market pressures to drive the cost of cameras down and to design for long battery life. Also, with evolving technologies, some people claim that the sensitivity and noise issues associated with CMOS sensors are becoming less apparent.

All that said, the Foveon X3 sensor that will be used in the Sigma DP1 is a CMOS sensor. In short, that means good battery life and a lower price (as if… – the DP1 will not be cheap). However, there is another factor that sets it apart from both CCD and other CMOS sensors. Without getting into a lot of technical details, suffice to say that the Foveon X3 uses a different array of photosensitive diodes on the sensor; an array that is said to provide better light sensitivity and image quality by mimicking color negative film in that the red, green, and blue (RGB) color sensitive pixels lie stacked on top of each other, in layers, instead of spread on a single layer as is found in conventional, mosaic sensors.

Foveon vs. Conventional Sensors

Conventional sensors (both CMOS and CCD) use what’s called the Bayer filter array. According to this Wikipedia entry, the differences between the Foveon X3 and Bayer array sensors can be summarized as follows:

In the Bayer sensor, each photosite in the array consists of a single light sensor (either CMOS or CCD) that, as a result of filtration, is exposed to only one of the three primary colors, red, green, or blue. Constructing a full color image from a Bayer sensor requires demosaicing, an interpolative process in which the output pixel associated with each photosite is assigned an RGB value based on the level of red, green, and blue reported by those photosites adjacent to it. The Foveon X3 sensor creates its RGB color output for each photosite by combining the outputs of each of the stacked photodiodes at each of its photosites. This operational difference results in several significant consequences.

In the case of the Foveon array, the consequences are essentially:

  • Color artifacts normally associated with Bayer sensors are eliminated.
  • Light sensitivity is increased (although color noise may also be increased in low light situations).

Here’s another view of the differences:

Foveon vs. Conventional Sensors

In this illustration you can see that with the Foveon X3 array, there are actually three pixels in each location (stacked on top of each other). This is tricky, because it can confuse the whole pixel count issue, which is highly over-rated anyway. In short, the DP1 bills itself as a 14 megapixel camera, but you need to divide that by three to arrive at a real sense of the dimensions of the sensor. In other words, it’s really a 4.6 megapixel camera times three (or, three layers of 4.6 megapixels).

Wait a sec… before you scream “What? Only 4.6 megapixels?” keep in mind that we’re talking about 4.6 megapixels, times three, on a really big sensor. And remember, as I mention in another post, sensor size is more important than pixel count – you are better off with fewer pixels on a big sensor than lots of pixels on a small sensor. In other words, forget about counting pixels; what counts is sensor size and (according to Foveon), pixel array.

There is still the question of noise on the Foveon X3, since it is a CMOS sensor. That issue is contentious; some reviewers of cameras using those sensors claim the resulting images are noisier than with comparable non-Foveon sensors. However, it is unclear if that is due to the sensor itself, or the image processing algorithm of the camera’s software. This review of the Sigma SD-14 on PopPhoto.com implies the images have very little noise. And remember; the Foveon array supposedly eliminates the color artifacts (a type of noise) that one gets with Bayer array sensors.

When it comes right down to it, the big deal as it pertains to the Sigma DP1 is not so much that it comes with a Foveon X3 sensor as that it comes with a big sensor. A “DSLR-sized” sensor in fact, which is revolutionary for any pocket digital regardless of whether it’s a Foveon or a Bayer array.

After all, when it comes to digital, size does matter. (I’ll say more about relative sensor sizes in a future post.)

Further reading:

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