Counting Pixels

2007.02.21

Just after Sigma announced their 14 megapixels SD14, people complained about the way megapixels are counted, again. In fact, this is not a new complaint, people complained about the Fuji Finepix F700, the predecessor of the current Fuji Finepix S5 Pro.

The truth is that Sigma is just as wrong as every other digital camera manufacturer. A pixel is short for pixel-element and should have full-color information. On standard computer monitor, a pixel is composed of 3 sub-pixels. Each sub-pixel being one color component. Therefore, a 3 megapixels monitor has roughly 3 million pixels or 9 million sub-pixels.

When most digital camera makers measure megapixels, they count the locations on the sensor which individually accumulate photons. An accurate term for these is photosites. A standard sensor is monochrome by nature, so a Bayer filter is placed over it. This turns each photosite into a sub-pixel. Therefore a camera normally labelled as having 6 megapixels actually has 6 mega-sub-pixels.

The Sigma SD14 is different because it uses a Foveon sensor which does not use a Bayer filter. For that reason, Sigma multiplies the number of photosites by 3. After all, everyone else counts sub-pixels as pixels, and their photosites are therefore worth 3 pixels! The truth is that the SD14 has 14 million thirds-of-pixels. And a standard 10 megapixels DSLR has 10 million sub-pixels. The different is simply the spatial arangement of the sub-pixels. In the case of a standard sensor, each sub-pixel is at a different location relative to the image. In the case of a Foveon sensor, there are 3 sub-pixels at each physical location.

As for Fuji, their SuperCCD SR uses two photosites per pixel and a Bayer filter with each color covering one pixel and therefore two photosites. The two photosites are used to capture information for a single sub-pixel but with a greater dynamic range because the pair have different light sensitivities. Originally, Fuji used their first SuperCCD SR in the Finepix F700 which was labelled as a 6 megapixels digital camera. In fact, it had 6 million photosites. These photosites were combined into 3 million sub-pixels which were used to interpolate an image. Later Fuji introduced the S3 Pro with the same type of sensor and labelled it as 12 megapixels. Recently, with the announcement of the S5 Pro, Fuji refined its terminology to clarify that the S5 Pro has 12 million photosites which are combined into 6 million pixels (which are in fact sub-pixels). Its not entirely correct but its no more wrong than Sigma's description of the SD14.

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