End of Megapixel wars and EF-S buying decisions

Started Feb 23, 2006 | Discussions thread
Arjen van Andel Contributing Member • Posts: 594
Not 24 Meg photosites!

Steve Deutsch wrote:

Fovean tech.

Hi Steve,

I'm not sure what you mean. Foveon technology, or you are a Foveon tech?

An 8Mp sensor has in total 8 million separate photosites. 50% are green, 25% red and 25% blue (RGBG). Those make up 8/4 = 2 million full colour sensors. Per full colour sensor two green, one red and one blue.

What it basically does is create 8 million full colour sensors by shifting the pattern. Normally the pattern can be something like: RGBGRGBGRGBG... The first sample is taken from the full colour sensor comprised by RGBG, then it shifts the pattern one position and uses GBGR as a full colour sensor, then it takes BGRG, and the next is the standard RGBG again. This is an oversimplification, of course, the next and previous lines are also included.

If one, for instance, photograph a 100% frame filling, pure green patch, the sensor records 50% correct pixels and the other 50% has to be interpolated from the red and blue photosites. This is the best scenario, since there are twice as much green sensors.

In this example the result will be 100% correct; all pixels will be green in the final image. It gets different when there are all sorts of colours in the frame, the guessing gets worse, and the smaller the detail, the sharper the edges and the farther colours move from green, the more it goes wrong.

Even if its clear that there are in fact 8 million full colour sensors on the chip (plus some extra around the frame to be able to reconstruct colour at the edges), those sensors still are four times larger than a single subpixel, and the software has no choice but to guess what's really there. It records a smudged pixel; four channels mix information of detail that is 1/4 of their combined size.

A Foveon-like sensor similar to Canon's APS-C format with stacked RGB channels would have a total of 8 million photosites, not 3x8 = 24 as you mention (but it has 24 million sensors). I don't think a stacked sensor can be seen as 3 separate sites; they're all in the same 'site' or location. But they record true full colour at that single site. The output file size would remain the same, but it's contents is much more accurate and the images look much sharper and cleaner and more detailed since there's no interpolated information to mess things up.


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