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There are different type of digital camera sensors, those sensors that can capture only one color at each photosite and those who can capture more than one color at each photosite.

Which one is better for image quality and why do most of today's camera digital sensors use the first type (the one that capture only one color at each photosite)?

mattdm
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1 Answers1

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Capturing three colours per photosites is in principle far superior to capturing one colour and interpolation. So much so that a three colour sensor will produce an image with equivalent detail to a one colour sensor with twice as many total pixels.

Why not three times as many? Well the colour channels in an image are not independent but correlated with eachother that means knowing the red value often gives you information about the green and blue values.

Three colour sensors have the additional advantage that colour aliasing artifacts don't occur during raw conversion, meaning manufacturers can do away with anti aliasing filters to improve sharpness.

However in practice three colour sensors have problems with light sensitivity leading to poor performance in low light. In addition to this there practical/economic problems with producing three-colour sensors with high pixel densities at low cost, which is another reason why Bayer sensors despite their apparent inferiority have gained almost ubiquitous adoption.

Matt Grum
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  • Isn't the anti-aliasing filter the main cause of loss of detail with Bayer sensors? (Rather than the color-per-photosite thing per se?) – mattdm Apr 23 '12 at 22:52
  • Also, what about color accuracy for Foveon, particularly at higher ISOs? – mattdm Apr 23 '12 at 22:57
  • @mattdm A Bayer sensor with no AA filter will still have less detail than a Foveon sensor. AA filter-less cameras often seem to have more detail due to aliasing (which with natural or organic textures looks like detail). Think about a 12MP Bayer sensor shooting a scene with only red objects, the blue and green pixels receive no light so you really only have a 3MP image. A 12MP Foveon (36MP in Sigmas marketing speak) would get 12 million red samples and give a true 12MP level of detail. – Matt Grum Apr 23 '12 at 23:16
  • But in practical photography, as you point out, there's usually correlated information. Between that and the lack of an AA filter, doesn't that make a pretty close horserace? – mattdm Apr 23 '12 at 23:23
  • Not to mention that Bayer sensors — for reasons of market share and research, or otherwise — seem to have less trouble increasing pixel density, especially for the cost. The new Nikon D800 is a 36Mpix Bayer sensor, whereas the top-of-the-line Sigma is 15Mpix — which is 45Mpix in their terms, but "only" equivalent to 30Mpix in the probably-more-realistic terms you've given. – mattdm Apr 23 '12 at 23:26
  • Not trying to argue here — I just think there's a lot to the story! – mattdm Apr 23 '12 at 23:27
  • @mattdm In the totally uncorrelated case (red/blue detail) the Foveon has resolution equivalent to a Bayer with up to 4x the pixels. In the totally uncorrelated case (black/white/grey objects) the Foveon has no advantage. In the semi-correlated case empirically it has been shown the advantage is 2x. Which is not a close horserace in my opinion! However I have ignored the economic argument so far, and that is that Bayer sensors with 2x resolution are often still considerably cheaper than Foveon sensors, as you point out, which in practical terms makes them better. – Matt Grum Apr 24 '12 at 12:14
  • I agree that 2× isn't a close race. But doesn't removing the AA filter (either just with standard Bayer as in the Leica M9 or Nikon D800E, or with a new color array like Fujifilm's) make it closer? – mattdm Apr 24 '12 at 12:57
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    @mattdm a little closer, yes, but part of the perceived sharpness gain from removing the AA filter is actually aliasing, and then there's the disadvantage of colour moire in fabrics. What you actually want is to cram so many megapixels in the sensor that you can do away with the AA filter, and bin different coloured pixels together if necessary to get Foveon style dense colour sampling, but without the low light penalty or huge cost. With 24MP crop sensors arriving that day is close! – Matt Grum Apr 24 '12 at 13:42