This is a comment from Dan (@ZDP189) on my last Sigma SD1 post that I think it is worth sharing in a separate thread (you can check also Dan’s Fuji X100 review):
Ah, you’ve run into the old Foveon X3 pixel count vs resolution chestnut. Some say Foveon called their technology ‘X3′ because they overstate their resolution by three times.
Foveon’s argument is that Bayer sensors count one sensor element (each of which are monochrome, but tuned to red, green, or blue) as one pixel, so Foveon counts each sensor element as one pixel, even though red, blue and green are stacked on top of each other. In terms of file size alone, this is true.
However, it’s not a count of resolution as we know it. To most people, resolution represents how finely the image grid is laid out. By this measure, Foveon overstates their resolution by three times. The additional colour information is nice because it avoids certain colour artefacts, but it doesn’t represent extra resolution.
The reason it’s less than perfectly clear cut is that Foveon is less susceptible to moiré and the sensor has no anti-aliasing filter. AA filters reduce moiré by slightly averaging out detail, so the sensor is a little sharper without it. Some people claim to be able to see 2/3 of the resolution that Foveon claim. On the other hand, it is a matter of scientific fact that the difference of adding/removing the AA filter does not reduce/increase the resolution by a factor of two, let alone three times, so I think this claim of superior sensor resolution is bunkum. Indeed, many Bayer sensors have no AA filter these days; none of the Super Compacts have one. The X1 does not, the X100 does not. The M8 and M9 do not either. I have never read a complaint about these cameras having moiré issues. These cameras have great resolving power, but I wouldn’t claim that my X100 had 18MP, let alone 36MP.
