DPReview member, Dogonit, demonstrates quite clearly how the 800$ Sigma pulls out more information from the same low ISO exposure than the twice-expensive Nikon D610.
“NOT ALL PIXELS ARE CREATED EQUAL: The Foveon sensor in the Sigma and the Bayer-type sensor in the Nikon are very, very, VERY VERY VERY different beasts. When Sigma claims a 48-Megapixel sensor it’s an overstatement, but one that’s based, at least partially, in comparative reality. The Foveon sensor does indeed have 48 Megapixels! They are just stacked over top of each other, like how film is made. The RGB sensors are translucent, so that the sensor takes a red, a green, and a blue reading for every pixel it ends up rendering, which comes out the other end as a 16 Megapixel image. How is this significant? Well, consider how a Bayer sensor works. The D610 is listed as a 24 Megapixel sensor. Does is have 24 Megapixels? Yes, BUT...each of these pixels picks up only red, green OR blue information. The Bayer filter then takes that information, calculates probable values based on averages from neighboring sensors, and estimates what each pixel’s color should be. What comes out the other end? A 24MP image, but it’s not really a 24MP image because it hasn’t really gathered full 24 Megapixels of Red, Green and Blue data; it’s captured somewhere in the neighborhood of 14 Megapixels (because there aren’t equal numbers of R, G, B pixels - there are far more Green pixels to record information). If you don’t understand this simplified explanation I encourage you to read more technical articles that explain this whole thing in detail. I assure you though that this is an accurate representation of what’s going on with each of these sensors. What I’m getting at is this: Don’t cry foul when Sigma claims 48 Megapixels. Nikon and Sigma are both over-representing their sensors in terms of the data it collects vs. the final output.”
And I agree. But then, you can count me in with those that feel the next logical step is 56 megapixels in a 35mm FF digital camera.