cameraphones - What does interpolation mean in the Nokia 808 Pureview?

The new Nokia 808 Pureview claims a 41MP sensor when shooting still images. Engadget tells us that it is due to "interpolation jiggery-pokery that condenses four or five pixels into one pixel". I thought surely this was just marketing nonsense, but I don't know of any other cases where manufacturers were advertising such high resolution on a small sensor such as this. I also found references to "pixel-oversampling", which may be just another name for interpolation.

Then, I found example images from the camera, which confused me even more. These examples seem very impressive to me, for any cell phone camera, and potentially any camera.

So what exactly is interpolation, and are these results as great as the initial examples appear?

Answer

The white paper in your link explains this very nicely.

The "jiggery-pokery" that Engadget speaks of is not faking the high resolution, but rather going the other way around: the sensor really does appear to have that many tiny little photosites, but under normal use, it pixel bins. (Presumably, the image quality is pretty atrocious at the pixel-peeping level.)

Nokia says:

Pixel oversampling combines many pixels to create a single (super) pixel. When this happens, you keep virtually all the detail, but filter away visual noise from the image.

The sensor they're using is relatively big (for a compact camera) — they're saying it's a 1/1.2" format sensor, which would be a 13mm diagonal, which is only slightly smaller than the Nikon CX.

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Digital Photography Review now has a blog post explaining this, with pretty pictures. One key thing that they note (and which I didn't bother to work out but should have) is that the larger sensor means that the photosites-per-area is the same as for a typical 8mpix cell phone or ultra-compact camera.

And, I'm going to re-quote something they take from Nokia's blog (the link you posted):

5Mpix-6Mpix is more than enough for viewing images on PC, TV, online or smartphones. After all, how often do we print images bigger than even A4? [It] isn’t about shooting pictures the size of billboards! Instead, it’s about creating amazing pictures at normal, manageable sizes.

I sure hope DSLR makers take that same philosophy as large-sensor cameras increase in megapixels as well. (Canon's on the right path with sRAW.)