41MP Nokia 808 smartphone hints at pixel-combining future for small sensor cameras

Feb 27, 2012 at 20:56 GMT

Nokia has made the startling announcement that it has created a 41MP smartphone, the Nokia 808 PureView. Interestingly, in most shooting modes the camera will output 3, 5MP or 8MP stills, rather than offering its full resolution - promising greater quality and offering some clever features. And this isn't a trade-show concept model, this is a product that will be offered to the public, though details of when and in which countries haven't been announced. What's interesting isn't so much the pixel count as how it's used, so we took a closer look.

The first thing to realize is that this isn't a standard 1/3.2" mobile phone sensor, it's an unusual and remarkably large 1/1.2" type (five times larger). In fact, it's almost three times the size of the sensors in most compact cameras. As a result, its photosites are the same size as those in most 8.2MP cameraphone but the 808 doesn't try to create an image of the same quality, 5 times bigger. Instead it oversamples the image and then combines pixels (using proprietary algorithms Nokia says optimize detail retention) down to a smaller size (though there is a special 'creative' shooting mode if you want the full resolution - 38MP at 4:3 aspect ratio, 36MP at 16:9).

Diagram showing the size of the Nokia 808 PureView's 1/1.2" sensor in comparison to those used in various compact cameras and mobile phones. A Four Thirds sensor is included for scale.

This pixel-combination means that noise (which occurs randomly) is averaged-out across multiple pixels (around 7-to-1 in the 5MP mode). The high native pixel count also means that it's possible to effectively 'zoom' by cropping into the center of the image and reducing the number of pixels you average together. Consequently the 808 can offer a roughly 2.8x 'zoom,' while maintaining 5MP output, despite having a fixed lens. The image quality will drop (since the noise is no longer being averaged out), but it does mean you get a roughly 28-78mm equivalent zoom, without the need to have moving lens elements, making the process fast and silent. It also means the lens' 15cm minimum focusing distance is maintained.

And, although the benefits of pixel-binning are lost as you magnify-in, because its photosites are the same size as contemporary 8MP phones, the resulting 5MP should offer the same pixel-level quality even at full magnification.

The same process allows 1080p video to be shot with a 4x cropping zoom.

Much like the Panasonic LX and GH cameras, the Nokia 808 uses an over-sized sensor to maximize the area used to offer different aspect-ratio images.

Despite the large sensor and comparatively large f/2.4 aperture, you won't get much control over depth of field (it'll be equivalent to setting an APS-C DSLR's kit lens to 18mm f/5.6). The depth-of-field control is reduced still further when magnified-in, because it doesn't gain the shallower depth of field that longer physical focal lengths usually bring. So, while it's an improvement over most phones, we wouldn't put much faith in the Nokia white paper's talk of bokeh.

The interesting thing for us, though, is not the Panasonic-esque multi-aspect-ratio use of the sensor, nor the astonishing pixel count, it's the idea of using that high pixel count to offer lower noise or non-interpolated digital zooming, while maintaining a constant image size. As Nokia's blog points out:

'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.'

And that's something that might be interesting to see in future compact cameras - models that will concentrate on output of a sensible size so that the user can easily get the benefit of them oversampling the scene.

Click here to read Nokia's blog post about the 808 PureView, which includes more detail about the phone's other features.

And click here to read the company's white-paper on the technology underpinning it.

Nokia 808 PureView lens and sensor specifications

Nokia's sample images:

ISO 800 - 5.0MP, 4:3
ISO 114 - 5.3MP, 16:9
ISO 58 - 38.4MP, 4:3 note the extremely close focus distance required to offer such shallow depth-of-field