Nokia PureView Pro explained: 41-megapixels isn't the headline

Nokia's 41-megapixel camera phone has certainly captured the headlines coming out of Mobile World Congress 2012 in Barcelona. It sounds a bonkers idea, killing the squabbling over 5-, 8- or 12-megapixel camera phones dead. But just how crazy is it?

Knowing that those familiar with digital cameras would be waiting to pounce on the 41-megapixel claims, Nokia have put pen to paper and provided plenty of detail on what they're actually planning to do with this new sensor technology.

The solution tackles a problem that affects camera phones specifically, and that's size. Having a bulky phone isn't acceptable, but as more and more people capture pictures with their phone, camera performance has become the new battleground for smartphones.

Nokia's solution isn't about capturing huge images, it's about using those high pixel numbers to make the sensor work more effectively in your phone. 

Nokia is calling the new sensor technology PureView Pro and it makes its debut in the phone of a similar name, 808 PureView, which we've had a play with already

Dispelling the myth

Photography, no matter what technology you introduce, has to rely on a basic principle: light passes through a lens on to the capture medium, in this case a digital sensor. The size of the sensor and lens dictates the light path; a larger sensor needs larger lenses to preserve that light path.

With phones, the small sensor means you can have a small lens and the distance from the lens to the sensor be kept small. Increase the sensor size and you have to increase the size of the lens proportionally to let the light through. The sensor of the Nokia system is 1/1.2-inch - smaller than a compact camera, but larger than most smartphones.

The desire to keep things small also makes it difficult to incorporate more complex optics, like a concertina lens as you'd find on a compact camera, which is one of the key points here.

Although there are 41-megapixels, you'll only ever be using 38-megapixels when shooting in 4:3, or 34-megapixels when shooting in 16:9. But these are still crazy numbers of pixels on a small sensor.

That's because Nokia's intention isn't that you pump out 38-megapixel photos, as if you had a medium-format studio camera. Instead, Nokia is introducing what it is calling "pixel oversampling", which in crude terms is combining pixels to reduce their numbers.

There may be 41 million pixels on the sensor, but you'll be telling it to combine those pixels and behave as though there are 8 million or 5 million, a much more conventional number for a camera phone. Nokia is stating that they can do this while keeping "virtually all the detail" and without introducing any noise. So, in reality, you will probably be using your phone camera very much as any other.

It's all about the zoom

But there is another benefit and that's zoom. As we said previously, putting a zoom lens on a phone will make it impractically bulky. Traditionally phones use digital zoom instead. In doing so, they use less of the sensor, but give you the same size image and upscale to try to preserve the detail. The results are poor, lacking in detail and introduce noise. 

Nokia's solution does away with the need to upscale. Instead, as you zoom the level of pixel oversampling reduces, so you start using fewer grouped pixels to produce your image. You still use a smaller part of the sensor, but because there are more pixels in place, you can still have a full complement of 5 million pixels for your zoomed image.

This will give you 3x zoom in a different way to conventional camera phones, without introducing the noise from upscaling and preserving the detail because you're still using a full array of pixels. You're using fewer physical pixels, however, so the real magic hangs in how effective the oversampling is at giving you a good clean image and how the pure pixels perform once they're acting individually.

Considerations

The company does make some valid claims about avoiding vignetting (dark corners introduced when the lens housing blocks some of the light falling on the sensor) and distortion (caused by the glass of the lens, often digitally corrected in the camera) as benefits over a compact camera. Beyond that, Nokia claims that you can preserve the f/2.4 aperture when at full zoom as, again, the lens isn't introducing limitations on the light path. True, this is an advantage over compact cameras.

Naturally, there is greater versatility and control in "real" cameras. Nokia extols the virtues of constant f/2.4 and "striking" bokeh, but doesn't consider you might want sharp landscapes at a narrower aperture, or a bulb exposure for your low-light shots. But that's not really the fight here.

The real point is how well the PureView Pro system copes with the demands of mobile phone photography. How well does it sidestep the problems of soft low-light images? How well does it deal with flash use? Does the oversampling produce clean results? Do the pure pixels at full zoom give you a clean image?

Nokia's sample images certainly look good, but don't they always? The real proof of the pudding will be how well the system copes in the hands of real people, taking real pictures.

It might sound outlandish at 41-megapixels, but the system could mark a change in phone cameras. Pure megapixels aren't the headline, it's how the system is going to be put to use. Nokia should be congratulated for pushing the envelope and trying something different because it isn't as crazy as it sounds.