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(Pocket-lint) - Nokia PureView is a term getting bandied about quite a bit. The Nokia Lumia 920 is the latest smartphone to feature it but it was the Nokia 808 PureView that was the one that heralded it after the announcement at Mobile World Congress 2012. So exactly what is “it”? What is Nokia PureView?

Step a little closer, and let PL tell you a story.

Once upon a time in Barcelona, Nokia held forth the Nokia 808 PureView cameraphone in front of the people. It was a phone with the biggest image sensor in the back of it that the world had ever seen. And the people saw it, and it was good.

With 41 megapixels at the user's disposal, there was so much detail in each and every snap that it effectively became possible to zoom into your own pictures without the need of a zoom lens on your phone. You could crop, and crop, and crop without any loss. At least, that was the assumption. In actual fact, PureView - or PureView Pro as it actually was - was not supposed to work quite this way. Yes, there is a setting on the Nokia 808 to take full, 41MP images for you to play with in post-production, but that’s not everything that the company was trying to achieve with what it’s now terming PureView Phase 1.

Aside the ability to effectively give you 3x lossless zoom on your mobile phone stills images, and even 4x lossless zoom when shooting 1080p video (very handy indeed), came the quest for better image clarity and, specifically, picture sharpness in the same kind of non-dedicated camera device. The way Nokia set out to do this is with pixel oversampling.

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So, imagine you have this 41MP sensor inside the Nokia 808, which is lucky because you do. Now, rather than shoot a massive 41MP image, how about shooting one that’s the same size as an 8MP image instead? You’ve then got many more pixels capturing light than you need. Rather than ditching the information, however, you instead point the five or so pixels at exactly the same place and combine their outputs. The result is that you can reduce the effects of noise and distortion by comparing and averaging out the information for each given portion of the picture.

Normally, of course, that might not be an issue but, at low light, when noise is a big problem, the end shot should be much improved. Obviously, the more you zoom, the fewer pixels you have to play with for oversampling purposes but you can certainly achieve both effects on the same image to some degree.

So, that was the story for the Nokia 808 PureView as part of PureView Phase 1. It was all about getting better picture quality and getting some effective optical zoom on a mobile phone, something that had never been done before.

Then along came the Nokia Lumia 920. With PureView.

The trouble is that the PureView on the 920 is not PureView Pro. It’s just PureView. And what’s PureView? Well, it’s not a technology that you can nail down, according to Nokia.

“PureView is a promise of versatile capture capability and leading edge innovation,” says the official line. Make of that what you will.

In real terms, the PureView you’ll find in the Nokia Lumia 920 is part of PureView Phase 2. The aim of that drive is specifically to hit new highs in low-light performance and to provide smoother video in all conditions.

What you don’t get in the Lumia 920 is a 41MP sensor. It’s a 8.7MP sensor. You can still do some lossless zooming by sampling just the middle part of the sensor but it’s obviously nothing like as powerful as the Nokia 808.

While pixel oversampling was a good way to improve low light photography results in the 808, that’s obviously not possible in the Lumia 920 with far fewer pixels at your disposal. Instead, PureView Phase 2 focuses on three main different areas to achieve the same ends.

Back-side illuminated sensor

A BSI or back-side illuminated sensor (no sniggering) is something that’s been around in digital photography for quite a few years. Front-side illuminated sensors have much of the wires and circuitary right between the light-sensitive diodes on the sensor’s surface - the parts which caputre the photons of light - and the light coming in. It didn’t take some bright spark very long to realise that this wasn’t a very good idea and instead redesigned sensors to get all of the electronic gubbins out of the way. Hey presto, better light impact and less noise on your stills.

Wider aperture

A bigger possible aperture means more light can pass through to the sensor at any one time, and that means a better possible exposure in any given conditions. So, the PureView system in the Nokia Lumia 920 goes to a maximum of f/2.0. That’s only half a stop better than the f/2.4 you’ll find in other camera phones. Not a massive deal then.

Optical Image Stabilisation

The OIS is the clever part as far as PureView goes in the Nokia Lumia 920. Camera phones don’t have OIS. Proper cameras do. They generally have it to stop camera shake when zoomed in. Instead, Nokia uses it in the Lumia 920 to allow them to drop the shutter speed to as long as ¼ of a second - three stops lower than the slowest you’d dare go handheld on a proper camera. What that does for the 920 is allow much more light to enter the camera for any given shot, and Nokia is confident that their PureView OIS system will prevent any blur. The results from Nokia's demo certainly back up the claims.

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Normally, OIS mechanisms on digital cameras are all about using gyroscopes to feedback any movement readings which the camera can then compensate for. However, the lens and sensor camera array in a mobile phone is so small and light that, instead, Nokia has floated the entire optical assembly which keeps them far more steady and unaffected by slight shake. What’s more, the speed at which the system can adjust is much faster than a gyro-based OIS and, according to Nokia, the upshot is that the Lumia 920 can offer up to 8x longer shutter speeds.

So, better exposures with no increased camera wobble. The only issue, of course, is if your subject is moving as well. Short of floating them too, there’s not a lot PureView can do about that one.

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In short

So, what is PureView? PureView is a brand label; like Ultrabook. It’s Nokia’s promise that the company has put some effort into the camera on your phone, but buying a mobile with PureView does not mean you necessarily get OIS or a BSI sensor or indeed a 41-megapixel camera with the lossless zooming. You have been warned.

Nokia 808 Pureview review

Nokia Lumia 920 pictures and hands-on

Writing by Dan Sung.