Nokia: The Nokia 808 PureView packs a Carl Zeiss f/2.4 lens and a 41-megapixel image sensor, capturing 16:9 photos with 7152×5368 pixels. By oversampling up to seven adjacent pixels into a single pixel, 5-megapixel photos can be generated. The gargantuan image sensor allows lossless 3x digital zooming. Continuously-focused video is captured at 1080/30p with 4x lossless digital zoom. That’s the amazing part; now the not-so-amazing part: The 4-inch ClearBlack AMOLED is limited to 640×360.
NOKIA REDEFINES DIGITAL ZOOM
When it comes to mechanics in the Nokia 808 PureView, we went overall the approved limits. This was a tremendous engineering achievement by a big team of passionate and dedicated people. When we started this project in 2007, we kept it very secret. The imaging experts at Nokia and in our partner companies did an incredible job. None of us were willing to give in when facing this incredible challenge.
We made dozens of optical designs trying to build the sensor, tried many different options for the algorithms, and worked intensively with our partners on the sensor design. We weren’t sure what kind of risks were involved, so we started concepting the technology for one device only. The device itself wasn’t actually relevant for us: we were more interested in the technology as a prototype.
The optics in Nokia PureView provides amazing sharpness. Few people will believe how sharp it is until they see the pictures. There is an old myth in mobile imaging that small lenses can’t produce good quality images. This is simply not true, since the performance depends on design, materials and the precision used in the manufacturing.
First, the specs. The Nokia 808 PureView sports a 1/1.2-inch 41-megapixel (7728×5368) image sensor. Each pixel is 1.4 microns. In 4:3 mode you get 38 megapixel images with a pixel format of 7152×5368. In 16:9 mode it’s 34 megapixels and 7728×4354. The sensor is coupled to a 26mm (16:9) / 28mm (4:3) f/2.4 Carl Zeiss aspherical lens system, which includes one high-index low-dispersion glass mould lens. The camera sub-system incorporates a neutral density filter and a mechanical shutter. The specifications are impressive, especially considering it’s used in a smartphone. But there is one particular reason why Eero Salmelin and Juha Alakarhu decided this must be developed: To vastly improve the poor image quality on zoomed photos using camera phones.
Digital zoom simulates optical zoom by cropping a portion of the image and then enlarging it. Let’s say you have the Canon S95. The CCD image sensor has 10.0 effective megapixels for a pixel format of 3648×2736. A 2x digital zoom means you’re image is now reduced to having just 1824×1368 pixels. A 4x digital zoom will further reduce the pixel format to 912×684. With digital zoom image quality degrades. Nokia:
When you zoom with the Nokia 808 PureView, in effect you are just selecting the relevant area of the sensor. So with no zoom, the full area of the sensor corresponding to the aspect ratio is used. The limit of the zoom (regardless of the resolution setting for stills or video) is reached when the selected output resolution becomes the same as the input resolution.
In other words, when you’ve set the image size to 5 megapixels or 3072×1728 the 808 PureView uses all 7728×5368 pixels to capture that image. All those pixels are oversampled to generate a 3072×1728 5-megapixel photo. When you apply a 2x digital zoom the pixels that are capturing the image is halved to 3864×2684. At this point you can no longer zoom, and there is no oversampling, but you have a 2x zoomed-in 5 megapixel photo with no degradation in image quality. In fact the image quality is claimed to be better than camera systems with optical zoom.
More lens elements are required for optical zoom and for correcting aberrations. But more lens elements interfere with image definition and light transmission. The simple structure of the Nokia 808 PureView allows for 10x greater manufacturing precision compared to SLR lenses, better image definition, and enhanced light transmission. I see great potential for professional digital SLR manufacturers to adopt lossless digital zoom to significantly improve photographic quality over optical zoom.
The team at Nokia responsible for the camera sub-system in the 808 PureView has achieved something remarkable: By completely rethinking digital zoom they have thoroughly redefined it. Digital zoom in turn is poised to redefine digital photography.