Wednesday, March 21, 2012

Nokia 808 PureView

                  The Nokia 808 PureView is a Symbian Belle powered smartphone, first unveiled on 27 February 2012 at the Mobile World Congress. It is the first smartphone to feature Nokia's PureView imaging technology based on a new 41 megapixel oversampling 1/1.2" sensor and a high resolution f/2.4 Zeiss all-aspherical 1-group lens. See examples.
The 808 won the award for "Best New Mobile Handset, Device or Tablet" at Mobile World Congress 2012.(...)

PureView Pro  Camera

                  PureView Pro is an imaging technology used in the Nokia 808 PureView device. It is the combination of a 1/1.2" large,very high resolution 41Mpix with high performance Carl Zeiss optics. The large sensor enables pixel oversampling, which means the combination of many pixels into one perfect pixel. PureView imaging technology delivers high image quality, lossless zoom, and improved low light performance (see below). It dispenses with the usual scaling/interpolation model of digital zoom used in virtually all smartphones, as well as optical zoom used in most digital cameras, where a series of lens elements moves back and forth to vary the magnification and field of view. Instead, it will give around 3x lossless zoom for stills, and 4x zoom in full HD 1080p, for 720p HD video, 6x lossless zoom and for nHD (640x360) video, 12x zoom.

PureView Pro specifications

§  41 megapixel CMOS image sensor, 1/1.2" image sensor format with 7728 x 5368 pixels, pixel size: 1.4 microns.
§  On-chip image processor performing image scaling with oversampling, giving lossless (digital) zoom: 3x for stills, 4x for full HD 1080p video with on-chip video processor performing image resolution processing with over 1 billion pixels per second[4] enabling the use of all pixels for improved image noise and dynamic range
§  Carl Zeiss optics with F-number: f/2.4. Focal length: 8.02mm: 35mm equivalent focal length: 26mm @ 16:9 and 28mm @ 4:3.Construction: 5 elements in 1 group. All lens surfaces are aspherical, partly extreme aspheric,one high refractive index, low-dispersion glass mold lens. Focus range: 15cm – Infinity (throughout the zoom range).
§  Mechanical shutter with short shutter lag and ND8 (3 f-stops) neutral density filter

Resolution processing image sensor

               PureView Pro sensor has an active area of 7728 x 5368 pixels, totaling over 41Mpix. Depending on the aspect ratio chosen by the user, it will use 7728 x 4354 pixels for 16:9 images/videos, or 7152 x 5368 pixels for 4:3 images/videos. What happens next depends on the settings and whether or not zoom is used. But to give an idea, the default still image setting is 5Mpix at 16:9, and for video, 1080p at 30fps. Using these settings, the zoom is around 3x for stills and 4x for video. Conventional digital zoom tends to scale up images from a relatively low resolution, resulting in poor image quality.

The image sensor is a new resolution processing image sensor, which is capable not only of delivering parts of its pixels, but ofdown- or oversampling its resolution by having its own on-chip image processor, highly reducing external processing needs anddata rates as well as image noise (see noise shaping) when lower resolutions (or HD video) are needed. Additionally this provides very high image resolution. Images up to 38 megapixels can be taken at full resolution at 4:3 aspect ratio and 34 megapixels at 16:9 ratio. The PureView is a pixel oversampling technology used by Nokia that converts an image taken at full resolution into 3, 5 or 8 megapixels to eliminate noise in the image.

Zeiss 1-group lens

                 The optic is a 1-group lens, which is based on a shiftable fixed-focus lens: identical to the highly regarded prime lenses in mostZeiss Planar or Tessar optics, focus is achieved by varying the distance to the image sensor (unit focusing lens). This construction has the advantage that no movable focus group is needed. Considerable movable (focus-range) lens groups need a minimum of one additional adaptive lens element in both the moved group and the stationary group, increasing the number of elements by at least two. This increases unwanted reflections as well as overall tolerances and therefore decreases sharpness.

Any 1-group lens is additionally an aperture-less lens, further increasing the freedom of lens element arrangement and allowing the designer to optimize the lens to only one f-stop. The lens consists of only 1 group with molded elements, which gives a highly stable, precise mechanical alignment. The lenses are partly made of plastic, which provides sufficient stability at this size and as a 1-group lens and has the significant advantage of making it possible to use extreme aspheric shaped lens elements.
5 all-aspherical lens elements are used, making it possible to increase border-sharpness and lower distortion and astigmatism. The high refractive index, low-dispersion glass additionally helps reduce chromatic aberrations. The disadvantage of a 1-group lens is that no aperture is possible; a neutral density filter with approximately ND8 (3 f-stops) is instead used. Although the lens is named a Vario-Tessar, it has almost nothing in common with the 4 element in 3 group, non-aspherical original Tessar.
Due to the comparatively large image sensor format of 1/1.2" and the comparatively fast lens with f/2.4, the camera has a quite shallow depth of field of approximately f/8, equivalent to a 35mm full-frame or approximately f/5, equivalent to a APS-C DSLR with the same angle of view (without zoom).


                Zoom with the PureView Pro works in a manner only partly related to the digital zoom principle. With no zoom, the full area of the sensor corresponding to the aspect ratio is used. Although it is only possible in this case to use the full resolution, pixel oversampling can be used to combine many pixels to calculate a single pixel and reduce image resolution. This will filter away visual noise from the image and greatly reduce noise in low-light conditions.
The limit of the zoom is reached when the selected output resolution becomes the same as the input resolution. That means once the area of the sensor reaches 3072 x 1728, the zoom limit is reached. So the zoom is always provides the true image resolution the user wants. The level of pixel oversampling is highest when zoom is not used. It gradually decreases until the maximum zoom is hit, where there is no oversampling. At this stage, PureView Pro optics and pixels start behaving in a more conventional way. But because only the center of the optics is used, the best optical performance is achieved – including low distortion, no vignetting and highest levels of resolved detail.


                    PureView Pro comes with improved autofocus also for video as the optics with the larger image sensor provide a relatively shallow depth of field. PureView Pro gives continuous auto focus in all shooting modes, close-up (Macro) focus, Face detection, Touch focus with easy manually selected focus point and Hyperfocal distance focus for defined depth of field, for extreme focus speed or when no reliable focus is possible due to darkness.


                      Most smartphone manufacturers crop off a section of the sensor to ease the processing load. In contrast, the on-chip oversampling image processor of PureView Pro enables oversampling of all 38Mega-pixels even at the high data rates of 1080p with 30 fps. Plus, it provides lossless zooming capability, which is output resolution dependent. Full HD 1080p gives 4x zoom; 720p HD video, 6x lossless zoom; and for nHD (640x360) video, 12x zoom. In addition, encoding is up to 25Mbps in 1080p H.264/MPEG-4 HD video format. The PureView Pro sensor integrates a special video processor that handles pixel scaling before sending the required number to the main image processor.

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