PureView 2 Explained: How Good Is The Lumia 920 Camera

Nokia Lumia 920

The word is finally out. Nokia announced the Lumia 920 with a different form of PureView camera. The camera specs don’t look extraordinary at first glance. It has no oversampling or lossless zoom. Neither can it produce highly detailed 38MP images. So what’s so special about this new PureView?

Looking at the camera specs

  • Sensor size: 1/3 inch
  • Sensor type: BSI CMOS
  • Resolution: 8.7 Million pixels; 8MP in 4:3 mode, 7.1MP in 16:9 mode
  • Aperture: f/2.0
  • Image stabilization: Optical (Barrel shift type)
  • Flash: Short pulse high power dual LED
  • Video: 1080p

Coming to the sensor it is about 10% larger than sensors used in other smartphones. However, pixel size and pixel density are same because of the 0.7 Million extra pixels. This allows the Lumia 920 to shoot in true 16:9 and 4:3 aspect ratios.

Needless to say, Nokia has done this earlier in N9 as well as the 808 PureView. BSI CMOS is where things begin to get interesting. Although many manufacturers have used BSI CMOS before in smartphones with varying degree of success, Lumia 920 is the first Nokia device to use it.

Before explaining BSI sensors, I would like to explain normal or Non BSI sensors commonly known as FSI sensors (Front Side illumination).

Basically the sensor in camera perceives light (and converts it into electrical signals). But light has to pass through various wanted and unwanted obstructions before it reaches the photodiode. Once light enters a camera with traditional FSI models it has to pass through three elements -

  1. On-chip lens (to collect light)
  2. Colour filters (otherwise the image will have no colors) and,
  3. Metal wiring.

In BSI sensors, the first two elements – on-chip lens and colour filters retain their position. However, the position of metal wiring and photodiode is interchanged (you can say that metal wires + photodiode combo is flipped towards its back, hence the name backlit!) This design reduces reflections from metal wiring thereby allowing more light to reach the photodiodes as illustrated below.

credits VisEra

The BSI sensors claim to capture twice the amount of light captured by FSI sensors.

Aperture also plays an important role. Larger aperture allows more light to enter the camera. No rocket science here. Nokia used a fairly large f/2.0 aperture to compensate for the smaller sensor size.

The big thing about this PureView camera is Optical image stabilization (OIS). Before diving deep into what this new OIS system does, it is important to understand why image stabilization is necessary?

While taking pictures, the camera shutter opens for a certain fraction of a second to allow light to enter the camera. While the duration is too small in good lightning conditions, it is prolonged in low light conditions to allow enough light to enter the camera. However even very fine movements (shaky hands) may cause blurring as the position of sensor keeps on changing while the image is being “captured”. OIS reduces this blur by compensating for the “shaky hands”  It detects vibrations and moves the sensor and/or the lens so that there occurs no RELATIVE movement.

Traditional OIS systems allow us to shoot two stops or four times slower. For example OIS would allow to capture a scene at 1/30seconds that can only be shot without blur at 1/120 (4times) without image stabilization.

Nokia has introduced a new type of Optical image stabilization which they claim is much better than traditional OIS solutions. According to Nokia (whitepaper)

Nokia’s new OIS system can cater for around 50% more movements per second than conventional OIS systems – up to around 500 movements every second! Besides the high frequency compensation, the system also needs to be able to respond extremely quickly to unintended movement to avoid so called “phase shift” or compensation lag.
Adding up all of the advantages of Nokia’s OIS system means camera shake in lower light can be compensated for to lower lighting levels than conventional OIS systems, ultimately resulting in low light photography. As a point of reference, and depending on the user’s ability to hold the device still, shutter speeds slower than 1/30th second typically results in camera shake. Depending on the amount of camera movement requiring compensation we’ve found in testing that shutter speeds as long as 1/4th second can be used.

Since Nokia is using a fairly large aperture ¼ second shutter speed would indeed be more than adequate in low light conditions.

Nokia also decided to dump Xenon flash in favor of LED flashes. These new  Short pulse high power dual LED flashes are much better than LED flashes used in other smartphones. According to the whitepaper

It provides the benefit of being able to be used as a continuous light source e.g. for video or as a highly popular secondary function; a torch and now for the first time – a pulse flash burst. This provides a similar capability to xenon flash where the flash fires a short single flash of light. This flash is short enough in duration to effectively freeze subject movement. Whilst the duration of the flash is still not as short as a xenon flash, it is short enough to provide a sharper picture of moving subjects than conventional LED’s which provide no such benefit.

Now this is something absolutely new so I won’t be able to comment on the efficacy of these flashes without using the device.

To compensate for oversampling Nokia has introduced “Nokia Proprietary Image Processing Technology” that reduces noise but retains the natural look of the image. Again something new that will only be put to test when we get real devices. However, Damian has been emphasizing about the software part in PureView so I believe that this isn’t just ordinary noise reduction algorithm.

How does this compete with original PureView? I really doubt that Lumia 920’s camera can match 808’s performance. Larger Sensor comes with many advantages like great dynamic range and better Noise performance. The LED flash is still slower than Xenon. I would like to emphasize a common misunderstanding that Xenon’s brightness makes it special. It is the speed of Xenon that really does the job. The WP8 camera UI will most probably be minimal and pro-users might find it a bit disappointing after using 808. Last but not the least there is no ND filter.

Even with some drawbacks, Lumia 920 may still match or even surpass the standards set by N8. We’ll see the real performance of this camera when it comes out in market.