Nokia's 41 Megapixel Camera Mobile Phone
Nokia ignited a bit of a controversy on Monday when it unveiled a smartphone with a 41 megapixel camera sensor dubbed the 808 PureView. Yes, you read that right—41 megapixels, not 14, or 4.1. It will soon be possible to buy a smartphone with as many megapixels as some low-end, medium-format digital SLRs.
Has Nokia completely succumbed to the megapixel myth? Well, no. Instead of positioning the PureView as the smartphone equivalent of a professional digital camera, Nokia is turning the conventional thinking that originally drove the increasing pixel counts of digital cameras on its head, and developing novel ways to exploit an overabundance of pixel data.
While we think there is room to criticize Nokia's specific implementation, there are plenty of good ideas wrapped in the "PureView technology" rubric that we think other smartphone makers—or for that matter, digital camera manufacturers—should consider.
Ridiculous megapixel counts
Forty-one megapixels is, on its face, a seemingly ridiculous number. Many current smartphones struggle to produce decent images even at a more common 8MP. Current compact cameras max out at about 16MP, and even top-end digital SLRs rarely reach pixel counts beyond 20 million or so.
The highest megapixel count on a conventional DSLR to date is Nikon's recently announced D800, which tops out at 36MP. And even that camera, which contains a full-frame 35mm sensor (24 x 36mm), has been criticized for having an overabundance of pixels crammed into the sensor.
The reason for the criticism is fairly simple. With all other factors being the same, capturing a higher number of pixels with a given sensor size means the individual photodiodes at each pixel location are smaller. Generally speaking, the smaller the photodiode, the less sensitive it is to light. Smaller sizes also mean that a smaller number of photons can saturate the diode, reducing dynamic range. And smaller sensors also mean decreased sharpness due to diffraction.
So, if the relatively large sensor on the D800 can suffer these effects with full-size professional lenses at 36MP, how can we honestly expect great performance from a much smaller, smartphone-sized sensor, its corresponding tiny lens, and a seemingly astronomical 41MP count?
If the 808 PureView were intended to be a 41MP camera, you would be justified in criticizing Nokia engineers' sanity. However, Nokia made some design decisions, such as utilizing a relatively large 1/1.2" sensor, to improve performance. Also, it's exploiting the high megapixel count to do interesting things such as oversampling to reduce noise, and creating one of the first usable "digital zoom" features to grace a compact camera.
Nokia says that its 808 PureView is merely the first smartphone to launch with its PureView technology, so it stands to reason that the company will exploit the technology in future models. We also think the ideas behind the technology are smart ones—more on that shortly—so don't be utterly surprised to see the megapixel counts of smartphones suddenly skyrocket if other vendors decide to implement similar features.
Bad news first
The 808 PureView runs Nokia's ancient Symbian operating system. Not only does this mean you'll be stuck in the relative smartphone dark ages if you opt for its impressive photography specs, it also means this particular model will probably never officially make it to US shores.
Though the iPhone 4 and 4S have gained significant popularity among casual shooters for its image quality, the iPhone is still a smartphone first, with camera capability added on—and the same can be said of most Android and Windows Phone 7 handsets. Many serious photographers would still prefer to have something more like a camera first, with smartphone features grafted on.
The Nokia N8 was a step in this direction with its 12MP sensor, Carl Zeiss lens, and proper xenon flash. The 808 PureView is another step in this direction, with the smartphone's size largely dictated by its internal hardware. To maintain comparable noise response to today's smartphones, Nokia had to use a relatively gargantuan sensor. Most smartphones today use a 1/3.2" sensor, roughly measuring about 4.5 x 3.4mm, and with 8MP that makes each pixel about 1.4 microns apart. To keep the same pixel pitch with 41MP, the 808 PureView has a 1/1.2" sensor that's roughly 10.8 x 7.5mm in size. That's over 5 times larger than typical smartphone sensors, and larger than the sensors used in most compact cameras sold today.
A larger sensor also requires a larger lens relative to smaller sensors. And, the 808's xenon flash tube and its power capacitors likewise require far more physical space than the meager white LEDs that pass for flashes on most smartphones. With such beefy hardware, it's no surprise that the 808 PureView is a bulky 18mm thick where the camera is positioned. That makes other smartphones seem wafer-thin in comparison, with the Samsung Galaxy SII measuring 9.9mm at its thickest point, and the iPhone 4S just 9.3mm.
So, if you want one of the most advanced smartphone cameras on the market, you'll have to settle for a brick in your pocket running yesteryear's operating system.
But boy, what an upside
On the other hand, Nokia has made, judging from specs and early samples alone, one of the best smartphone cameras on the market. For one thing, the 808 PureView is actually, by default, a 5MP camera. If the camera is making images with just 5MP, though, why bother with the huge 41MP sensor?
The answer is oversampling. At its default 5MP setting, every pixel in the finished image corresponds to about eight pixels on the sensor. This oversampling helps reduce noise, increase color accuracy, and increase sharpness.
Digital photo sensors are typically only sensitive to brightness, not color, which would result in a monochromatic, black and white image. To record color, sensors are covered with a patterned array of red, green, and blue filters known as a Bayer filter. By filtering the light that hits each pixel, some pixels record the intensity of red light, some blue, and some green. To determine a red, green, and blue value for each pixel location, the missing values for each pixel are interpolated from surrounding pixels.
Since each pixel in a finished 5MP image on the 808 PureView gets data from eight different pixel locations on the sensor, a value for red, green, and blue can be determined without extra interpolation. Furthermore, noise caused by some pixels returning random or inaccurate values is effectively averaged out by combining data from more than one pixel location. This results in more accurate color and reduced noise overall.
Additionally, oversampling reduces the softening effects of the Bayer filter, the anti-aliasing filter, and diffraction of the tiny lens, resulting in sharper images.
This type of oversampling is actually something you have probably seen quite often without even realizing it. If you have ever viewed an image in Photoshop at less than 100 percent view, you have seen the same effect. It's also the same kind of oversampling that is done when you reduce an image to a smaller size. Anyone with enough experience knows that if an image isn't super sharp or if it looks a little noisy at full resolution, that image will look much better printed at a small size or displayed on a screen at, say, 600-800 pixels wide. (It's also the same reason that images that may be noisy or soft can look sharp on the screen of your camera or smartphone.)
A sample 41MP image captured in bright daylight at ISO 55. Detail is quite sharp.
Nokia
Cropped at 100 percent, detail is still quite sharp. However, even at ISO 55 there is visible noise. Oversampling would make the noise virtually disappear, though, as seen above.
In fact, the 808 PureView isn't even the first camera to implement such oversampling—it's just more commonly called "pixel-binning." Some compact cameras use this technique to offer special "high ISO" modes for low light, as long as you're wi lling to settle for reduced pixel resolution. The difference is that by default the 808 PureView uses pixel binning all the time.
Resource: arstechnica.com
