When I first heard about the Nokia 808 PureView and its seemingly impossible 41-megapixel camera I was, to say the least, somewhat sceptical. 41 megapixels is a greater resolution than even a professional-grade medium-format DSLR such as the Pentax 645D, and definitely not what one expects to find in a snapshot camera built into a mobile phone. Like most people I assumed it was just marketing hype, that it would use some sort of image interpolation, of the sort that used to bring Fujifilm a lot of criticism back in 2002, to produce a 41MP image from a lower resolution sensor. However, I was wrong; it really does have 41 million photocells, all crammed onto a sensor that will fit inside a mobile phone. How on Earth has Nokia achieved this apparent miracle, and more importantly, why?
To understand the 808 PureView properly we need to take a look at that unusual sensor, particularly its physical characteristics. Unfortunately, this means I'll have to talk mostly in numbers for the rest of this paragraph, for which I sincerely apologise. Most mobile phone cameras use extremely small sensors, usually 1/3.2in, or in real terms approximately 4.54 x 3.42mm, with a surface area of 15.5mm2. Most compact cameras use 1/2.3in sensors, which typically measure 6.17 x 4.55mm giving a surface area of 28.5mm2. Advanced compacts such as the Canon G12 use a 1/1.7in sensor measuring 7.44 x 5.58mm, an area of 43.4mm2. The Nokia 808 PureView uses an even larger sensor than that; it's a 1/1.2in chip measuring 10.67 x 8.00mm, with a surface area of 85.33mm2. In other words, its sensor has nearly double the surface area of an advanced compact, and over five times the surface area of a typical phone camera sensor. However it's still only about a quarter the size of an APS-C sensor (typically 370mm2), and less than one-seventeenth the size of the 40-megapixel sensor (44 x 33mm, 1452mm2) in the Pentax 645D, the only camera with a comparable resolution.
What this means of course is that the individual photocells are extremely small, in fact they're 1.4 microns, roughly the same pixel density as a compact camera with a 13-megapixel 1/2.3in sensor. In fact, if you think of the sensor as three 13-megapixel 1/2.3in sensors bolted together it makes a lot more sense. Unfortunately, such overpowered compacts are very prone to excessive image noise. Photocells that small are relatively poor at capturing light, especially when there's not a lot of light around to be captured, so they tend to produce very poor image quality in low light situations. To try and get around this the 808 camera combines the output of several photocells into one pixel, a technology which Nokia calls "Pixel Oversampling", but which regular readers will recognise as "pixel binning", a common high-ISO noise reduction technique used in cheaper compact cameras. From its 41MP sensor, it produces images of 8MP, 5MP or 3MP, depending on the quality setting, combining the output of different numbers of photocells into one pixel in the final image.
The problem of course is that it's a trade-off, and below a certain light level, with photocells that small it really doesn't matter how much you "oversample" them, they're still not very good at collecting light. Zero multiplied by eight is still zero.
But then the 808 can also produce full-resolution images of 38 megapixels in 4:3 aspect ratio, or 36MP in 16:9, which is quite a party trick, and we'll be taking a look at these later.
Another technological breakthrough that makes this larger sensor design possible is the 808's high quality Carl Zeiss lens, which was specially developed to match the sensor. Usually a larger sensor requires a greater distance between the lens and the sensor, which is why more advanced cameras are usually quite big. By using a non-zoom, fixed aperture lens designed specifically to match this sensor, the camera unit on the 808 is able to remain relatively small, although it does still make quite a bulge on the back of the phone. The lack of optical zoom is something of a limitation, but the 808 does offer a "lossless" digital zoom, adjusting the pixel oversampling to compensate for the cropping of the image and retaining much the same quality.
So how well does it all work? Actually, not as bad as I'd feared, but not as good as the hype would have you believe. If you've seen some of the previews of the 808 that have been appearing in recent weeks, as well as the sample pictures on Nokia's website, you'd be forgiven for thinking that it's the greatest camera in the world, and that professional photographers will soon be abandoning their Canons and Nikons in its favour. That's fairly unlikely, I'm afraid. Don't get me wrong; it's an impressive camera, but it's only impressive for a phone.
In terms of performance it's not at all bad; from the phone's homescreen a press of the shutter button on the side of the case switches on the camera in approximately 1.5 seconds, and it has a shot-to-shot time of approximately two seconds in 8MP PureShot mode, which is comparable to a good compact. The fact that it actually has a shutter button immediately puts it a couple of rungs further up the quality ladder than most mobile phones.
The contrast detection autofocus system is fast enough in good light, and it has an LED lamp that helps at close range in low light, although at longer ranges it doesn't focus at all well once light gets below late evening levels. It has a tap-to-focus function that is always on, which can be a bit annoying if you don't notice that a careless touch has accidentally moved the focus point to the corner of the frame.
In fact, the 808's biggest limitation as a camera is the fact that it's in a mobile phone. It's probably quite a nice phone (that's not really my field), but even with a shutter button and a proper Xenon flash, a phone is just not a good shape for a camera. It's awkward to hold it without accidentally touching the screen, which as I mentioned will move the focus point, and despite its relatively compact size it's difficult to shoot one-handed. Even the cheapest budget compact has better handling.
Most compact cameras also have a better list of features. The 808 has no spot metering, no manual white balance setting, no continuous shooting mode, and no delete-all option in playback; the single AF point is too big for accurate focusing, the fact that the flash is right next to the lens makes it prone to red-eye, and the exposure metering is terrible, under- or over-exposing a large percentage of shots. It seems to have only centre-weighted metering, and even minor backlighting such as the sky will cause the foreground to be very under-exposed. The lens has a fixed aperture of f/2.4, but it does have an optional neutral-density filter.
In terms of image quality, it's a mixed bag too. Some people have been raving about it, but to be honest I wasn't all that impressed. One of the main claims is that it's supposedly noise-free, but this is simply not true. Sure, at the 50 ISO minimum sensitivity setting the image quality is undeniably very good, but noise starts to appear even at 100 ISO, and by the 1600 ISO maximum setting it performs no better than any mid-price compact camera.
One area where the sensor does score fairly well is in dynamic range. In the PureView oversampling mode it's certainly above average for both shadow and highlight detail, roughly what I'd expect from a high-end compact. In full resolution mode however the dynamic range is, not surprisingly, extremely poor.
The level of detail at the full-resolution 38-megapixel setting (or 32 megapixels in 16:9 format) is undeniably impressive, showing up tiny details in the scene amazingly well. In my usual sample shot it could easily pick out wood grain from thirty feet away, at least in good light.
However that's not really the way the camera is intended to be used. It's at its best in the 8-megapixel PureView mode, using the pixel oversampling technology to its best effect. In this mode the colour rendition is very good, with bright vibrant hues in good light, and plenty of fine detail, but it's really still not any better than a good 8-10-megapixel compact camera.
It also suffers from a perennial problem of phone cameras. Because the delicate Zeiss lens is covered with a Gorilla Glass screen to prevent damage, it quickly picks up finger marks and general pocket dirt. Unless you remember to carry a lens cloth with you and clean it before every use you'll frequently find that a slight haze of reflected glare drastically reduces the quality of your pictures. Even with a clean lens, the glass screen is very prone to lens flare in bright sunlight, assuming we ever get any.
Like everything else in the known universe, the 808 can also shoot 1080p video, and it has to be said that the quality isn't bad, although again it bears the 'for a phone' caveat. It suffers from the usual CMOS sensor "Jello effect" if you pan too quickly, but it has that bright LED light for extra illumination, and the sound quality - mono only of course - is surprisingly good.
While there's no question that the Nokia 808 PureView is technologically interesting and represents something of a breakthrough in photographic technology, at the end of the day it's still only a phone camera, just one with a larger than average sensor. Yes, the images can be amazingly detailed at full resolution, and yes the "oversampled" images do look nice at lower ISO settings, but the trade-off between resolution and photocell size really doesn't pay off as well as the hype would suggest, and as a camera it's handicapped by the usual phone camera problem; the lack of an optical zoom lens, a sluggish AF system that doesn't work well in low light, and inaccurate and unreliable exposure metering.
Add to that the handling limitations of the phone as a physical form and the conclusion is that despite all its clever technology, the 808's PureView camera is still just novelty feature on a so-so smartphone. If someone uses the oversampling technology on a real camera, it might prove to be more interesting.
These test shots were taken using the 8-megapixel PureView mode, since that is supposed to produce the best results. The Nokia 808's lens has a fixed aperture of f/2.4, so I'll just list the shutter speed and ISO for each shot. The camera is also zoomed in a little to fill the frame, and I've applied +0.7EV exposure compensation, since that's how I shoot these with every camera. Above, is the full view of the test shot at ISO 50.
1/9th of a second, 50 ISO. At the lowest ISO setting the picture quality is undeniably excellent, with good contrast, rich colours and almost zero noise.
1/15th, 100 ISO. Already at 100 ISO there is a light dusting of overall image noise, especially in the green areas.
1/35th, 200 ISO. At 200 ISO we can see some odd artefacts along the edge of the bumper of the green car. Also, more noise.
1/60th, 400 ISO. At 400 ISO the noise reduction has caused quite nasty banding in the colour gradients, but still not bad on the overall detail.
1/125th, 800 ISO. At this point it's basically guessing where the edges are, and the colour is very blotchy. Still just about OK for a Facebook update though.
1/270th, 1600 ISO. At the maximum setting we're pretty much in webcam territory. This is really no better than any half-way decent compact camera. Not bad for a phone camera though.
The fixed-aperture f/2.4 prime lens does produce some optical distortion, but it's ironed out internally with software. The result isn't quite perfect, but it is very good.
Centre sharpness is excellent, but at 200 ISO there is some noise in the shadows.
Corner sharpness is also excellent, with no visible chromatic aberration.
This is the usual scene we use to assess detail. We captured two versions - one using the 8MP PureView setting (on a slightly cloudy afternoon in Britain), and the other using the full 38-megapixel resolution (on a brighter day). Below, are the full-size crops taken from each photo. (A link to download all the main, full-size photos can be found on page 4).
Full-size crop from the 8MP PureView setting - the level of detail is very good, at least the equivalent of a decent 8MP compact camera.
Full-size crop from the full 38-megapixels on offer - it has to be said, that's pretty impressive.
Dynamic range is actually very good, but it's about the same as an advanced compact camera. This was shot at the 5MP PureView setting.
Another 5MP PureView shot. Colour rendition and detail are consistently good, but the shot is somewhat over-exposed.