26 January 2014

Dark matters: the reason your smartphone photos are better than ever

If 2013's going to be remembered for one thing in the smartphone world, it's as the year where cameras got good. Almost any device you'd care to name now has a shooter that takes more-than-decent photos in daylight.

That leaves only one battleground where smartphone cameras don't fare so well – in the dark. On some of the biggest phones of the year, the main boasts haven't been about processing power or even the number of megapixels in the camera; but rather how good they are at taking photos once the sun goes down.

The HTC One, for example, was launched in March with only a 4MP camera, but one with much larger pixels (what HTC calls 'Ultrapixels). The reasoning behind that was clear: larger pixels means less noise, better light-gathering and all-round superior low-light performance.

Nokia, as well, has been focused on low-light performance, with both the Lumia 925 and Lumia 1020 featuring innovations like optical image stabilisation and oversampling to improve the phone's low-light performance.

Even Apple put a wider lens and better flash into its iPhone 5S, innovations that are almost exclusively geared towards taking better photos in the dark.

These approaches are a little different from those taken by other manufacturers: Sony is a prime example, with both of its 2013 flagships, the Xperia Z and Xperia Z1, sporting cameras that were big on the megapixel count, but suffered badly compared to the competition once things got dark.

The number one reason why 'proper' cameras – DSLRs – are so much better at taking photos in the dark is that they're a lot bigger. The camera's sensor, the little rectangle that 'sees' the outside world and turns light into electrons, is orders of magnitude larger in a professional full-frame camera like the Nikon D3S, or even a more enthusiast-level mirrorless camera like the Sony NEX C-3.

That matters because the bigger the sensor, the more light that hits it, and the brighter the picture is. It's like leaving a Post-It and an A3 sheet of paper out in the rain – far more water's going to hit your giant sheet of paper.

The end result is that low-light photos, like the one taken above, look great with a large-sensored DSLR, but are a poor-detailed mess on smartphones. In addition to just not being bright enough, there's another problem that plagues smartphone photos taken in the dark: noise.

To compensate for the small sensor size, smartphones often increase the sensitivity of their sensors, a value often referred to as ISO – the higher the ISO setting, the more sensitive the sensor is to light.

However, higher ISOs also generate 'noise', random pixels that light up or change colour, making the photo look grainy and downright horrible. It's not unlike turning your speakers up past where they were meant to – everything turns into a muddy mush.

Bigger (and more expensive) sensors can generally go up to much higher ISO settings without noise, which is one of the reasons DSLRs can shoot much better in low light. Whereas an iPhone or Samsung Galaxy can dial up to around ISO 3200, the very best DSLRs go all the way to 204,800, and even more pedestrian mirrorless cameras hit an ISO around 25,600 with ease.

To get around the problems of noise at higher ISO levels, manufacturers – in particular Nokia – have been turning to clever software tricks. Thanks to the 41MP sensor in the Lumia 1020, Nokia are able to employ oversampling, a trick that essentially combines pixels, reducing the overall megapixel count of the camera, but in turn cutting out those rogue pixels that cause noise.

According to Juha Alakarhu, Head of Imaging Technologies at Nokia: "With oversampling, we can avoid many of the problems that traditional cameras have, and we not only overcome the problems but make things much better".

Another factor in capturing good low-light images is the quality and size of the lens. For light to actually get to the sensor in the first place, it has to go through the optics, and for low-light photography, one number is important above all others: the aperture size. This number, measured as an f-stop (where, bizarrely, smaller is bigger), determines how wide the aperture on the lens goes at maximum, and therefore how much light goes through to hit the sensor.

Again, aperture size is something phone manufacturers are pushing hard. Whereas the iPhone 4 and Samsung Galaxy S2 both featured aperture sizes of around f2.7, the iPhone 5S goes all the way down to f2.2 – and Nokia goes even bigger, with even its midrange phones like the Lumia 720 sporting a f1.9 lens. That's close to the performance of full-sized cameras, where even the best lenses, costing thousands of pounds, rarely get beyond f1.4.

In addition to the aperture size, there's something else manufacturers can play with: optical image stabilization. The reason many hardcore photographers use tripods to capture images in the dark is that by increasing the exposure time (how long the shutter's open for), you increase how much light hits the sensor.

Sadly, long exposures also mean one other thing: blur, from our ever-fallible shaky human hands.

Optical image stabilization is a means of staving off blur: by essentially giving the lens a bit of suspension, minor shaking can be eliminated, and slightly longer – and brighter – exposures can be taken. Although optical image stabilisation has been a staple in high-end camera lenses for around a decade, it's a relatively new (and welcome) addition to the smartphone world, with Nokia adding it since the Lumia 920, and the HTC One following close behind.

However, optical image stabilisation, being a physical process, adds bulk to phones – one of the reasons the camera is so prominent on phones like the HTC One and Lumia 925. That's the reason why many manufacturers are now looking to a software solution to low-light photography – post-processing.

Just as Photoshop can fix red-eye and a wonky horizon, software can also fix the noise: it's just a matter of processing power. With phone processors now routinely quad-core monsters like those packed into the Samsung Galaxy S4 and Lumia 1520, processing power is abundant – and image processing, historically a task out of reach of mobile devices, is now pretty simple.

Nokia has been "building its own propriety imaging algorithms", and it's now even added support for the lossless RAW image format, which is far more friendly to post-processing on computers. Hopefully, that means upcoming smartphones will hit higher ISO levels with less noise, all meaning there's a better chance of you getting a half-decent selfie in the next bar you choose to frequent.

Across the board, then, there's no indication of the low-light improvements slowing down. Although some things – like the size of the aperture – are almost as good as they can physically get, there's constant improvement across the board, all minor changes that promise major rewards in the near future.

According to Nokia, "The overall image quality is the combination of all these things coming together, and we continue to work hard and push the boundaries in all of these areas".

But with the best cameras still costing thousands of pounds, not to mention being larger than a whole flock of iPhones, the challenge is reducing these breakthroughs down to a size – and more importantly, price – where they can be crammed into our pockets.

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