The key to the smartphone camera revolution: Fewer, bigger pixels

Apple has exactly the right idea when it comes to smartphone cameras.

When it introduced the iPhone 5s, Apple increased the sensor size by 15 percent but left the resolution at a relatively paltry 8 megapixels. As I explained in my previous post, 8 megapixels is still enough to make an excellent-quality 8″ x 12″ print…which is more than enough for most consumers, especially those taking photos with smartphones. And even a 1080p television or monitor can only display about 2 megapixels full screen.

A comparison of various sensor sizes. The dark orange square represents a 35mm "full frame" camera. The light orange square represents the 1.6X crop APS-C sensor found in Canon's entry-level camera line, and the light green square represents Four Thirds and Micro Four Thirds. Meanwhile, the smaller sensors are what you would find in point-and-shoot cameras and, at the smallest point (the dark blue), the cameras in smart phones. Image courtesy of http://www.macrumors.com/2014/07/17/iphone6-sony-13-megapixel-imx220-sensor/
A comparison of various sensor sizes. The dark orange square represents a 35mm “full frame” camera. The light orange square represents the 1.6X crop APS-C sensor found in Canon’s entry-level camera line, and the light green square represents Four Thirds and Micro Four Thirds. Meanwhile, the smaller sensors are what you would find in point-and-shoot cameras and, at the smallest point (the dark blue), the cameras in smart phones. Image courtesy of MacRumors.

The advantage of this approach is that the individual pixels are larger (1.5 microns), and that means more light and better image quality.

A missed opportunity

Unfortunately, most smartphone manufacturers are moving in the opposite direction by increasing megapixel counts. For example, the Sony Xperia Z1 has 20 megapixels. Although the Sony’s sensor is a bit larger (matching many point-and-shoot cameras at 1/2.3″), the individual pixels are just 1.1 microns…and that means less light per pixel and reduced image quality. It also means larger file sizes — which is a major challenge given the relatively limited storage space on smartphones and the increasing trend to move away from card-based storage in smartphones. Even if users back up their photos regularly onto computer hard drives or cloud services, even those have limited storage space.

Just who is making 12″ x 18″ “high-quality” prints from a smartphone camera anyway? And would any photo from a camera with such small pixels be worth printing at this size anyway?

For too long, digital camera manufacturers have leapfrogged each other on megapixel counts as a selling feature for consumers, and this has carried over into smartphones. As a result, the real technological improvements in camera sensors for image quality and high-ISO performance have been stymied by smaller and smaller pixels. But the people who really need higher megapixel counts are professional photographers, not consumers. Yet the professional Nikon D4S with a full-frame sensor only prints 16.2-megapixel photos…because that’s all they need. But because of the larger sensor, the D4S has a pixel size of 7.3 microns — massive compared to even the best smartphone.

So why not just add bigger and bigger sensors to smartphones? Why not a full-frame sensor?

Certainly moving from 1/3.2″ on the iPhone 5 to 1/3″ on the iPhone 5S was a reasonable improvement, and the relatively obscure Nokia 808 Pureview offered a whopping 1/1.2″ sensor. But larger sensors cost significantly more in terms of manufacturing and require larger and larger lenses. That means more bulk. Not exactly something you want to slide into your pants pocket or replace when your 2-year wireless contract ends. Without a major revolution in lens design (which is dependent on the laws of physics), we can only realistically go so big with smartphone sensors.

Polka Boy performs at the biergarten at the historic Rathskeller restaurant in downtown Indianapolis. Shot with LG Google Nexus 5 smartphone. f2.52; 1/20 shutter; ISO 1531.
Polka Boy performs at the biergarten at the historic Rathskeller restaurant in downtown Indianapolis. Shot with LG Google Nexus 5 smartphone. f2.52; 1/20 shutter; ISO 1531; pixel size 1.3 microns.

Fuzzy (literally) math

So imagine if Sony had opted to go for 8 megapixels instead of 20 on the Xperia Z1 with its 1/2.3″ image sensor.

  1. A 1/2.3″ sensor measures 6.16 mm wide x 4.62 mm high.
  2. An 8-megapixel image measures 3,456 pixels wide x 2,304 pixels high.
  3. Divide 6.16 mm by 3,456 pixels to get 0.0018 mm per pixel.
  4. One micron is equivalent to 1/1,000 of a millimeter, so multiply by 1,000 to get 1.8 microns per pixel. That’s significantly larger than the iPhone 5S, and that would mean significantly better image quality.

As good as that is, what if the Nokia 808 Pureview only had 8 megapixels of resolution instead of 41? Let’s do the math again.

  1. A 1/1.2″ sensor measures 10.67 mm wide x 8 mm high.
  2. An 8-megapixel image measures 3,456 pixels wide x 2,304 pixels high.
  3. Divide 10.67 mm by 3,456 pixels to get 0.0031 mm per pixel.
  4. Multiply by 1,000 to convert millimeters to microns, and you get 3.1 microns per pixel.

That’s substantially larger than the pixel size of the Sony RX100 (2.4 microns per pixel), which is an award-winning, high-end compact camera with 20.2 megapixels of resolution.

Other tradeoffs – cropping and digital zoom

So part of the appeal of the 1/1.2″ Nokia 808 Pureview and its smaller 1/1.5″ sensor successor the Nokia Lumia 1020, was the 41-megapixel resolution that enabled “reinvented zoom.”

What the cameras in these phones are really doing when they “zoom” is cropping the image to a more reasonable 5 megapixels without any loss in image quality. This is quite useful for smartphones, most of which don’t have optical zoom lenses. But when the pixels are small to begin with (1.4 microns on the 808 and 1.1 microns on the 1020), “lossless” doesn’t mean a whole lot in terms of image quality.

But, as someone who regularly crops photos, if the full photo drops to 8 megapixels, then my crops could take them down even smaller. It leaves less flexibility…but even if I crop all the way in to 2 megapixels, that’s still 1080p resolution…great for displaying on screen (or Facebook, Twitter, Instagram) or making 4″ x 6″ prints. The addition of a real optical zoom lens on a smartphone minimizes the need for digital zoom or cropping in the first place.

What does the future hold?

I don’t know if we’ll ever get to DSLR image quality or lens quality with our smartphones, but considering the rapid pace of advancements the industry has already made, I would also not rule it out. As it stands today, the best smartphones can go toe to toe with point-and-shoot cameras in terms of image quality, although the new generation of bridge cameras like the Olympus Stylus 1, Sony RX10, RX100 and Panasonic FZ1000 are raising the bar higher than ever for what a point-and-shoot camera can offer. There are only a handful of smartphone cameras that offer optical zoom at all (the Samsung Galaxy K Zoom is basically a point-and-shoot camera first and a smartphone second, so it’s a bit awkward as a phone), but massive efforts are underway to bring real zoom lenses to smartphones while still allowing them to fit comfortably in your pocket.

The smartphone and camera industries just need to focus their energies in the right areas…and Apple has certainly set the precedent for that. As advancing sensor technology continues to squeeze more and more out of every pixel, then we can start to talk about larger and larger print sizes with more and more megapixels.

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