Smartphone Cameras : The Technology Inside

The most important features of smartphones is the camera. Buyers nowadays demand a good quality camera on the back of their handset. As they get better each year, for many they’ve replaced standalone point-and-shoot cameras as the go-to device for everyday photography, as they’re easier to access and more compact to carry with you. The front-facing camera is increasingly important too with the trend of ‘selfies’ across social media.

Basics of Camera Hardware

For smartphone cameras, like pretty much all cameras available on the market, there’s two main components that form the camera module: the sensor and the lens. They’re typically packaged together into a single unit that adds on to the smartphone’s main board through a ribbon cable.

This is the camera module from the Galaxy S5; a Samsung S5K2P2XX 1/2.6” sensor with an f/2.2 lens.

The sensor is the part of the camera that actually ‘captures’ the image. It’s a complex integrated circuit that typically includes photodetectors – the key component that captures light – plus amplifiers, transistors, and often some form of processing hardware and power management. When the smartphone’s camera software requests an image, the sensor provides all the necessary data.

The amount of megapixels a camera has refers directly to the amount of photodetectors in the sensor’s array. An eight megapixel sensor, for example, means that there are eight million photodetectors in the array.

The lens focuses light onto the sensor so the image looks crisp and clear. While it’s possible to use a camera without a lens, the resulting picture will be just a blur of colors as photons from all angles hit the sensor. Basically, you need a lens so the light from the large scene in front of the camera can be reduced and focused down to fit the small size of the sensor.

A breakout of the construction of Nokia’s PureView camera

Lens is a collection of multiple plastic or glass elements, with glass usually providing a higher quality, sharper result. Each element has a specific function in focusing the light onto the sensor, whether that’s generally shaping the light to fit the size of the sensor, correcting issues, or providing the final focus point.In a camera with autofocus, the final lens element (or collection of a few elements) will move closer to or further away from the sensor, thanks to the assistance of a motor. This allows different areas of the image to appear in focus, and is one of the key aspects of a practical camera system.

The distance between the lens elements and the sensor is what’s known as the focal length. This distance determines both the effective magnification of the camera system, as well as its field of view. A short focal length equates to a wide-angle lens with little magnification, and vice versa. How far the lens needs to be from the sensor changes depending on the size of the sensor itself, and what function you want the lens to perform. Smartphone cameras almost universally use wide-angle lenses and small sensors, meaning focal lengths are below 5mm.

Megapixel Count

This is the one most people are familiar with, and the area that usually falls onto marketing materials and spec sheets. That’s because it’s the easiest to understand: a higher megapixel count equates to more detail, which can be used for creating lifelike images, or for cropping and zooming. On a smartphone camera, zooming can be particularly important due to fixed-focus lenses, where you want a large megapixel count so detail is preserved.

Having a high megapixel count is all well and good, but it doesn’t even begin to tell the story of how a camera performs overall. A typical tradeoff with having a sensor packing many millions of pixels is a small pixel size, but conversely having too few pixels makes images look bad through a lack of detail. All camera manufacturers understand this trade-off, which is why on smartphones you’ll typically find sensors packing between five and 20 megapixels.

Comparing a 41-megapixel image (Nokia Lumia 1020), 20-megapixel image (Lumia 1520) and 5-megapixel image all at 100% crops

Megapixel (MP) count can be deceptive as well. Jumping from 13 MP (LG G3) to 20 MP (Sony Xperia Z2) may sound like a large jump, but it’s only 1.5 times more pixels and images are just 25% wider (5248 pixels wide versus 4160). At full resolution there’s not a massive difference between the images both cameras produce.

To truly get a large difference in quality and ‘zoomability’, you need at least four times the pixels: like going from 5 MP to 20 MP.

Pixel Size

Sensor size is useful for getting an idea of how much space in the smartphone camera module is consumed by the sensor, but less useful for gauging total light collection as megapixel counts vary between smartphones. This is where pixel size comes in, giving a direct measure of how large the individual photodetectors are in the CMOS sensor.

This is how the camera looks inside the One M8’s body looks.

Pixel size for smartphones fits into a narrow range between one and two micrometres (or microns, abbreviated as µm) in either the horizontal or vertical direction. Again, the larger you go, the more light each pixel can collect. This is why the HTC One M8’s camera, with a 2.0 micron pixel size, performs a lot better in dark conditions than the Samsung Galaxy S5, with 1.12 micron pixels. It’s simply because the pixels are larger and can capture more light.

Calculating these differences becomes very easy. The Apple iPhone 5s’ camera has a pixel size of 1.5 µm, which can capture approximately 88% more light per pixel than a 1.12 µm sensor found in the Sony Xperia Z2, for example. This is despite the Z2 having a larger sensor overall (1/2.3” versus 1/3.0”), as the Z2 has a much larger megapixel count (20.7 MP versus 8.0 MP).

This is Samsung’s ISOCELL sensor, a variation of which is used with the Galaxy S5.

ISOCELL is Samsung-specific technology for their BSI sensors, which places barriers between each photodetector to reduce crosstalk, improving sharpness and color accuracy, especially in low-light situations. Crosstalk is where photoelectrons bleed between the pixels, causing bloom and halo effects in certain conditions, so reducing these effects is important in producing clean images.

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