Megapixel Count, Sensor, Pixel Size, Focal Length

Standalone cameras come in a huge range of types, which are often specific to certain uses: superzooms are great for photographing birds and airplanes, medium format provides the detail needed for magazines and posters, macro cameras are for getting up close, and broadcast cameras are equipped for streaming video to your TV.

Smartphone cameras, though, fall into the one bracket designed to be the most versatile without being complex. Sensors are small due to the small footprint of a handset, lens focal lengths and apertures are fixed to reduce the number of moving parts, the lens is wide-angle to be most useful for normal shooting situations, and there's not a lot of extra hardware to accompany the sensor and lens.

The Duo Camera on the back of the HTC One (M8), a 4-megapixel OmniVision OV4688 paired with a 2-megapixel OV2722 for depth sensing

That said, there's a range of areas in which phone cameras do differ between vendors and models. Each one of them is outlined and detailed below.

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.

Sensor Size

Sensor size anchors a lot of the other important values relating to a camera system, such as the f-number, necessary focal length, and its crop factor. Luckily smartphone manufacturers figure all of those things out for you, leaving the only one thing you need to worry about: the light gathering properties of the sensor.

Taken with a 20.7-megapixel, 1/2.3" Sony Xperia Z1 at ISO 50, 1/160s, f/2.0

This one should be pretty easy to understand. A larger sensor has more area for the light to fall on, equating to a greater ability to gather the light, assuming the megapixel count stays the same.

The size of a smartphone sensor is typically given as a fractional number in inches (eg. 1/2.3", 1/3.06"), which may appear to give the diagonal dimensions of the sensor, but actually doesn't. Instead it refers to a type of sensor, with a diagonal size around one third smaller. For example, a 1/2.3" sensor typically has a diagonal of 7.66mm, rather than the expected 11.04mm indicated by the number itself.

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. Photo: iFixit.

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.

The technology behind the design of the CMOS sensor can affect the light gathering properties of each individual pixel, but the easiest way to compare is just by going on size. A camera with 1.4 micron pixels captures twice the light (per pixel) of one with 1.0 micron pixels, calculated by comparing difference in total area. Another way of saying this is that the 1.4 micron sensor is one stop brighter.

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).

Taken with a 4-megapixel, 1/3" HTC One (M8) at ISO 200, 1/25s, f/2.0

As I mentioned briefly earlier, this trade-off between megapixel count, sensor size and pixel size is extremely significant. Some OEMs like Sony and Samsung push for megapixel count, others like Apple and Nokia like a balance, while HTC goes all out on pixel size.

There's no correct answer to which one is better, as it comes down to what you're after from your smartphone camera. You'll typically get better low-light images with larger pixel sizes, but a higher megapixel count may be more appealing if you shoot mostly during daytime. As always, I'd recommend checking the camera specifications and sample images for any smartphone you're considering purchasing, to see what sort of camera you'll end up with.

BSI, ISOCELL, Stacked CMOS, RGBC Filter, etc.

All of these terms refer to the construction of the sensor, the sensitivity of each pixel, and the susceptibility to noise.

BSI stands for backside illumination, and is a method of producing a camera sensor where the photodetectors are layered above the transistors and other components. It's a more complex method of CMOS production, but it reduces reflectivity, which in turn improves the light capturing ability of the sensor. BSI sensors are found in nearly all high-end smartphones.

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.

Stacked CMOS technology is found in Sony sensors, again improving the ability to capture light by pushing some parts of the circuitry below the pixel array. It's found in Exmor RS sensors alongside BSI technology.

Comparing the top four flagship Android smartphones in low-light conditions.

The RGBC filter made some noise at the time of the Moto X, and is found in some of OmniVision's high-end sensors. Instead of a Bayer RGBG filter, OmniVision adds in a clear pixel to the filter, which improves low light performance by passing through full brightness information to the processing hardware. The camera then translates this into Bayer photos for the SoC's image signal processor.

There are other forms of technology found in camera sensors as well, but these are the main types you'll find in 2014's products.

Focal Length

Focal length is the distance between the lens and the sensor, which determines the field of view and magnification. The actual focal lengths for most smartphone cameras aren't very useful for people familiar with photography terms due to the small sensors. This is why in many reviews, including ours on TechSpot, tend to mention 35mm-equivalent focal lengths instead.

35mm-equivalent focal lengths tell you what focal length the camera's lens would need to have if it were to produce equivalent images on a DSLR with a 35mm-format sensor. It's then easy to bracket the camera lens into different types based on what's already known about lenses for traditional 35mm cameras: 18-35mm designates a wide-angle lens, 35-60mm are 'normal' lenses, and above 60mm are long-focus (otherwise known as telephoto or zoom lenses).

Taken with a 20-megapixel, 1/2.5" Nokia Lumia 1520 at ISO 100, 1/1100s, f/2.4

All smartphones fall into the wide-angle lens bracket, typically somewhere around 24-30mm; the larger the number, the less wide angle the lens is.

Calculating the 35mm-equivalent focal length requires a ratio known as the crop factor, which tells you how small the sensor is relative to a 35mm sensor. The HTC One M8, for example, has a 1/3" sensor with a crop factor of 7.21 - this means the diagonal dimension of a 35mm sensor is 7.21 times larger than a 1/3" sensor - and if you multiply the One M8 camera's actual focal length (3.82mm) by the crop factor, you land on an effective focal length of 27.54mm.

There's not a great deal of difference between the focal lengths of smartphone cameras, although Samsung phones tend to use lenses slightly less wide than their competitors. It doesn't make a huge difference in practical use, though.