Sony have a great reputation in the smartphone camera sensor market. You'll find a Sony sensor at the heart of many of today's flagship smartphone cameras, or those phones with a reputation for good image quality. This includes the Apple iPhone, Samsung Galaxy S-series, the HTC 10, the Google devices from last year and this year, and a number of other handsets too. Logically enough, you'll also find a Sony image sensor inside the Sony Xperia devices, too, but for 2016's Sony Xperia XZ, the Japanese company has decided to use the IMX300 sensor rather than a higher numbered part, such as the IMX377 (as used by a number of devices ranging from the LG-built, Google Nexus 5X to the HTC 10). These sensors are a similar physical size but a key difference is in the number of pixels that each uses to capture images: the IMX300 is a 25-megapixel sensor and the IMX377 is a 12.3-megapixel sensor. This means that each light capturing pixel in the IMX300 is smaller than the pixel in the IMX377, but there are almost twice as many. By the numbers, the IMX300's pixels are 1.1um compared with 1.55um for the IMX377. The chief advantage of going for larger pixels is that they capture more light, which is important for low light photography. The chief advantage to using more pixels is that it results in greater detail in the image. But does this make one sensor "better" than the other?
When it comes to handset components, the best part for the device in question very much depends on the context. The best mobile chipset today depends on what we are measuring it on: Apple's latest chipset has the highest benchmarks, but should this make it the best chipset today? It is only available for Apple products; we are not going to see a Google Pixel device based around an Apple processor. Samsung's AMOLED displays are reckoned to be the best in the industry, but the South Korean electronics giant has 99% of the AMOLED market so there is not so much competition. And there will be many reasons why Sony picked the higher megapixel sensor for the flagship Sony Xperia XZ rather than follow the current market trend of going for a lesser number of pixels, but making them larger to capture more light.
Although we will likely never know the full reasons why Sony opted for the IMX300 rather than the IMX377, or the IMX378, camera sensor in the Sony Xperia XZ, we should consider a number of factors. The first is that the sensor used to capture images is only one part in the imaging process: manufacturers need to provide a lens assembly, which makes a big difference. Once an image has been captured, the device software needs to be applied to result in the end image. There are additional technologies that may be used to improve matters, such as optical image stabilization, intelligent focusing systems, smarter flash timing and similar. However, putting these aside, these three elements of the lens, sensor and software need to be carefully optimized for the best image quality and each is equally important. Sony has massive experience in using its higher pixel camera sensors and the smartphone software it uses has benefited from many generations of Sony Xperia devices. It would be logical that Sony used the slightly newer IMX300 image sensor (it was released in September 2015, rather than April 2015 for the IMX377) because this represents an iteration of an established and successful line of sensors.
Another consideration may be that Sony's software produced very similar results between the IMX300 and IMX377 sensors: the IMX377 produced brighter images when taken in low light conditions, but the IMX300 produced sharper images in good lighting, but the difference might have been minimal. Should Sony produce another "same as" smartphone camera, or instead offer an alternative and offer a camera capable of taking sharper images some of the time at the expense of weaker images at night? Sony will have also considered the cost of the sensors, the marketing benefit to having a camera with more pixels, and any differences in the dimensions of both components beyond the light sensor. For an industry complaining of a lack of innovation, Sony probably made the right choice in trying something different to the established flagship camera route.