system
Given the following post, respond as if you're commenting on Reddit:
human
Why are megapixels on smartphone cameras irrelevant?

Every year smartphone manufacturers are releasing smartphones with wildly different megapixel counts thus promising better image quality. And yet some manufacturers just cant keep up in the race for the best smartphone camera. 

Why is it so difficult to make a smartphone with a camera which can keep up with the leaders in this market?
gpt
Responses so far have largely skirted a rather more fundamental problem with ever-increasing pixel counts, and it is this:

Since we were getting up to 12-15 MP, the individual sensors are so small that in short-exposure photos they simply do not receive enough photons to give a good quality signal.  When pixels are large, they have the opportunity to detect a more statistically valid number of photos relative to the overal picture. As pixels bacame smaller the chance of a particular sensor picking up a particular photon That means that whereas before you had one pixel with a meaningful brightness value, you now have 3 with a range of values. This results in noise in the image; a grainyness due to some pixels being triggered more than others as a result of photon interation.

What many cameras now do as a result is use processing techniques such as comparing a particular pixels value to that of those around it, and smoothing the signal response. In effect they're simulating larger pixels with many smaller pixels. The results are somewhat better than the lower pixel count equivalent, but twice as many pixels does not equate to an image with twice the clarity.

The particular issue with smartphones is that they are incredibly thin; that means the focal length between the lens and the sensor is incredibly short, which in turn means the sensor has to be really tiny. You then start trying to cram huge pixel counts in and the opportunity for high noise and low light sensitivity becomes obscenely large. Together with the processing required for high-pixel count sensors, cheap camera options on phones produce crappy results.  

This is why a 10 year old 8 MP DSLR can produce photos which blow most modern cameraphones out of the water - even with a basic lens; it has a sensor that is vastly bigger, and individual pixels which are much much larger. This means it is far more light sensitive. So even though the internal processing is a lot more basic, the output of the camera is much more responsive.

That's not to say that more megapixels is always bad, but it's a balancing act. Using MP to judge camera quality is like using GB of RAM to judge a computer.