Two dimensions to your image
You probably heard the term “megapixel war” and that manufacturers try to top the number of megapixels their camera or smartphone offers. So you could get the impression that more is merrier, but there is more to the equation than that…
Resolution
If you look at a chessboard, you can see it has 8 squares across by 8 squares up, in total 64 squares. So if your chessboard was a camera sensor, it would have a resolution of 64 pixels. Indeed not a lot, but it goes to show the principle of each “dot” that enables the camera sensor to register the amount of light it is exposed to.
My Nikon D750 camera has 6016 pixels across and 4016 up, giving a total of 24.160.256 pixels, often abbreviated 24 megapixels or 24MP. For most photography this is more than sufficient resolution unless you crop your picture, which in effect takes away some of the pixels and hence the resolution.
Bit depth
The bit depth tells how much information your camera is able to store per pixel, i.e. the size of the number per pixel. JPG files store very little information per pixel – only 8 bits known as a byte, and hence it is able to store 2^8 = 256 different values (This is per colour channel, but lets leave that aside for now).
If your image is a RAW file, the format allows you to store 12 (or 14) bit of information. This gives a much finer granularity in the tones and colors that can be stored, but it also comes with a price: the file is much larger, as the information stored per pixel is dramatically increased.
No chain is stronger than…
Many only consider the resolution as an important parameter, but the bit depth is equally important, as it enables you to store the specific reading of light from the sensor. However, you also need a good lens to let through good light with all the details required to capture the scene, so if you put a poor lens in front of a high resolution sensor that captures in RAW, the only thing the sensor will register is precisely how bad the lens is.
So you need all the elements in the chain to work together: lens, sensor resolution and sensor bit depth, in order to get the best possible image captured, with good resolution and contrast.
So if you have a good camera that can capture say 24MP in RAW format on a good sensor, the limiting factor is probably more the lens that you put in front of it than the resolution of the sensor. And that is why any experienced photographer will tell you: invest in good glass before anything else.
Related reading
What is hard light vs soft light?
Is mirrorless cameras better than DSLRs?
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