Likely this will be the same as the color depth in bits per channel.
For digital cameras it can be 8bit or 12bit and sometimes as high as 14bit. What is it for the IPhone 5?
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For all intents and purposes, that would be 8-bits per channel or 24-bit per pixels because what you get out from the camera is a JPEG image and that is its limit.
The sensor internally is highly likely to have a greater bit-depth, maybe 10 or 12 bits per channel. This is actually needed to produce an 8-bit-per-channel JPEG because sensor output is linear but JPEG levels follow a gamma curve. If you have an 8-bit linear sensor, you would not be able to fill out all possible values of a JPEG image.