I'd like to point out that if we assume that the infrared wave travels at the speed of light in empty space (I know it's somehow slower but good enough for the

I'd like to point out that if we assume that the infrared wave travels at the speed of light in empty space (I know it's somehow slower but good enough for the back of this virtual envelope) to a subject 1.5m from the camera and comes back, the total time would be 10.007 nanoseconds, so the timer should be accurate enough to measure fractions of nanoseconds to perform a correct autofocus. So, I don't think that's how it works. Now, for the sound wave, maybe?