Nope, not laser focus. The focus and tracking coils on an optical pickup assembly are servo-locked.
However, there is one thing that has to be correct, and thats the laser power, which is what he adjusts. Any time you have to play with that adjustment, its because the diode is weak, just like a picture tube when you gotta crank up the screen control after a few years in order to see a good picture. its getting weak, only to burn out.
Anyway, the optical sensor is a semi-PSD (position sensitive device) photodiode. it has diodes placed at 12oclock, 3, 6, and 9 o clock. When the laser its out of focus in one direction, the laser will become oblong vertically. Therefore the lens is corrected until it becomes oblong horizontally, then its too far. it backs off until the laser beam locks directly onto the center diode. If the tracking shifts, the laser beam will begin to track onto the nearby diode, therefore telling the tracking servo to move the coil to correct for this. If its outside of the lock range, it will step the sled motor until it becomes in the lock range.
3-beam pickups work differently, where the servo mechanisms use differential amplifiers to take the sum and difference of the 2 side beams relative of the center beam to figure out if tracking/focus error is too far to the left or right, and by how much.
CD players use EFM, (eight-to-fourteen modulation) to lock the disc servo, and its also used to demodulate the digital bit stream. Then the bit stream is used for further demodulation (like PCM for CDAudio, etc..)
Thats why you hear CD drives/CD players "hiss" when they read a disc. its because of the high frequencies of the servo lock keeping the laser beam exactly in the right orientation on the PSD pickup photodiode assembly, that and the spindle motor. You can always tell the health of the laser by "how" it makes its hissing. if its too loud, thats because the servo's current lock is operating near the end of its lock range to keep it locked. (beam too weak or too strong). If you hear no hissing, and alot of clicking, its not making a lock at all, the beam current is below the lock hysteresis (laser weak or dead.)
Oh, in case your wondering, no i havent repaired or modded any CD players before,
