An autofocus mechanism includes a light source emitting light through an objective lens at a work piece; a first detector detecting a portion of light reflected by the work piece and generating a first signal; a second detector detecting a portion of the reflected light and generating a second signal; a first amplifier amplifying the first signal and generating a first amplified signal; a second amplifier amplifying the second signal and generating a second amplified signal; an amplification rate definer defining an amplification rate of each of the first amplifier and the second amplifier based on the first amplified signal and the second amplified signal, respectively; and a calculator identifying a focal position of the objective lens based on the first amplified signal and the second amplified signal.