An autofocus mechanism comprising: a light source configured to emit light through an objective lens at a work piece; a first detector configured to detect a first portion of light reflected by the work piece and generate a first signal; a second detector configured to detect a second portion of the reflected light and generate a second signal; a first amplifier configured to amplify a voltage value of the first signal by a first amplification rate and generate a first amplified signal; a second amplifier configured to amplify the voltage value of the first signal by a second amplification rate greater than the first amplification rate, and to generate a second amplified signal; a third amplifier configured to amplify the voltage value of the second signal by a third amplification rate, and to generate a third amplified signal; a fourth amplifier configured to amplify the voltage value of the second signal by a fourth amplification rate greater than the third amplification rate, and to generate a fourth amplified signal; a corrector configured to correct one of the first amplified signal and the second amplified signal based on a first correction function expressing a relationship between the voltage value of the first amplified signal and the voltage value of the second amplified signal to generate one of a first corrected signal and a second corrected signal (CA'), the corrector further configured to correct one of the third amplified signal and the fourth amplified signal based on a second correction function expressing a relationship between the voltage value of the third amplified signal and the voltage value of the fourth amplified signal to generate one of a third corrected signal and a fourth corrected signal (CB'); and a focal position identifier configured to identify a focal position of the objective lens with respect to the work piece based on the first amplified signal and the third amplified signal or the first corrected signal and the third corrected signal in a case where one of the first signal and the second signal is equal to or greater than a first threshold value, wherein the focal position identifier (41 c) identifies the focal position based on the second amplified signal and the fourth amplified signal or the second corrected signal (CA') and the fourth corrected signal (CB') in a case where one of the first signal and the second signal is not equal to or greater than the first threshold value. a light source configured to emit light through an objective lens at a work piece; a first detector configured to detect a first portion of light reflected by the work piece and generate a first signal; a second detector configured to detect a second portion of the reflected light and generate a second signal; a first amplifier configured to amplify a voltage value of the first signal by a first amplification rate and generate a first amplified signal; a second amplifier configured to amplify the voltage value of the first signal by a second amplification rate greater than the first amplification rate, and to generate a second amplified signal; a third amplifier configured to amplify the voltage value of the second signal by a third amplification rate, and to generate a third amplified signal; a fourth amplifier configured to amplify the voltage value of the second signal by a fourth amplification rate greater than the third amplification rate, and to generate a fourth amplified signal; a corrector configured to correct one of the first amplified signal and the second amplified signal based on a first correction function expressing a relationship between the voltage value of the first amplified signal and the voltage value of the second amplified signal to generate one of a first corrected signal and a second corrected signal (CA'), the corrector further configured to correct one of the third amplified signal and the fourth amplified signal based on a second correction function expressing a relationship between the voltage value of the third amplified signal and the voltage value of the fourth amplified signal to generate one of a third corrected signal and a fourth corrected signal (CB'); and a focal position identifier configured to identify a focal position of the objective lens with respect to the work piece based on the first amplified signal and the third amplified signal or the first corrected signal and the third corrected signal in a case where one of the first signal and the second signal is equal to or greater than a first threshold value, wherein the focal position identifier (41 c) identifies the focal position based on the second amplified signal and the fourth amplified signal or the second corrected signal (CA') and the fourth corrected signal (CB') in a case where one of the first signal and the second signal is not equal to or greater than the first threshold value.