Abstract: In a system which automatically analyzes laboratory slides, the adjustable objective lens is automatically focused. A beam splitter splits light from the slide into two parts. One slide image is projected to an actual image plane where it is converted into electrical signals representing the optical characteristics of the slide. The other slide image is projected to a parfocal image plane at which two light sensing devices for the focusing system are positioned. One light sensing device is in front of the parfocal image plane and the other is behind the parfocal image plane. These light sensing devices produce electrical signals representing the position of the actual image plane with respect to the desired image plane. The electric signals are differentiated and then applied to a comparator which supplies pulses to a stepping motor which moves the objective lens.