Apparatus (10) for detecting containers (12) of predetermined diameter that includes a pair of optical proximity sensors (28,30) positioned adjacent to a container conveyor (14) such that light energy from the sensors is incident sequently on the containers as the containers pass on the conveyor adjacent to the sensors. The sensors (28,30) are adjustable positionable with respect to each other such that the sensors are spaced from each other in the direction of container motion (60) on the conveyor. An electronic circuit (52,54,56) is coupled to the sensors (28,30) for detecting passage of containers (12) on the conveyor (14) while ignoring dithering of the containers. In the preferred embodiment of the invention, the sensors (28,30) are mounted on a common support (20) having a scale (44,46) for measuring separation between the sensors. Printed indicia in units of container diameter, preferably in both English and metric unit, is affixed to the support (20) adjacent to the scale. One (28) of the sensors is mounted on the support adjacent to a "zero" reference point on the scale indicia, and the other (30) sensor is adjustably positionable on the support adjacent to the scale. The support (20) in the preferred embodiment of the invention takes the form of a housing enclosing the sensors and having an elongated window (34) parallel to the conveyor (14) through which containers on the conveyor are exposed to the sensors.