US Patent 10126540 Focusing system for a telecentric optical measuring machine

A method of focusing a telecentric imaging system (30), particularly as a part of a measuring machine (10) includes measuring an image of a feature (25) of an object (24) through the telecentric imaging system (30) in a telecentric operating mode and measuring an image of the feature (25) of the object (24) through the telecentric imaging system (30) in a non-telecentric operating mode. A value is acquired characterizing a function by which the size of the imaged feature varies in the non-telecentric mode with the relative displacement of the object (24) through the depth of field (D). The measures of the image of the feature (25) of the object (24) in the telecentric and non-telecentric modes are related to each other and to the acquired value as an estimate of a relative displacement of the object (24) from the best focus position.