*[[Focus]] the [[image]] of a "distant object" onto a [[screen]].
*[[Measure]] the [[distance]] between the [[lens]] and the [[screen]].
: The [[Image Distance|image distance]] (the distance between the [[focus]]sed [[image]] and the [[lens]]) is the same as the [[Focal Length|focal length]] for a "distant object".: If the [[Object Distance|object distance]] (the distance between an [[object]] and a [[lens]]) is too small, then the [[Focal LengthImage Distance|focal lengthimage distance]] is not the same as the [[Image DistanceFocal Length|image distancefocal length]].
===Ray Diagrams===