ˌīkəˈnäməd.ə(r) noun
Etymology: International Scientific Vocabulary icon- + -meter
1. : an instrument for determining the distance of an object of known size or the size of an object at known distance by measuring the image of it produced by a lens of known focal length
2.
a. : an instrument that determines the proper objective to be used in taking a picture of given size from a given standpoint and that consists of a diopter and an open rectangular frame sliding on a graduated rod
b. : a direct viewfinder having a metal frame