Final answer:
Objective optometers are usually referenced to the focal plane of the objective lens. The objective lens forms a real, inverted image at the focal plane of the eyepiece. This image serves as the object for the eyepiece, which then forms a virtual, magnified image.
Step-by-step explanation:
Objective optometers are usually referenced to the focal plane of the objective lens. The objective lens is the lens that is closest to the object being examined. In a telescope, for example, the objective lens forms a real, inverted image at the focal plane of the eyepiece. This image serves as the object for the eyepiece, which then forms a virtual, magnified image.