Final answer:
The statement about canthus to canthus measurement being entirely exact is false; measurements can vary and do not solely determine eye health or vision capabilities. Accurate vision involves the proper focusing of light onto the retina, usually facilitated by the correct curvature of the cornea and the converging power of the lens, particularly in close vision.
Step-by-step explanation:
The statement that canthus to canthus measurement is entirely exact, because the inner canthi of the eyes encroach farther across the sclera with younger children, is false. The positioning of the canthi can vary and is not always an exact measurement tool for determining eye health or vision capabilities. Visual acuity and the health of the cornea do correlate with how the cornea refracts light, ensuring that the light is focused properly onto the retina. This relation reflects the principle that the cornea's shape can significantly influence vision quality and is most effectively controlled in corneal surgeries or treatments, aiming to restore normal distant vision in a high percentage of patients.
The principles highlighted here are demonstrated in the concept that the eye's lens must become more converging (thicker) for close vision than for distant vision, which involves the action of the ciliary muscles. Objects at a distance do not require as much convergence since the rays from these objects entering the eye are less divergent. The eye is most relaxed when looking at distant objects, implying that vision correction aims to emulate this relaxed state as much as possible for clarity and comfort.