July 13, 2010

Acuity of Vision

20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance.

This article is for Medical Students & Professionals
This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Anatomy of the Eye article more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.
In this article:
Visual acuity
MCQ exams: clinical scenario
MCQ exams: answer
MCQ exams: explanation

Visual acuity

Visual acuity (VA) commonly refers to how sharp or clear the vision is, but technically it rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e., (1) the sharpness of the retinal focus within the eye, (2) the health and functioning of the retina, and (3) the sensitivity of the interpretative faculty of the brain.

Visual acuity is usually measured on a scale that compares a person's vision at 20 feet (about 6 meters) with that of someone who has perfect vision. Thus, a person who has 20/20 vision sees objects that are 20 feet away with the same clarity as a person with normal vision, but a person who has 20/200 vision sees at 20 feet only as clearly as a person with perfect vision sees at 200 feet (about 61 meters).

MCQ exams: clinical scenario

What specific part of the eye is mainly responsible for acute vision?

a) zonnule of Zinn
b) limbus
c) optic disc
d) macula
e) fovea

MCQ questions & answers on medicalnotes.info

MCQ exams: answer

The correct answer is E.
Fovea is the answer.

MCQ exams: explanation

The fovea lies slightly below and to one side of the optic disk. It is found in the centre of a shallow depression or pit (the macula).

Only cones are present at the fovea, which is an area approximately 0.2 mm in diameter: all other parts of the retina including blood vessels are pushed aside.

The cones here have individual connections with the bipolar and ganglion cells, hence the fovea gives us our most sensitive and acute vision.

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