Vision-What’s Behind the Numbers

By William Benevento, M.D.

 Every day in the office my patients ask me, “How’s my vision?”  They expect an answer with a “twenty” in it, but in fact the truth of the matter may be much more complex.

The measurement of visual acuity utilizes a chart at a standard distance with characters of different sizes.  The chart is viewed from twenty feet away, either in a straight line or through mirrors (to give the same distance in a short room). Someone with normal acuity will just be able to read the “20/20” line.  In most states, to get an unrestricted drivers license at least one eye must see 20/40 or better.  The letters on the 20/40 line are twice the size of those on the 20/20 line.  In other words, the patient needs to be at twenty feet to see the smallest thing a normal person could see at forty feet; for 20/80 acuity, the normal person could see these larger letters at eighty feet, but the patient can only see them from twenty feet.

Visual acuity in the distance is only one component of the picture.  Plenty of happy 20/20 thirty year olds become unhappy 20/20 forty-five year olds when they try to read.  Near visual acuity changes in the forties, as the muscle that moves the lens inside the eye can no longer pull hard enough to allow comfortable reading.  Bifocals or reading glasses lessen the load on this muscle and sharpen the near vision.

Many patients have normal visual acuity in certain lighting conditions but perform poorly in dim or bright light.  This is a function of contrast sensitivity, the ability to distinguish black and white from gray.  Contrast is naturally decreased in low lighting and situations which cause glare (e.g. headlights at night); some patients are much more susceptible to this because of cataracts, retinitis pigmentosa, optic nerve disorders, or other ocular diseases.

Color vision is also independent of visual acuity.  Almost one out of every ten men have inherited some problem with their color vision. There are three different types of color sensitive cells in the eye; one for red, one for green, and one for blue.  By far, the most common color deficiency is to red.  It is possible to lose color sensitivity later in life, frequently in association with optic nerve disease or cataracts.

An extremely important aspect of vision which is frequently taken for granted is visual field, or peripheral vision.  The ability to see things off to the side allows us to navigate in our world and acquire targets on which to focus. Diseases that limit visual field can be devastating, significantly limiting independence.  In its late stages, living with poorly-controlled glaucoma can be like seeing the world through drinking straws; central visual acuity may be fine, but the area seen is so small that it’s effectively useless. Strokes can also affect visual field, sometimes eliminating all the side vision to the right or left and making reading particularly difficult.

Finally, stereo vision implies an ability to judge depth or “see in 3-D”.  Though most people associate depth perception with using both eyes together, this in fact is only a small component in determining the distance to an object. Other clues include relative size, interposition (overlaid objects), aerial perspective (color fades in the distance), shading, geometric perspective (converging lines in the distance like train tracks), and relative motion (closer objects appear to be moving faster).  Artists are familiar with these concepts, using all but the last to give their flat paintings depth.

So how is your vision?  Clearly, seeing letters on a well-lit chart twenty feet away is only part of the answer.

Eye Surgeons Associates located in Bettendorf, Muscatine, Rock Island, Silvis and Geneseo, provides comprehensive eye care for the entire family, for a lifetime. Our medical doctors are fully fellowship trained in their specialties and board certified. For more information visit us online at www.esaeyecare.com.  Dr. Benevento attended Stanford University, graduating with departmental honors. He first came to the Midwest to attend Washington University in St. Louis, where he completed both his medical training and his residency in ophthalmology. As a board certified ophthalmologist, Dr. Benevento has special interest in diabetes and the surgical treatment of cataracts with the latest techniques.