How do Optometrists Measure Vision?

Common Causes of Poor Vision

The most common cause of poor visual acuity is refractive error, or how the light is refracted in the eyeball. Causes of refractive errors include reduced flexibility of the lens due to age (presbyopia), irregularities in the shape of the eye (myopia, hyperopia) and cornea shape abnormalities (astigmatism).
If you feel that your vision isn’t what it used to be, or if you’re over 40 years old, you should go to an eye doctor to get your vision measured and tested. Many age related eye problems start to show in your 40’s, so it’s better to be prepared for it. Some diseases such as diabetes can also pose problems to your eyesight.

How is Vision Measured?

Visual acuity is usually measured using a Snellen eye chart. Eye charts can measure how well you can see at a distance, and this is where the term “20/20 vision” originated from.

• During an eye exam, your ophthalmologist will ask you to read a set of letters set in multiple lines, from biggest to smallest. If you can read the smallest letters at the bottom row, your visual acuity is very good. Standard eye charts are placed 20 feet away from your eyes. In some cases where space at a doctor’s office is an issue, the eye chart is reflected using mirrors to simulate the 20 foot distance.
• The term 20/20 vision is used a lot and a common misconception is that your eyesight is perfect when you score this high on an eye test. 20/20 vision is actually what “normal” vision should be, i.e you can read at 20 feet what most people should be able to read at that distance, especially if you’re healthy. It’s also not necessarily perfect vision, either. 20/20 vision only indicates the clarity or sharpness of vision at a distance of 20 feet.
• If you have 20/40 vision, what you see 20 feet away is what a person with normal vision sees at 40 feet away, meaning your vision is only half as good and objects need to be half the normal distance for you to see them. A person with 20/15 vision can read letters at 20 feet that a person with 20/20 vision can only read at 15.
• A legally blind person who has 20/200 vision can read letters at 20 feet that a person with normal 20/20 vision can read at 200 feet.
• It’s very rare that a person scores 20/10 or lower. Many animals, especially birds of prey, have even better eyesight than humans could ever have, usually scoring a 20/5 mark or lower, which enable them to accurately spot their prey from hundreds of feet in the air.

Types of Eye Charts

There are many different eye charts available to eye doctors. The most commonly used, is the Snellen eye chart that you often see in clinics and eye-related ads. The top letter is a bold E, followed by progressively smaller letters. This chart was developed by Dutch Ophthalmologist Dr. Hermann Snellen in 1862, because of the need for a more standardized eye chart.

Other charts include the Monoyer or decimal scale chart developed by French Ophthalmologist Ferdinand Monoyer where he incorporated his own name and is read in reverse, starting with bold letters Z and U. The Tumbling E chart is used for young children or people who don’t know the alphabet. It’s basically structured like the Snellen, but only uses a bold capital letter E that rotates in all four directions.

Patients are instructed to tell the doctor where the three fingers of the “E” are pointing by using their own three fingers. The Jaeger eye chart is a handheld chart used to test near vision. In most European countries, the Landolt C is the standard eye chart used in most clinics. It starts with three large “Cs” or a ring with a gap that rotates in different directions. Just like the Snellen, every line features smaller Cs until barely visible.

Levels of Vision – From Best to Worst

20/20 – Considered Normal vision

Minimum vision requirement for fighter pilots. Can easily read numbers in the telephone book.

20/40

Can pass driver’s license test and drive, provided he/she wears contacts or glasses. Most printed material can be read.

20/80

News headlines are this size. Can read the dials of a clock or digital alarm clock at 10 feet.

20/200

Legally blind. Can read signs, but will need special lenses.

The next time you get your vision tested, check out what type of eye chart your doctor uses and surprise them now that you know a thing or two about it!