by Jessica Philbrook
Q: My daughter was having trouble seeing the board at school and when I brought her to the eye doctor she had 20/50 vision. The optometristtold me that she was nearsighted and needed glasses. What is 20/50 vision and what is the difference between nearsighted and farsighted?
A: These are common questions that many people have after leaving their optometrist’soffice. Visual acuity, defined as clarity of vision or how well a patient sees, is measured using a chart called the Snellenchart that has letters calibrated to a specific size based on the patient’s distance from the chart. 20/20 vision refers to a specific size of letter that should be seen at 20 feet. In other words, “perfect vision.” When a patient is found to have 20/50 vision, that means that, at 20 feet away, the patient can only see letters that the “perfect vision” patient could see from 50 feet away. So, 20/20 visual acuity is better than 20/50 visual acuity and 20/50 visual acuity is better than 20/80 visual acuity. These numbers also relate to the amount of nearsightedness or farsightedness the patient has.
Farsightedness, otherwise known as hyperopia, means that a person sees well in the distance, but needs to focus more than the average person to see up close. This is the result of a shorter eyeball. Many people who are farsighted need glasses to help with reading only, but sometimes the amount of farsightedness is enough that glasses are needed to see at all distances because even things far away are too close for easy focusing in these special cases.
In general, babies are born farsighted (due to their small eyeballs) and as they grow they become less and less farsighted. In the case of the perfect eye, the baby would be born farsighted and as they grew, their eye would elongate, becoming less and less farsighted until around age 7, when they would have the perfect eye size and no refractive error or prescription needs.
Occasionally you see a child that needs glasses at a young age and then they grow out of their need for correction. That is because they were born with an abnormally high amount of farsightedness that needed correction to aid in visual development and as they grew, their eyes became longer and no longer needed help focusing.
Nearsightedness is just the opposite. Nearsightedness, also known as myopia, results from longer than average eyeball length and allows for clear vision at a certain point close to the person, but blurry vision in the distance. Nearsightedness tends to develop during the school years when children are growing quickly and tends to progress until age 20 or occasionally 30. Most people that are nearsighted wear their glasses or contact lenses all of the time, but they could take their glasses off to read if they so choose. Next month we will discuss astigmatism and presbyopia (the need for reading glasses or bifocals, that occurs with aging).
Editor’s Note: Dr. Jessica Philbrook is an Optometrist. She works with Dr. Bob Ellis at 40 Court Street in Houlton.
