“What is my vision?” Understanding the numbers 20/20

 “What is my vision?” This is a common question asked by many patients. Vision, or visual acuity, can be measured in the form of a fraction, such as 20/20, which many consider to be ‘perfect’. But what does 20/20 vision mean?

First of all, I need to explain what the numbers in the fraction mean. The top number refers to the distance the vision is tested, or 20 feet. The second number is the distance at which a ‘normal’ eye can see the target. So, if someone’s visual acuity is 20/80, and they are 20 feet from a target to see it clearly, someone with perfect vision can be 80 feet away and still see the same target clearly. 20/80 visual acuity is 4X worse than 20/20. 20/40 vision in at least one eye is needed to have a driver’s license. That’s why you can still “pass” your driver’s test without perfect 20/20 vision.

This way of evaluating vision can be performed either without any correction, such as glasses or contacts, or with best correction. Visual acuity is also tested in each eye separately.

So how do we measure this in the exam room? Each room is fairly small so how do we get the target 20 feet away? Mirrors! If you look at the wall in front of you during your exam, you’ll notice a couple mirrors on the wall above your screen. This enables us to reflect the letters back and forth across the room until it enters your eyes. We usually use a projector or a computer monitor as the source of our target. The letter sizes are calculated so they are equal no matter where you had your exam.

What about people that see great far away, but need reading glasses? Visual acuity can also be tested up close with a near card. If over the age of 40, usually some type of bifocal is needed. The power of this is determined after the distance prescription is found. The “ADD power” is placed on top, or added on, to that prescription. If no prescription is needed for far away, many people find using cheaters or over-the-counter readers to be useful.

Once we’ve determined your visual acuity, another common question is “how much did my vision change?” You’re wondering if you need to purchase that new pair of glasses or not. As long as we have a previous prescription or pair of glasses to compare, we can let you know at the time of your exam. It depends on the person whether the change is “a lot”. Some consider a little change not worth a new pair of lenses, while others want to see the best they can. We can explain to you your vision change based on the improvement in your visual acuity, as described above.

Keep in mind that a new prescription may not be the only consideration in purchasing a new pair of glasses. Your current pair may have scratched lenses, the tint may have faded, or the frame may be crooked and unrepairable. We will do our best to help you decide if a whole new pair would be warranted, or if new lenses for your current frame would be sufficient.

When you come in for an exam, we want you to leave feeling that you understood the results of your testing, and feel comfortable purchasing that new pair of glasses if you need them.

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