Eye Post #7: How Does the Optometrist Determine My Prescription?

Monday, August 3, 2015

Taken from myeyes.com

Have you ever wondered how the optometrist determined your prescription after looking at some letters and telling the doctor "1" or "2"? (If you've had an eye exam before then you'll know what I'm referring to :P)

The doctor moves a big apparatus in front of your eyes, a machine like the image above. There are many dials and knobs on the front face of the apparatus. This apparatus is called a phoropter.

When our eyes have problems seeing clearly, a common reason is because the light passing through our eyes is bent (also known as "refracted") but not focusing at a single point on the retina at the back of our eyeball. When light is able to focus on the retina, everything we see is sharp and crisp. If light rays focus in front of or behind the retina, our vision becomes blurry.

Insider the phoropter there are many lenses with a different power. As the doctor asks you whether you like "1" or "2", they are testing to see which of lenses in the phoropter-- when placed in front of your eyes-- help improve your vision as you look at the letters or numbers on the screen. The lenses that correct your vision are able to help further refract the light so that they will end up focusing on the retina.

The optometrist will write down the prescription based off of the lens power that was needed to correct your vision, and the value is obtained from the phoropter. Whatever lens power that was able to help each eye see sharp and crisp is what the optometrist will write down as your prescription. When you purchase your glasses, looking through those lenses according to the values from the prescription will be just like how you looked through the lenses in the phoropter, nice and clear!

Now you have an idea of how the optometrist gets your prescription! Hope you learned something new today :)

Until next time, Des


  1. This is a detailed, yet easily understandable, explanation of what eye doctors do during exams. Many optometrists offer patients the opportunity to buy eyeglasses at their office, so patients can have an exam and find the perfect frames all in one place. This one-stop arrangement is particularly helpful for moms who are getting eye exams and eyeglasses for their children.

    Beulah Jackson @ 96th Street Eye Care

    1. I agree! I work in an optical that does exactly that.


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