,
9 tweets,
3 min read
Read on Twitter
I’m so glad you asked. Allow me to answer in excruciating detail...
This is how the eye is supposed to work. The cornea and lens refract light directly onto the fovea, allowing you to see 20/20. This is how us normals are able to see well without glasses, and lemme tell you, it’s pretty awesome. 
Lots of people either have gigantic eyes or steep corneas that refracts TOO well. The light is focused in front of the fovea (near-sighted) instead of ON the fovea and you end up seeing like garbage and sleeping in your contacts.
Some people have pathologic near sightedness with a refraction around -8 or -9 or worse. One thousand years ago, before glasses were invented, these people would have been eaten by a bear by age 10
Fortunately, glasses change the focal point of the eye, allowing you to see clearly and live beyond your tween years
When you squint, your eyelids close over all but the very center of your cornea. This blocks the peripheral light rays which would normally cause the light to be focused in front of the fovea. Only the central rays can pass unabated through the cornea...
These central rays are NOT refracted and hit right on the fovea, giving you that sweet sweet 20/20.
Unfortunately squinting takes away your peripheral vision, so if you spent every day walking around squinting, you would once again be susceptible to bear attacks.
One last thing, pinhole is an incredibly useful test. If a new patient comes in with blurry vision and only sees 20/80 on the chart, I will use a pinhole. If vision improves to ~20/20, I know the patient just needs glasses. If no improvement, there’s something wrong with the eye.
