Perhaps you’ve had your vision checked recently. You were looking through the occluder (the black thing that looks like a really poorly made carnival mask) with your right eye looking through the hole. You were not been able to read down to the bottom of the chart. Letters became blurry, you stuttered, squinted your eyes, and made a last ditch effort to make it farther down the eye chart.
Your tech then says to you, “Try not to squint. Here, let’s see if this helps…” as he / she pulls the occluder away from your eye and drops down an extra black plastic attachment. “Now, hold this back up and find a little hole to look through.”
“Hold it up like this?” You ask holding what now seems like a solid mask of black plastic against your brow. You move the occluder around, find a hole to focus through, and exclaim, “Oh, WOW! I can read it now, what’s going on? Can I have these made into a pair of glasses?”
This is what we call the pinhole acuity. Essentially, we are causing you to squint (I know, just what we told you not to do).
When we first checked your vision, we want to know your best acuity without squinting. When you begin to have trouble, we may drop the pinholes down to see if a change in lenses can possibly help.
I tell people it acts just as those pinhole cameras from childhood did. Many of us have made those out of cereal boxes or cans as a kid. The materials needed were an enclosed box (your eye) and a tiny hole through which only a small ray of light can pass through. A small hole that only lets a tiny ray of light through keeps the image in focus longer when it hits the back of the box. Therefore, a small ray of light will keep the image in focus longer when reaching the back of the eye. This is why we naturally squint when we are struggling to see something. With the pinhole test, your tech is telling the doctor whether the image can be clarified before it reaches the retina, thereby improving your vision. If so, it acts as a good bench mark as to where we expect you to land on the eye chart after your refraction – or glasses check.
This does not ensure that a glasses change will match the exact result of the pinhole, but it does let us know what part of the eye is being affected. If your vision improves by use of the pinhole, it is likely the front part of the eye that is causing the blurred image (the cornea or the lens). If vision is not improved by the pinhole test, it is likely the back of the eye (the vitreous or the retina) that is affected.
This is the same phenomenon that occurs when we ask you to take off your glasses and look at the big “E” while we shine a bright light in your eyes. We are checking for pupillary reaction, but in so doing we are causing your pupils to constrict creating a pinhole effect. Then you say, “Oh, the E got much clearer when you shined that light into my eye!”
“Fantastic!” we say. “We are already improving your vision and we haven’t even prescribed you a new pair of glasses.”