A recent visitor to our WebMD Vision & Eye Disorder message board posted an inquiry about his mother who had just recently had cataract surgery. I could immediately sense the worry in his posting.
During the post-op clinic visit the woman could see very the vision chart very clearly when using the familiar pinhole occluder. It uses a movable plastic flap with lots of small holes. Once the pinholes were removed the woman’s vision was very blurry. Was there cause for concern? I was able to offer some much-needed reassurance.
As I teach ophthalmology residents, the answer to every question regarding the eyes and vision is based entirely on either anatomy or optics – sometimes both!
The pinhole occluder imitates the most precise pair of eyeglasses ever made. The individual pinholes (Trivia Alert! Each hole is 1.2mm in diameter!) only permit straight rays of light to pass through. Therefore, only straight light rays enter the eye.
The optics of corrective spectacles work to straighten bent light rays – to reverse a refractive error. Now, if the rays are already straight no refracting is necessary. This explains why folks often see better when using the pinhole occluder (with or without their eyeglasses). People who can already see 20/20 without correction do not experience any improvement with the pinhole. A person with spectacles who sees better with the pinhole likely needs a change to their correction. In the case of the post-op cataract patient, she can expect an excellent visual result once healing is complete and postoperative astigmatism resolves.
Can you make a pinhole occluder at home? Sure! Unbend a standard paper clip and use one end of the wire to poke multiple holes in a index card – perhaps one-quarter inch apart. When you peek through the pinhole you should be able to read the time on a clock across the room.
We instinctively create a pinhole every time we squint. By narrowing the opening through which light can enter the eye we eliminate many nonaxial (not straight) light rays.
Pinholes and squinting are not a long-term solution for improved vision. They markedly constrict the visual field and reduce total illumination. Even so, both help us see better until our refractive errors can be corrected.