During a recent eye exam my longtime patient Beth asked: “Why can I see the alarm clock at 3 am by squinting?”

Beth asks a great question and brings up a very cool vision phenomenon! A small amount of the increased clarity of vision is due to changing the shape of the eye as you squint. However, the main optical principle at work, and one that is quite fascinating, is called the “pinhole effect”. Clear vision is a result of light rays that properly focused on the retina located in the back of the eye. Typically, this is done with glasses or contact lenses; both of which bend light into focus and therefore correct vision.

When we squint it creates the same effect as looking through a pinhole. Basically only a small amount of focused central light rays are allowed into the eye. This prevents the unfocused light rays in the periphery from reaching the retina. The result is better vision. Photographers understand this as “depth of field”.

The pinhole effect is an excellent way to diagnose eye injuries

We teach athletic training students at Radford University to use the “pinhole effect” when evaluating athletes with eye injuries, that are also complaining of blurry vision. The pinhole test is quite simple but extremely helpful. We ask the athlete to look through a small eye cover containing many tiny pinholes. If the vision improves it is unlikely that the blurred vision is related to an eye injury. It is more likely to be an existing vision problem that can be corrected by lenses. On the other hand, if the blurry vision does not improve through the pinholes then injury is much more likely to be the cause of the blurred vision.

The pinhole effect is unfortunately used to exploit and scam individuals

Some scam artists have tried to sell pinhole glasses by making claims that they will permanently improve your vision. In this scam, the pinhole effect is used to sell the much grander idea of “vision improvement”. Of course it comes with a hefty price–plus shipping & handing fees! The reality is that the pinhole effect does clarify your vision but it has no lasting or long term effect.  The Federal Trade Commission (FTC) filed lawsuits against companies to ban these false claims all the way back in 1993. Be aware and avoid falling victim to these lofty claims that have no basis in reality.

Try this at home. To really understand the pinhole effect try this: take a piece of paper and poke several holes in it with a needle, tack or paper-clip. Remove your glasses or contact lenses and try to view something like a TV from across the room. The clarity you see is due to the pinhole effect. Isn’t the eye and vision amazing!