Very few clinicians besides eye specialists realize how valuable a Pinhole test is. A pinhole essentially corrects nearsightedness & farsightedness, both classified as “refractive error,” meaning nothing more than that you need glasses.
Light bounces off whatever image we see, carrying its reflection through the lens to land on the retina (upside-down). Near-sighted eyes are too long, so the image lands further back than it should, and appears blurry. Eyeglasses correct the focus. This can also be achieved with a pinhole, which directs all the light to a single spot on the retina, so it doesn’t matter how long or short the eye is (see picture below). If you’re ever on the top of a mountain & break your glasses, poke lots of tiny holes in a cardboard and it may work well-enough to navigate home.
Visual loss, i.e. blurry vision, can be divided into 2 broad classes: refractive error, & true eye damage. The latter can be serious, may require surgery, lead to blindness, etc. We need to tell the difference.
Interestingly, there are a lot of people who have refractive error in just one eye, & never realize it. I, for example, have 20/400 vision in my left eye, & 20/20 in my right. Now that my right eye is beginning to get farsighted with age (needs reading glasses), I do all my reading with my left eye, & all my driving with my right, without even realizing it. No problem.
Suppose someone like me isn’t aware of their own difference, & gets hit in the left eye. They cover their right eye to test for damage, & lo & behold, their vision is blurry. They rush to an ER or urgent care (understandably).
The clinician has to determine if the blurry vision is due to the injury (potentially bad!), or if the person was simply always nearsighted in that eye, without knowing (not bad). So they test vision through a pinhole. If the blurry vision improves (often normalizes) with use of a pinhole, it’s the latter, & the person can go home happy. If there’s no difference with a pinhole, we say “it doesn’t pinhole,” i.e something’s significantly wrong with the eye, & call an ophthalmologist.