The cheapest instrument in the room

A black disc with a hole in it costs less than a pound and decides whether the next thing you need is a lens or a referral.

There is a black plastic disc in every consulting room drawer with a small hole punched through the middle. It costs almost nothing and it answers the most important question in a sight test: is this blur a focusing problem, or is it something else?

Why it works

A blurred image is a point of light spread into a disc on the retina. Narrow the beam that is allowed in — which is all a pinhole does — and that disc gets smaller, because you have thrown away the rays that were arriving at the wrong angle. Depth of focus goes up. Blur from long sight, short sight or astigmatism largely disappears.

Nothing else about your eye has changed. The optics have simply been given an easier job.

What it rules out

If acuity improves through the pinhole, the fault is refractive and a lens will fix it. That is the ordinary case and it is the one we can sell you something for.

If acuity does not improve, the fault is behind the optics: a cataract scattering light, a change at the macula, something on the optic nerve. A stronger prescription will not touch it, and anyone who sells you one is selling you a new pair of glasses you will not get on with.

What happens next

We look. Pressures, fields, a dilated view of the retina, an OCT scan through the macula. Most of the time there is an explanation we can monitor here. Sometimes there is a letter to write, and we write it that day.

The pinhole is thirty seconds of a forty-five minute appointment, and it decides which of those two mornings you are having.