I had to go to the hospital recently to have my eyesight checked. I sat in the waiting room and I struck up a conversation with 78-year-old Peter who’d undergone a corneal graft 10 years ago and comes in for annual checkups. My PhD focused on treating difficult to heal wounds by using different phases and concentrations of silver nanoparticles. Whilst firmly chained to the lab during my research, I got the opportunity to see my fair share of yucky wounds and because of that, nothing medically freaks me out. Because what he told me was horrific. Interesting but really gross.
I had loads of tests done at the hospital. The field view test, the Snellen Chart, a couple of scans to check for fluid build up and I had my eye pressure checked too for a signs of glaucoma. That was a strange one. Rather than the usual puff of air they give you at the opticians, I had some numbing drops put in each eye then, without much warning, a probe touched the middle of each pupil. ‘You’d have never let me do that without them’, the nurse told me. Feeling your eyeball squish backwards is an unusual feeling – not one I’d recommend for the squeamish. I also was asked by every nurse if I’d already had my pupils dilated because apparently…ahem… I have the biggest pupils they’d ever seen. I bet they tell that to all the girls.
Finally, as you can see, I had the dilating drops. These stung a bit and made the world a peculiar place for a few hours. I couldn’t focus on anything due to the relaxation of my lens and because my massive pupils were letting in loads of light, everything was painfully bright. So as I talked to a vague shape of a doctor, I was told the good news – there’s nothing wrong with my sight. He then told me something pretty amazing. I have 6/4 vision. I’d heard of 20/20 but no idea what 6/4 meant so he explained it to me, unhelpfully with diagrams that I couldn’t focus on.
Think back to the Snellen Chart with the letters of varying sizes to measure visual acuity, measured as a fraction (chart below). The numerator (top number) refers to the distance you stand away from the chart which is fixed at 20 feet. The denominator (bottom number) indicates the distance at which someone with normal eyesight could read the bottom line of the chart. Normal vision is therefore 20/20. If you’re 20/40 then the line you can correctly read could be read by a person with normal vision at 40 feet and it goes all the way up to 20/200 and after that you’re registered as blind.
Switching to metric, the equivalent of 20/20 is 6/6 (6 metres = 20 feet). Being 6/4 (actually 6/3.8) means I’m 20/12.5 on the old scale, the line marked 10 on the chart above. 6/3 is the best human vision can get (the bottom line on the chart, number 11). It’s rather exciting to think I’m only 80 cm from ocular perfection!