|Snellen eye chart|
This morning on BBC TV news Ed Milliband, declining to criticise the ineffective police response to the London riots, told the interviewer “I'm not going to engage in 20/20 hindsight”. What did he mean?
From the context in which it's normally used I've always assumed 20/20 vision meant perfect vision or maybe, as Milliband appears to think, all-round vision; but actually no, it signifies normal vision, for which the technical term appears to be normal visual acuity (VA).
To an optometrist, 20/20 vision means that when you stand 20 feet away from a wallchart, you can see what someone with normal vision can see at that distance.
If you have 20/40 vision, your visual acuity is rather poor. It means at 20 feet, you see what a person with normal vision can see at 40 feet. 20/10 vision means your visual acuity is good; at 20 feet you can see what a person with normal vision would need to step up to 10 feet to see.
20/100 vision means that at 20 feet you can read no more of the wallchart than a person with normal vision could at 100 feet. And 20/200 is the cut-off for legal blindness in the United States.
The information comes from the American Optometric Association. For all I know 20/20 vision may be a specifically American expression, and perhaps something different is used in Europe. It certainly seems to have entered the language as an instance of American business speak. Annoying at the best of times. Doubly so when it's misunderstood even by those who use it.
In metric terms 20/20 becomes 6/6. So if Milliband wanted to be right up to date with his jargon, perhaps he ought to have said “I'm not going to engage in 6/6 hindsight”. Lacks a certain rhythm, but would it have been more happening and abreast of the modern thing?