If you have ‘perfect’ visual acuity, you have 20/20 vision, as measured by the standard eye chart devised by a Dutch ophthalmologist called Dr Hermann Snellen in 1862. The charts, which are still in use, have a single large letter at the top and lines of progressively smaller letters below. The first figure refers to how far away (in feet) the person whose vision is being measured is sitting or standing from the chart. The second figure refers to how far away a person with good vision would have to be and still be able to read the same line of letters as the person being tested. If you had 20/30 vision, a person with perfect vision could read at 30 feet the same letters that you can just make out at 20 feet. These distances are now generally expressed in metres, so that ‘perfect’ 20:20 sight would now be written as 6:6.