Tomous wrote:Grumpy David wrote:Like Tiger Woods, I have better than 20/20 vision.
20/15 supremacy.
I only found out the other day 20/20 vision wasn't the best vision possible. Seems to be a popular misconception.
Europe tends to measure this in metres, the Americans tend to measure in feet. 20/20(ft) is therefore equivalent to 6/6(m) (give or take a few inches!)
The first number is the test distance (usually 20 feet or 6 metres). The second number is
the distance at which the smallest seen optotype subtends 5 arcminutes (read: how small the letter you can see is!)
Imagine a letter E plotted on a 5x5 grid. Each square corresponds to one arcminute. If you cannot resolve those five arcminutes (i.e. see the lines) then the E will not look like an E to you. The bigger the letter the longer the distance it needs to be away from the viewer to subtend 5 arcminutes. The biggest letter on these vision charts is usually 6/60, so that letter would need to be 60m away for it to subtend 5 arcminutes. This goes down gradually with each line as you can imagine. The 6/6 line is subtending 5 arcminutes at 6 metres. Lines below the line of 6/6 letters subtend 5 arcminutes at a distance smaller than 6m (i.e. 6/5, 6/4). That's how you can achieve better than 6/6 visual acuity.