Twenty/twenty vision
Discussion
Dr Jekyll said:
Why is 20/20 considered 'normal' vision when many people (perhaps with the right glasses) can see better than that, the 20/15 line?
Who decided what we 'ought' to be able to see at 20 metres anyway?
It's 20ft. Americans use this terminology as they haven't grasped metric yet. We use metres, ie 6/6 is equivalent. Who decided what we 'ought' to be able to see at 20 metres anyway?
The denominator indicates the size of the letters, specifically it denotes the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute, which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet. The metric equivalent is 6/6 vision where the distance is 6 metres. This means that at 20 feet or 6 metres, a typical human eye, able to separate 1 arc minute, can resolve lines with a spacing of about 1.75mm.
Edited by 968 on Sunday 30th October 00:07
Dr Jekyll said:
Why is 20/20 considered 'normal' vision when many people (perhaps with the right glasses) can see better than that, the 20/15 line?
Who decided what we 'ought' to be able to see at 20 metres anyway?
I assume it's based upon the average size of an eye/lens, and then you could do some sums to work out what a correctly constructed eye should be able to do. Who decided what we 'ought' to be able to see at 20 metres anyway?
968 said:
It's 20ft. Americans use this terminology as they haven't grasped metric yet. We use metres, ie 6/6 is equivalent.
The denominator indicates the size of the letters, specifically it denotes the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute, which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet. The metric equivalent is 6/6 vision where the distance is 6 metres. This means that at 20 feet or 6 metres, a typical human eye, able to separate 1 arc minute, can resolve lines with a spacing of about 1.75mm.
I appreciate that, but the point is that so many (most?) people can actually see at 20 feet what they 'ought' to be able to see at 15 feet. (Presumably this means they can also see at 20 metres what they should be able to see at 15).The denominator indicates the size of the letters, specifically it denotes the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute, which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet. The metric equivalent is 6/6 vision where the distance is 6 metres. This means that at 20 feet or 6 metres, a typical human eye, able to separate 1 arc minute, can resolve lines with a spacing of about 1.75mm.
Is one arc second just a convenient but arbitrary standard?
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