4 ms·
This notation 20/* is very interesting: in Brazil they never tell you a number like that. People tell you about how well they see by saying that they don't nee
by swah 14y ago
This notation 20/* is very interesting: in Brazil they never tell you a number like that.
People tell you about how well they see by saying that they don't need glasses, or by giving their prescriptions, like "hyperopia, 2.5 on right eye, 1.0 on left, plus astigmatism of 1.0 on the right eye..."
- lucian1900 14y agoIt's an American think afaict. I still don't quite get how it works, just that it has something to do with feet.
- taejo 14y agoIt's a ratio of distances -- the benchmark distance is 20 feet but that's non-essential. 20/10 means you can see the same level of detail at 20 feet that somebody with "normal" vision can see at 10 feet.
- lucian1900 14y agoThat seems problematic, how is normal vision defined? With dioptres, the definition is very clear and based on optical properties of the eye.
- zokier 14y agoPrescription and acuity are not the same thing. Many people do not get "perfect" acuity even with optimal prescription. Ie prescription tells what kind of corrections you need to get the best possible vision, but doesn't tell how good your best possible vision is. Acuity on the other hand tells you what's the smallest detail you can distinguish over some distance (such as 20 feet in the normal 20/xx expression).