up home page bottom

Wednesday, October 17, 2007

Eye exam results

I survived my first month without glasses! Well, I cheated a bit...heheeheh...sometimes I used my wife's glasses (OD:-3.5,OS:-3.75) for important things like winning a badminton game. I went to the opthalmologist/optometrician(?) last evening (Tue,16 Oct) for my eye exam and asked for a 20/40 and 20/80 glasses. She was surprised at my request since 'I am the first one who made such a strange request. Normally people ask for better (stronger) prescription i.e. 20/15'. I learned two things from her (i) the eye exam machine is not as accurate as doing it manually with the trial lens kit (ii) lasik eye surgery doesn't mean permanent cure - after 3-5 years you may need some adjustment i.e. prescription glasses (again!) or more surgery! By the way, I have been trying to search for explanation on how the eye exam machine works but could not find anything but endless list of where to get lasik surgeries. My last prescription is OD:-6.25-0.75x9 and OS:-6.5-1.5x173 while my eye exam results is OD:-5.75-0.75x10 and OS:-5.75-0.75x173. So it seems my eye power has gone down quite a bit. I will be getting -5.0 (20/40) and -4.0 (20/80) for my new glasses. 20/40 is the minimum requirement for DMV requirement for driving in California.

By the way, OD (oculus dexter) and OS (oculus sinister) stands for right and left eyes respectively. What do numbers like -5.75 -0.75x10 mean? Well, the first number (-5.75) is for spherical correction and the second number (-0.75x10) is for cylindrical correction (cylindrical diopters x axis in degrees) . The numbers are measured in diopters, D for short. The spherical term corrects for near- (negative values) and far- (positive values) sightedness while the cylindrical term corrects for astigmatism. What is a diopter? It is a value you obtained by taking the focal length of the lens and inversing it mathematically. For example, for -2 meters focal length, it will be 1/(-2) = -0.5D. And to make it simple, they will just drop the decimal point. So, -5.75D means a power or degree of 575 (near-sighted) in layman language.

A 20/20 vision is regarded as normal. Therefore, if you have a 20/40 vision after correction, it means that while you can read a particular line on the eye chart (Snellen) at 20 ft distance, a person with normal vision will be able to do so at 40 ft. I do recall seeing a table somewhere relating the diopter to the distance. When I can find it, I will post it here.

Wait, where's my susu kacang.....


CY said...

what is the equivalent power in Malaysia context (100? 150? 700?)

Susu Kacang said...

Drop the decimal point...e.g. 7.00D means 700.