How to calculate strength of computer glasses!?!
Question: How to calculate strength of computer glasses!?
2)For computer related work,he uses his reading glasses, having a prescription of (-)4.5 and 1.25 cylinder.What should be correct prescription for his computer glasses?
Answers:
The best way to determine this is for your friend to measure the distance to the computer screen and at the next eye exam also request a computer Rx or a computer bifocal. This way it can be exactly what he needs for his working distance. An optician can calculate a computer lens with a standard Doctors prescription simply by cutting the add power in half, but not everyone has the same distance to their computer so it might work fine or it might not. That would usually allow for arms distance, but with the use of desktop and/or laptop, the distances now can vary.
A single vision lens will allow for the screen distance but typically won't allow for near as well. So a bifocal will give him near and computer in one pair of glasses to see the screen, keyboard and any printed material he may need to read.
Your friend has quite a high Rx... -7.00D Sphere and -1.25D Cylinder... that's getting into the low end of the HIGH MYOPIA.
Now because he is myopic (near sighted), when he's not wearing any glasses, his eyes are naturally focused at aroun 15cm, or 1/2 in. So if he holds his reading material there, then he'll be able to read well... although he would have to converge or 'cross' his eyes a bit, which could lead to eye strain.
Now for Computer screen... if he's using -4.50 / -1.25 Cyl.. then this Rx is +2.50D sphere weaker than his distance glasses. That means when he wears this pair, his eyes will naturally focus at 40 cm or ~ 1 foot 3 in. Now if he has his computer screen closer than this, then it's no problems.
If he has his screen further away.. say at 60cm or 2 ft, then these glasses are too strong.
In the end there's no correct Rx... it depends on his working distance. Generally speaking, VDU screens are around 50cm away.. and so most people would only need an 'add' of +1.75DS which when placed over your friend's distance Rx, would be -5.25 / -1.25 cyl. But this Rx (although good for computers, may not be strong enough to read small detail.. say the phone book..etc...)
you should go to doctors immediately.