That is really cool - thank you Phil.
So presumably this applies equally when the eye lens is focussed at its other extreme (ie near)?
So if -3.50 dioptre (my far sight power) lets me view in focus down to
0.5m, then if I add 1.25 for reading (as the optician specified), then I should be able to focus down to:1.25 = 1/X - 1/0.5
1.25+2 = 1/X X = 1/2.25 X=0.44mSomething seems not right there - what did I do wrong?
You added 2+1 and got 2. ;)
Should be 1/3.25 = 0.31 m.
Cheers
Phil Hobbs
There's no great mystery to it, 1 dioptre is a lens with a focal length of 1 metre, 2 dioptres is half a metre. When I first thought about it I was concerned that "glasses + eye" is a compound lens, two lenses separated by a distance. On reflection, a full distance correction means that parallel light is focussed on the retina. So if you "reverse the process" and project parallel light through a 1 dioptre lens immediately in front of the glasses, it will come to a focus at 1 metre (etc).
Yes - that seems perfectly reasonable - and seems to be bourne out in reality...
In a nutshell, your prescription provides a focus at far distance, ie an infinite distance away from you.
You can focus at a particular point closer than that. you just need a lens with a dioptric power that is the reciprocal of the working distance (in metres) you want to focus at, in front of your distance prescription if you have one*.
Hope that makes some sense.
So what about the situation where the glasses I use for the computer screen at 60cm are -6 and -7 and I want reading glasses for reading physical books ? Up till now I have read physical books with no glasses at all, but now find that I have to hold the books too close to my face to be very convenient mechanically and so would prefer to have some single focus reading glasses for that.
Depends,
Are the above values just for computer use, or is it a distance prescription that also works for computer?
And your age / amplitude of accommodation should be taken into account,
But as a rule of thumb, assuming -6 and -7 SPH values for distance prescription, a 2.50 add would let you read at 40cm (but not beyond) meaning you might benefit from a prescription of -3.50 and -4.50.
No.
No. It?s a distance prescription adjusted so that it works best for the computer screen and is still adequate for the distance. That means that with the small street signs that have just the name of the street on them you do have to get closer to the sign to read it but you can still read them fine when driving around and the glasses still work fine when they test your eyes quickly when renewing the drivers license.
I haven't seen any evidence of any problem there and am heading into my 70s now.
I'm still getting a relatively slow worsening of the short sighted.
OK, I'll try that thanks. I can try them very cheaply using the online suppliers in china, basically $15.
I did try conventional reading glasses from ebay, both +2 and +3 and they were useless. Unsurprising when I thought about it.
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