On the subject of glasses, for a single focus lens for a short sighted person, how are intermediate lenses sepcified (ones you might use for reading plus computer work at up to say 1m)?
Is the far sight dioptre power changed (by a factor or actually prescribed by the optician) or is the far sight dioptre given to the lens maker along with a request for "intermediate"?
Don't know, but my optician very easily made me up a pair on the spot for that, and got it right. Fair enough, if you look up from the PC at someone who's come into the room, you can't see them properly, but that's not what they're for ... but I think you're less likely to run into problems getting a pair of intermediates right than with getting VFs right.
Last time I bought any my optician wanted to know the distance to the screen, so I presume they apply a correction factor.
People have varying requirements when it comes to intermediates, so you can't just specify according to some preset formula. It requires a separate sight test at the specific distance - one meter in your case; it was 24" in mine. Assuming you're short sighted, the prescription will be less negative. So for example if your distance script is -3.25 for each eye your intermediate script will be -2.75 or thereabouts give or take a bit. But you need to test at the appropriate distance to ascertain what the 'give or take bit' is.
So in principle, if you have your far prescription, it ought to be possible to calculate an intermediate based on "I want to focus upto 1m instead of infinity"?
When I started having difficulty with that sort of screen distance, I had a normal sight test, during which they decided for the first time I needed a reading addition and recommended varifocals, I said I wasn't ready to decide there and then on having them, but would buy a pair of "bog standard" single vision lenses but tweaked for screen distance, which they made from stock lenses/frames in 40 minutes, without having to re-test just for that distance, so yes - I'd say they can calculate what power for a given distance.
You can do a calculation but if you are with an optician getting a prescription the best thing to do is to hold a reading card at the right distance and have them show you the effect of various corrections.
To a reasonable approximation, a +1 correction on a "distance" lens will let you focus at 1 metre, +2 at half a metre, etc.
My vision is completely different but in principle I agree. It's not rocket science. Buying specs online means that I can "adjust" my official prescription for different purposes/distances, including specialised bifocals, without an optician sticking his oar in. And the prices are low enough that I could afford the odd mistake.
My advice is to keep careful records of your prescriptions and glasses. I find a spreadsheet ideal. That might seem a bit nerdish but over time it will become a valuable resource.
The first time I had those, the optician just used the usual system for determining the prescription, then appeared to do a mental adjustment, and checked that by getting me to read a chart on paper held at the distance the screen would be.
The most recent one just paused when I said that is what I wanted and wrote that on the prescription and didn?t bother with the final check with a printed chart.
Yes, and I know that because I also got him to do a prescription for my sunglasses that I don?t need to be optimised for the computer screen.
No, because I buy my single vision glasses online and supply them with the actual prescription.
You can actually. The most recent prescription did it that way.
It requires a
No it does not.
Assuming you're short sighted, the prescription will be
No.
Which of the on-line companies do you use?
Yes, and there is a newsgroup that covers that stuff, I've added it.
What about with a prescription of -7 and -6 for the computer screen at about 600mm if I want reading glasses to use for reading a physical book ?
I got some normal reading glasses from ebay for peanuts, +2 and +3 but they are useless for reading books with. I can read books fine with no glasses at all but need to hold the book too close to my face to be mechanically convenient now. Presumably I need something like
-2 or -3 for reading glasses.
Currently Glasses Direct.
That's quite a useful ready reckoner...
On 21 Apr 2015, Tim Watts grunted:
When I first tried to procure myself a pair a couple of years ago, I used one of the online opticians. Under the prescription, they had 3 pairs (ie left and right eye) of boxes to fill in for standard, reading and intermediate prescription. I only had the standard and reading prescriptions, so completed only those, and ordered a pair of intermediate glasses assuming that they would calculate the required prescription.
I was provided with a pair of standard glasses, without any queries being made. When I complained, I was told that I needed to have provided the intermediate prescription and otherwise it wasn't possible. We then had a row about why they'd gone ahead and made up a pair of unwanted glasses - can't remember why, but they refused to back down and having obtained an intermediate prescription I bought my next (proper) pair from a competitor.
When I could see it was the optician. Given that it is a loss of range of focus that afflicts people as they age, you do not want the focal distance to be trying to use the eye muscles at the extreme end of the range it can do, as this leads to fatigue and blurred vision. Brian
Care to name?
I'm long sighted, but corrected by contact lenses - obviously for distant vision. I'm of an age where the eyes hardly focus if at all. But having 'normal' sight as corrected, can use cheap ready reads. I find +2 specs fine for the computer and reading normal sized print. But not for fine work. ;-)
It probably depends on your age - you may well have more accommodation.
Yup. Optical power in dioptres is the reciprocal of the focal length in metres. This is useful because of the lens formula:
1/d_o + 1/d_i = 1/f,where d_o is the object distance, d_i is the image distance, and f is the focal length.
In terms of optical power P=1/f, this is
1/d_o + 1/d_i = P.Optical power just adds, i.e. for thin lenses, if you have two of them adjacent, P1 and P2, then P = P1 + P2. To change focus from infinity to
1 m, you could just hold a 1 m focal length lens in front of your distance glasses, right? So now you know how to compute the required correction: since the powers add, just add +1 dioptre to your infinity prescription.More formally, changing from d_o = infinity to d_o = 1m requires the addition of power
delta P = (1/new d_o) - 1/old_d_o)
i.e. delta P = 1 dioptre
So as I say, just take your distance prescription and add 1 dioptre--if you're -2.75 for infinity, -1.75 will make a focus at 1 m, -0.75 -> 0.5 m, etc. (When you get to really close distances, the position of the lenses starts to matter, so you have to be more precise, but the above thin-lenses-in-contact approximation is better than good enough for eyeglasses.)
Cheers
Phil Hobbs
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