Adjusting eyeglass prescription for 8 feet (2.5 meters)?

Question

Is there a way to calculate an eyeglass prescription for a specific distance? In this particular case I watch a TV set that's 8 feet away while sitting in a rocker. The rocking shifts the line of sight in and out of the progressive zone of my regular glasses (SPH -6    NVADD  + 2.5). Rather annoying. 

Is there a way to estimate a single vision prescription for 8 feet?   I asked my ophthalmologist, he just said that the far vision was "good enough". 

But what is the correction? I suppose if it's only + 0.25 diopter  that's "good enough" for me, If I get a pair of single vision glasses specifically for TV watching, I'd like for them to be dead on.

Judy B · Accepted Answer

There is a simple formula.  Prescription required to focus at a particular distance is given by
1 / distance in metres.  For eight feet that works out to about +0.40D. 

For single vision far glasses to focus at 8 feet you would add +0.40D to your -6.00D to get -5.60.  Glasses only come in 0.25D increments so you will need either -5.75 or -5.50.  If you choose -5.50, you will need to move the TV slightly closer to the rocking chair.

I think you understand now why your doctor told you that your current glasses were good enough for 8 ft.

Anonymous · Answer

Well, the - parts are short sighted and the + parts are long sighted. The Cyl means you have a mild astigmatism in the left eye. This means (if i remember rightly) that your eye is shaped more like a rugby ball than a football (a bit more elongated at each end) New glasses can take a while to adjust to but if things still aren't right go back and get the glasses check. They could just not be fit properly or a measurement a couple of millimeters out. 20/20 is perfect vision but hes probably given you a slight prescription to make it easier for you to see 20/20 without struggling and to make it easier to adjust to the higher prescription in the left eye. If you are having problems its always better to go back and get things checked out. The optometrist may tell you that you need to give your eyes a bit longer to adjust, but follow your instincts, if it doesn't feel right and you've been trying for a week or two then something may not be right. Some people can take up to 3 months to adjust to a new prescription/glasses, others do it within a couple of days

? · Answer

2.5 Meters In Feet

Emma · Answer

I am afraid there isn't any prescription that can adjust for the rocking.  I sit about as close as you do and I have blended trifocals.  My TV is 40 inches and I probably am using the middle range for viewing.
I have a rocker but do not rock it while viewing TV.  You could try asking your doctor to give you a single vision lens extra pair of glasses to use just for TV, good at the distance that you sit.

Anonymous · Answer

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Adjusting eyeglass prescription for 8 feet (2.5 meters)?

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