Wednesday, December 27, 2017

Presbyopia


Presbyopia is the combination of two Latin root-words.  "Presby-":  meaning arms-are-too-short and "-opia" meaning vision.

Originally thought to have been coined after a Christian denomination commonly found in Scotland, the condition was observed in congregations and was found to be exacerbated by a combination of the frugal and canny Scots printing church documents in the smallest font available (to conserve paper) and using the minimum number of candles and windows allowed by local building codes.

Bifocals
Many people who find their arms getting shorter as they age resort to bi-focal glasses.  The top prescription is suitable for distances while the lower prescription is useful for reading and working on computers.

The speedometer on vehicles is always between the two prescriptions which results in blue-haired ladies either driving 25 mph (driving by Braille) or 85 mph (everything is a speed-bump if your vehicle is large enough and fast enough).

Trifocals
Sometimes old farts are pressed into service working overhead.  Sometimes it is vehicle maintenance.  Sometimes it is recreational plumbing or wiring.

In addition to having arms that are too short, the oldster handiman finds that he cannot tip his head back far enough to engage the problem with the "near" portion of his bi-focal glasses.

If you perform enough of this work, close and overhead, it might pay to buy a pair of tri-focals.

Dedicated glasses
A less expensive solution is to purchase a pair of glasses that are 100% the lower, close prescription.

My pair arrived the day after Christmas.

It is easy enough to do.  Obtain your distance prescription and your Pupillary Distance from your eye doctor.  He might kick a little bit about giving it to you because selling eyeglasses is the profitable part of his business.  Remind him that the data belongs to you because you, or your insurance company paid for it.

***Note:  The following discusses how your eye doctor determines the prescription to put in the bottoms of the bifocals and is not intended as "practicing medicine" on the internet***

The way an eye doctor determines your "near" prescription is pretty straightforward.  He uses this formula.

Take your age in years.  Divide it by eight.  Then subtract 5.00.  That is the "add".

This is what the prescription entry dashboard looks like at Zenni Optical.
Lets suppose the prescription for your right eye is -3.75 spherical and -6.00 spherical for the left eye and that you are 60 years old.  Sixty divided by eight is 7.50.  7.50 - 5.00 = 2.50  Consequently you would add 2.50 the spherical for each eye which would result in

Same prescription but adjusted for "near" vision for our mythical sixty year old geezer.
The only other tweak to the prescription is to slightly reduce the PD.  Our eyes become slightly crossed when we are looking at objects that are close to us.  Reducing the PD by 2.0 millimeters is usually sufficient.

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