Are there tables of the right add to use for various working distances?

I'm 60 and nearsighted, currently about -6, and have been
getting less nearsighted for the last 10 years or so after
topping out at about -7.5. Anyway, I'm now using my
2-year-old reading glasses as my desktop computer glasses,
so it's that time again.

Up to now I've worn progressives as distance and
general-purpose glasses, and separate single-vision glasses
for my desktop computer and another set for serious reading.
So that's three so far. But now I'm considering getting a
laptop, which I assume would fit in there between the
desktop and reading glasses, but possibly one pair of
glasses might work for two of those three (I doubt it).

Going into this round of lens replacement, I would like to
get an independent source of information as to the
appropriate add for various close-in working distances, just
as a check on what the optician tells me. Since I have very
little accomodation left, it's important that I do this
right.

In addition, in case I continue to get less nearsighted, I'd
like to see if through careful selection this time I might
be able to "stage" the three single-vision glasses so that
they just shift in function from reading, to laptop, to
desktop, so that I only have to replace the reading glasses
as time goes on (well, and the progressives of course). The
left eye has been .75 less nearsighted than the right for
about 15 years now, and if that continues, then this might
work.

At first I thought I could just use the diopter to focal
length relationship to calculate this information, but then
I wondered whether there might be things that make that
formula less valid. Such as perhaps the fact that the
glasses lens is maybe an inch in front of the eyeball, or
the fact that the lenses are spherical instead of parabolic.
And I have this vague recollection that when I asked a
version of this question years ago (about computer glasses),
the optician said that the amount of add could vary somewhat
depending on the strength of the distance prescription. The
implication was that in actual practice the add for a
specific working distance might be different for a very
nearsighted person than for a moderately nearsighted one,
for whatever reason.

Well, I guess what I'm looking for is a table, or tables, of
distance-versus-add that eyecare professionals use in actual
practice to get the best results for people wearing glasses.
If there is such a thing.

As an aside, I've also thought about under-correcting on the
progressives this time by .25, just so they would last
longer. Any thoughts on that?

Thanks for any help.

 

you could use a table, but why not just use the formula:

d=1/f where d=distance in focus and f=power (add)

d is in meters and f is in diopters

w.stacy, o.d.

 

William Stacy, O.D. says...

I could. That would yield these values:

Reading - 16" +2.5D
Computer - 28" +1.5D (actually +1.4D)

It seems to me that both of these are at least .25D too much
add compared to what I've used in the past. My readers have
always been +2D add, and I think my computers were +1.0 add,
maybe even less.

I still wonder if these calculations aren't off a bit
because the lens is an inch in front of the eye. Doesn't
that make the lens effectively less minus than the
calculation would suggest?

When I've attempted to wear contacts in the past, the Rx for
the contacts was always a good bit less minus than my
glasses, and it was explained to me that that was because
of the difference in where the lens sits relative to the
eye. Well, I see the same thing when I slide my glasses
down to the end of my nose - they become less powerful, less
minus.

On the other hand, the adds are relative to the distance Rx,
so maybe it all comes out even anyway. It's just that the
calculated values look like they're too much add. That's
why I was looking for a table showing what opticians and
optometrists actually use in practice.

 

Sure. you probably have *some* residual focusing left (more than zero),
and don't forget there's a depth of focus depending on pupil size and
lighting, which adds a little range on either side of whatever you chose.

vertex distance indeed is a factor. how much, depends on the powers.
and yes, if it's minus, they are effectively less minus, if they are
plus, they are effectively more plus. this effect is based on a
different formula.

of course. the higher the minus, the greater this effect, and it goes
to zero as the power of the lens approaches zero.

actually, we use formulas some of the time, rarely use tables, and most
often we just dial the lens powers in and check the ranges physically...

w.stacy, o.d.

 

My little experience has been that I can do the job of specifying add for
computer glasses, at least for me, better than optometrists I went to. At
first, when personal computers were rather rare, they seemed to wing it more
than calculated it.

Because of other eye problems, I have been seeing ophthalmologists more
often then optometrists. I have had better luck recently getting the adds I
want. I don't know if it is because ophthalmologists are not that tied to
the quality of the glasses they prescribe or whether computer glasses are no
longer as novel as they used to be.

Bill
-- Fermez le Bush

 

(as I've said before), what I do, since I have virtually
zero accomodation, is borrow for a few minutes their
test-frames, and also a couple of .25 test-adds, and
play with those while looking at some detailed object
at the distance the test-frames are set for, and
find the extra (over their prescription) .25 or .50
on the left or right that will bring the left and
right "depth of field" near-far-range to be EXACTLY
the same.

Only *I* myself (or you yourself) can do this adjusting --
no machine and no "is it clearer this way, or that way"
procedure can do it as well as I can (on myself), moving
say a magazine nearer and farther and noting the distances
at which it stops being *perfectly* clear, first with
one eye, than with the other -- and getting those
distances to be equal.

Once I'm satisfied, they then adjust the prescription
to match what I've just discovered.

Anyway, that's what I do.

Before I started doing this, either reading or working
on the computer was uncomfortable for sure!


David

 

Actually, we do that all the time for patients. That's why the rod that
holds the nearpoint test card on a refractor or phoropter is graduated
in inches, centimeters and diopters. We can watch the numbers as we
move the card in and out, getting very accurate distances to first blur
with any lens combo in place. Way easier than your method, and probably
more accurate.

w.stacy, o.d.