The "Add" Factor

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MsBrainy - 30 Mar 2008 04:24 GMT

My understanding is that the reading portion of bi-focals (or progressives)
is determined by a formula, not by a real life test. But what if I can’t
read through it, whereas I CAN with the naked eye?

My problem is with my right eye, the one that has been thru 3 “successful”
surgeries – macular hole, retinal detachment and cataract. I do not expect
my vision there to be perfect – it is not and cannot be. But I do expect
glasses to improve it at all distances. In all my vision tests and
refractions, nobody has ever tested my near vision. Actually, it’s pretty
good (not “perfect” of course) without correction, i.e. I can read even very
small print. But my progressive glasses that enable me to see at distance,
as well as my Zeiss computer glasses, are useless for reading, though I can
read using my good (but myopic and presbyopic) left eye – with or without
glasses.

The “Add” factor in my Rx is +2.75, and +1.75 for the computer glasses, but
the print appears to my right eye as a mere blur, unless it’s quite large.
It works for my left eye, but I don’t think it’s proper for my right. On the
other hand, I understand that the Add must be the same for both eyes. Is
there a solution to this situation?

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MsBrainy

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Dave Bell - 30 Mar 2008 04:45 GMT

> My understanding is that the reading portion of bi-focals (or progressives)
> is determined by a formula, not by a real life test. But what if I can’t
> read through it, whereas I CAN with the naked eye?
>
> I understand that the Add must be the same for both eyes. Is
> there a solution to this situation?

That sort of makes sense, if you consider that the main lens is supposed
to bring both eyes to focus at some comfortable distant vision. Then it
follows from optical principles that adding a small positive lens would
give near focus vision, and the added prescription would be the same for
both eyes.
Are you quite near-sighted, without lenses? That would imply a strong
negative power for the main lens. It sounds like you may simply have
wrong prescription(s).

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Dr Judy - 30 Mar 2008 16:56 GMT

> My understanding is that the reading portion of bi-focals (or progressives) is determined by a formula, not by a real life test.

Not in my office. I determine add by measurement and checking real
life range. And I measure corrected near acuity.

> But what if I can't read through it, whereas I CAN with the naked eye?

If highly myopic, when unaided you may be able to read smaller print
by holding closer, thus gaining magnification. This can be duplicated
by using an add that is opposite in power to your myopia, ie -5.00
distance with +5.00 add --- problem will be needing to hold things
very close.

> My problem is with my right eye, the one that has been thru 3 "successful"
> surgeries - macular hole, retinal detachment and cataract. I do not expect
[quoted text clipped - 6 lines]
> read using my good (but myopic and presbyopic) left eye - with or without
> glasses.

The best you can expect at near in the right eye with correction is
the same corrected acuity as you have at far. As I said before, if
myopic then holding very close will improve and allow reading of fine
print. That won't work on the computer which is not held close.

> The "Add" factor in my Rx is +2.75, and +1.75 for the computer glasses, but
> the print appears to my right eye as a mere blur, unless it's quite large.
> It works for my left eye, but I don't think it's proper for my right.

Does the "quite large" correspond in near acuity measure to the far
acuity you have with that eye? If so, then that is the best you can
expect.

Due to surgery etc, you likely have distortion of vision in right eye
which will be more noticable at near and amplified by the distortions
of progressives. Single vision readers may be better.

> On the other hand, I understand that the Add must be the same for both eyes. Is there a solution to this situation?

You can have different adds for each eye, but the plane of focus will
not be same (ie one eye will be focused at 40cm, the other at 20cm)
and that can be uncomfortable. I have occasionally done this for a
patient who had a need to see at a variety of distances and did not
want progressives.

At near, the blurred, distorted right eye vision may interfere at a
the cortical, perceptual level with the better information from the
left eye, especially if you are right eye dominant. The answer then
is to patch the right eye while reading and rely solely on the left
eye.

Dr Judy

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