glasses & contact prescription differ in opposite directions for each eye

just had an eye exam and got new contacts and glasses

glasses prescription is:

R -4.00 -0.25 110
L -5.00 -0.25 095

contact prescription is:

R -4.25 8.6
L -4.75 8.6

the contacts are really comfortable vision wise, but the glasses are
making me feel slightly strained- i am wondering why they strength
would differ in the opposite direction on each eye? is this typical.

Just looking at the math, at appears that your right contact lens is
overcorrected compared to your glasses. If the contact vision is
comfortable, you might need a bit more power on the right side of the
glasses.
There are different ways to balance a patients vision. If this was
determined by testing you in a phoropter, these numbers might be just
fine. But, just looking at the math alone, you are right...the right
contact should be a -4.00 to match the specs.

(E-Mail Removed) <(E-Mail Removed)> wrote:
> Just looking at the math, at appears that your right contact lens is
> overcorrected compared to your glasses. If the contact vision is
> comfortable, you might need a bit more power on the right side of the
> glasses.
> There are different ways to balance a patients vision. If this was
> determined by testing you in a phoropter, these numbers might be just
> fine. But, just looking at the math alone, you are right...the right
> contact should be a -4.00 to match the specs.

thanks,

the contacts are actually quite comfortable vision-wise, i notice my eyes feel a bit "strained"
when wearing my new glasses. it just seemed odd that the difference between the two prescriptions
wasn't in the same direction. i am going to try the glasses for a few days and see if it is just
in my head before going back.

-
(the testing was done with a phoropter)

The reply by "doctor-my-eyes" is not correct, or at least we can not
know if it is correct because we do not know what kind of contact lens
you are wearing and how the fitting curve relates to your cornea curve.
Further, a contact lens power on the eye is not mathematically
equivelent to the spectacle power. For example:


your glasses prescription is:

R -4.00 -0.25 110>>>>> this relates to a theoretical contact lens power
of -4.00 or -4.25, the higher power more likely if it is
silicone-hydrogel type lens. That .25 difference may or may not matter
in any event.

L -5.00 -0.25 095>>>>>this relates to a theoretical contact lens power
of -4.50 or -4.75, because as mentioned above *and* the fact that a
spectacle lens power measured at 13mm in front of the eye needs to be
adjusted to less minus power when measured at the cornea (like a contact
lens) for the same net effective power.

your contact prescription is:

R -4.25 8.6
L -4.75 8.6

And therefor both prescriptions are darn near the same and likely
correct, especially if as you mentioned, the practioner measured the
powers in the phoroptor or with hand held lenses.

--L.B., O.D.

yep. I said that. I left out that silicone stuff, though. Nice
rebound. Wanna have a beer? (burp)

LarryDoc <(E-Mail Removed)> wrote:
> The reply by "doctor-my-eyes" is not correct, or at least we can not
> know if it is correct because we do not know what kind of contact lens
> you are wearing and how the fitting curve relates to your cornea curve.
> Further, a contact lens power on the eye is not mathematically
> equivelent to the spectacle power. For example:

> your glasses prescription is:

> R -4.00 -0.25 110>>>>> this relates to a theoretical contact lens power
> of -4.00 or -4.25, the higher power more likely if it is
> silicone-hydrogel type lens. That .25 difference may or may not matter
> in any event.

> L -5.00 -0.25 095>>>>>this relates to a theoretical contact lens power
> of -4.50 or -4.75, because as mentioned above *and* the fact that a
> spectacle lens power measured at 13mm in front of the eye needs to be
> adjusted to less minus power when measured at the cornea (like a contact
> lens) for the same net effective power.

Hi- thanks for the reply. The contacts in question are Optix O2.

My curiosity was really why the variation was in the opposite direction, i.e. the -4 for glass went to -4.25
for the contacts, but the -5 for glasses went to -4.75 for contacts. I would think they would both adjust in
the same numerical direction, but at the same time I don't understand the third number in the prescription
(the 110 and 95)

In article <d8co7j$oid$(E-Mail Removed)>,
Christopher Albin Edmonds <(E-Mail Removed)> wrote:

> Hi- thanks for the reply. The contacts in question are Optix O2.

You're welcome. Silicone hydrogel indeed!

> My curiosity was really why the variation was in the opposite direction, i.e.
> the -4 for glass went to -4.25
> for the contacts, but the -5 for glasses went to -4.75 for contacts. I would
> think they would both adjust in
> the same numerical direction, but at the same time I don't understand the
> third number in the prescription
> (the 110 and 95),

You'd think, but here's why and why not:

1. The vertex distance alteration in power occurs in powers above + or -
4.50 diopters at least in any noticable degree.

2. The "stiffness" of silicone lenses tends to create a plus lens tear
layer under the lens as the plastic vaults over the cornes apex. Hence
the corresponding increase in minus to neutralize the tear lens.

3. The third number in the rx is the astigmatism component, and it is a
minor amount and of little or not consequence due to the aspheric optics
and vaulting design of the lens.

Hope that answers your query!

--LB, O.D.