Relationship of eyeglass prescription to non-toric contact lens prescription?

Here's the situation - my girlfriend recently had her annual checkup,
and we
decided to have her try the Acuvue-2 contact lenses. Her prescription
(glasses) was -6.0-0.5 and -6.0-1.0. The optometrist fitted her with
a trial pair of contacts, but did not give her a prescription at the
time.

Last week, she returned for her follow-up visit. He pronounced the
fit good, and jotted down the formula for the contacts as -5.5 and
-5.0 on a new prescription form.

I had assumed that the spherical and cylindrical corrections were
somewhat additive, and was surprised to see the opposite. My question
is, obviously, does this look correct? Could he have actually
trial-fit her with -6.5 and -7.0, then made a hasty mistake a week
later when writing the prescription?

I'd like to feel that this looks normal before ordering a large supply
of her lenses.

best regards,

Martin

"Remove SPAM From Address to Reply" <(E-Mail Removed)> wrote in
message news:(E-Mail Removed) om...
> Here's the situation - my girlfriend recently had her annual checkup,
> and we
> decided to have her try the Acuvue-2 contact lenses. Her prescription
> (glasses) was -6.0-0.5 and -6.0-1.0. The optometrist fitted her with
> a trial pair of contacts, but did not give her a prescription at the
> time.
>
> Last week, she returned for her follow-up visit. He pronounced the
> fit good, and jotted down the formula for the contacts as -5.5 and
> -5.0 on a new prescription form.
>
> I had assumed that the spherical and cylindrical corrections were
> somewhat additive, and was surprised to see the opposite. My question
> is, obviously, does this look correct? Could he have actually
> trial-fit her with -6.5 and -7.0, then made a hasty mistake a week
> later when writing the prescription?

See my thread lower regarding CL powers.

The power requred for a CL is difference to that required for spectacles due
to the distance the lens is from the eye. As there is an astigmatism in the
prescription some compensation needs to be made to the spherical power.

The exact strength required will depend on a number of factors including
fit, lens material etc. A optomotrist should therefore fit the lenses and
check site with the lenses in.

A quick check with the calculator (you can find online at
http://www.eyedock.com/calcs/cl_calcs.htm) suggests that the lenses should
be -5.75 / -6.00. A prescription of -5.50 and -5.00 therefore appears to be
substantiall different.

Perhaps you should check with the optomotrist?

"James Smith" <James S=(E-Mail Removed)> wrote in message news:<zej8d.159680$(E-Mail Removed) o.uk>...
> "Remove SPAM From Address to Reply" <(E-Mail Removed)> wrote in
> message news:(E-Mail Removed) om...
> > Here's the situation - my girlfriend recently had her annual checkup,
> > and we
> > decided to have her try the Acuvue-2 contact lenses. Her prescription
> > (glasses) was -6.0-0.5 and -6.0-1.0. The optometrist fitted her with
> > a trial pair of contacts, but did not give her a prescription at the
> > time.
> >
> > Last week, she returned for her follow-up visit. He pronounced the
> > fit good, and jotted down the formula for the contacts as -5.5 and
> > -5.0 on a new prescription form.
> >
> > I had assumed that the spherical and cylindrical corrections were
> > somewhat additive, and was surprised to see the opposite. My question
> > is, obviously, does this look correct? Could he have actually
> > trial-fit her with -6.5 and -7.0, then made a hasty mistake a week
> > later when writing the prescription?
>
> See my thread lower regarding CL powers.
>
> The power requred for a CL is difference to that required for spectacles due
> to the distance the lens is from the eye. As there is an astigmatism in the
> prescription some compensation needs to be made to the spherical power.
>
> The exact strength required will depend on a number of factors including
> fit, lens material etc. A optomotrist should therefore fit the lenses and
> check site with the lenses in.
>
> A quick check with the calculator (you can find online at
> http://www.eyedock.com/calcs/cl_calcs.htm) suggests that the lenses should
> be -5.75 / -6.00. A prescription of -5.50 and -5.00 therefore appears to be
> substantiall different.
>
> Perhaps you should check with the optomotrist?

Yes, I will check. Thanks.

BTW, that online calculator appears to have a bug in it. When I enter
-6.0-1.0x180 for spherical lens, it responds with -6.0. When I enter
-6.0-1.0x0, it changes the "0" axis to "180" (the same as the first
example), but returns a different answer of -6.5. In one case, the
text generated says the astigmatism correction is 0.75 diopters, in
the other case it says it is 1.0 diopters.

Martin

> BTW, that online calculator appears to have a bug in it. When I enter
> -6.0-1.0x180 for spherical lens, it responds with -6.0. When I enter
> -6.0-1.0x0, it changes the "0" axis to "180" (the same as the first
> example), but returns a different answer of -6.5. In one case, the
> text generated says the astigmatism correction is 0.75 diopters, in
> the other case it says it is 1.0 diopters.

Ive just tried it and it give -6.00 in both cases with 0.75d astigmatism.
Perhaps someone qualified can comment?

Isnt Axis 180 the same as 0?

"James Smith" <James S=(E-Mail Removed)> wrote in message news:<LgB8d.163370$(E-Mail Removed) o.uk>...
> > BTW, that online calculator appears to have a bug in it. When I enter
> > -6.0-1.0x180 for spherical lens, it responds with -6.0. When I enter
> > -6.0-1.0x0, it changes the "0" axis to "180" (the same as the first
> > example), but returns a different answer of -6.5. In one case, the
> > text generated says the astigmatism correction is 0.75 diopters, in
> > the other case it says it is 1.0 diopters.
>
> Ive just tried it and it give -6.00 in both cases with 0.75d astigmatism.
> Perhaps someone qualified can comment?
>
> Isnt Axis 180 the same as 0?

Yes, 180 is the same as 0. When I noticed the bug, I emailed the contact on
the web site and had a response within about 30 minutes from Todd M. Zarwell,
OD, who said he was working on it. Probably he got it fixed before my
post on the subject actually appeared.

Martin