Does this Prescription look right?

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bthuynh@gmail.com - 21 Dec 2004 04:30 GMT

Hello,
I just got a prescription for some eyeglasses and contacts and am
trying to make sense of it. First let me give you the numbers:
For the Eyeglasses:
OD Sphere -1.75, no astigmatism
OS Sphere -4.25, Cylinder -050, Axis 015

For the Contact lenses:
OD Sphere -1.75
OS Sphere -4.50
B. Curve 87
Diam: 14.0

Now from the reading I've done I can figure out what some of these
numbers mean, but I need help with putting it all together.

It seems that because I have no astigmatism in my R eye, and because
the power is less than -4.75 (so no correction factor is needed for
glasses and contacts), the powers for the contacts and eyeglasses will
be the same. Is this right?

But for the left eye, I have read that at powers of -4.75 and above, a
correction factor starting at 0.25 needs to be added to the contact
lens Rx. But it seems my L eye power is listed as -4.25. However, the
astigmatism power is -0.50. Would I be correct in adding these two
values together, to get a value of -4.75 for the L eye, which the
optometrist then added 0.25 to arrive at the -4.50 value for the
contact lenses?

Thanks for helping this amateur optometry sleuth.

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Mike Tyner - 21 Dec 2004 04:52 GMT

> But for the left eye, I have read that at powers of -4.75 and above, a
> correction factor starting at 0.25 needs to be added to the contact
[quoted text clipped - 3 lines]
> optometrist then added 0.25 to arrive at the -4.50 value for the
> contact lenses?

Your spectacle prescription for the left eye describes a "football" shape
that is -4.25 along the flatter meridian and -4.75 along the steepest curve,
90 degrees away. Average these two and the "spherical equivalent" is -4.50.

I was taught that the correction factor should be applied at -4.25 and
beyond; i.e. -4.50 in spectacles means we should start with a -4.25 contact.

I've verified that rule of thumb countless times by over-refracting (eg.
finding a prescription of +0.25 when you wear a -4.50 contact) but in fact,
when you're young, an extra -0.25 does no harm and some actually prefer it.

Wearing -4.50 when you need -4.25 means your contact makes you a little bit
farsighted, but as our resident expert tells us so frequently,
farsightedness is the "natural state".

-MT

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RM - 21 Dec 2004 04:59 GMT

> Hello,
> I just got a prescription for some eyeglasses and contacts and am
[quoted text clipped - 16 lines]
> glasses and contacts), the powers for the contacts and eyeglasses will
> be the same. Is this right?

Correct.

> But for the left eye, I have read that at powers of -4.75 and above, a
> correction factor starting at 0.25 needs to be added to the contact
> lens Rx. But it seems my L eye power is listed as -4.25. However, the
> astigmatism power is -0.50. Would I be correct in adding these two
> values together, to get a value of -4.75 for the L eye,

No. you add 1/2 the astigmatism power to the spherical power to obtain what
is called the "spherical equivalent" power. It adds up to -4.50 in this
case. -4.50 is right on the breakpoint where you might consider adjusting
the contact lens power down by a quarter (to -4.25). For me it depends on
the patients age, and whether I get the "feel" that they are slightly higher
or slightly lower than -4.50. I pick up that "feel" from how they respond
during the refraction. Your doctor just left the contact lens power
at -4.50 which should be fine.

RM

Reply to this Message

- 21 Dec 2004 20:19 GMT

> Hello,
> I just got a prescription for some eyeglasses and contacts and am
> trying to make sense of it. First let me give you the numbers:
> For the Eyeglasses:
> OD Sphere -1.75, no astigmatism
> OS Sphere -4.25, Cylinder -050, Axis 015

snip....

> For the Contact lenses:
> OD Sphere -1.75
[quoted text clipped - 10 lines]
>
> Thanks for helping this amateur optometry sleuth.

For the left softlens S-4.00 will do.
When you are young of age you might consider S-4.25 which gives you most of
the time a bit more vision in the dark or at twilight.
The S-4.50 is to strong in power.

Signature

Jan (normally Dutch spoken)

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