Contact lens powers (spectacle Rx) thoughts and comments

Just reading on google and discovered that a Contact Lens Rx is different to
a Spectacle Rx due to the vertex difference.

Previously I had not noticed that power was different to my spectacle rx,
but this appears to be the compensation for cyl when useing a spherical lens
giving the same result.

I have compared by prescription using an "online calculator" at
http://www.eyedock.com/calcs/cl_calcs.htm and
http://www.thomson-software-solutions.com/

I understand that the actual power lens (soft spherical) required can vary
on various factor such as fit, however on previous prescriptions the lens
required matched exactly that suggested by the eyedock and Optometric Tool
box software. However I was interested to see how the cylinder/sphere etc
affect lens choice. Obviously the end choice will be with consultation with
a optomotrist, but is is nice to know why he is getting to the result!

for example prescription 2002
-5.50-1.00x150
-5.50-0.50x20 Both calculator suggest -5.50 for both eyes. I was
prescribed -5.50 both

next prescription: 2003
-5.50-0.5x150
-5.50 OS? Eyedock calculator both -5.50 / Optometic Toolbox
suggests -5.50/-5.25. Optomotrist kept prescription at -5.50 both

next prescription: Sep04 (1)
-5.50-1.25x155
-5.75-1.00x15 Eyedock calculator suggests -5.75 left, -5.50 Right.
Optometric Toolbox suggests -5.75 both eyes

prescription: Sep04 (2)
-5.50-0.75x150
-6.00-0.50x20 Both calculators suggest -5.50 left, -5.75 right.

Interestly both the last prescriptions were peformed by different
optomotrists on consecative days, so I am suprised by the variance of the
results. Both optomotrists seemed to take a similar time to get to there
prescription. The only notable difference to me what hat the second
optomotrist used a autorefractor (hot air ballon) before the exam, whilst
the first did not. Does this make the second Rx more accurate?

Both of the calculators show a Right -5.50 and Left -5.75 on the second
prescription. However of the first prescription the result are inverse. The
eyedock shows a Right -5.75 and Left -5.50 whilst the Optometric toolbox
shows -5.75 for both eyes.

Is there any specific reason why two seperate eye exams should have two
different Rx, and the use of two different calculators suggest lenses the
opposite way round? Are there different methods for calculating, or is this
just an error in the calculator?

Is there any reason to assume that one Rx is more accurate than the other?
As contact lenses are spherical is the Rx with a lower cyl the better want
to start with?

Based on your experience, and the info given from two eye test what lens
power would you start with (I assume you with then overrefract to confirm
the power). Out of interest I have two -5.50 lens at the moment and have
very similar vision in both eyes, either I cannot distingish a difference or
what I think is the slightly clearer eye changes. If I has to guess, I would
say that both eyes are very slighly shortsighted with my current lenses (i.e
maybe a slight halo on a light)

Am I right in thinking that Right -5.75 would be a good place to start (Both
calculators suggest on second Rx, Optometric tool box suggests on first Rx)
with another -5.75 in left as both calculators suggest -5.75 in the left
based on the first Rx and keep both lenses the same power?

Can the prescription results be averaged to reduce any error? Is so should
this be done after a CL power is calculated using the actual values rather
the rounded to 0.25D (as this will take into account the cyl correction that
is being corrected by a different in sphere). If the results from both
calculators are averaged, we get a result of Left -5.755 and Right -5.5975.
Thus by using a -5.75 in both eyes you are 0.1525D over.

Any thoughts?

cliveP wrote:

> Just reading on google and discovered that a Contact Lens Rx is different to
> a Spectacle Rx due to the vertex difference.
>
> Previously I had not noticed that power was different to my spectacle rx,
> but this appears to be the compensation for cyl when useing a spherical lens
> giving the same result.
>
> I have compared by prescription using an "online calculator" at
> http://www.eyedock.com/calcs/cl_calcs.htm and
> http://www.thomson-software-solutions.com/
>
> I understand that the actual power lens (soft spherical) required can vary
> on various factor such as fit, however on previous prescriptions the lens
> required matched exactly that suggested by the eyedock and Optometric Tool
> box software. However I was interested to see how the cylinder/sphere etc
> affect lens choice. Obviously the end choice will be with consultation with
> a optomotrist, but is is nice to know why he is getting to the result!
>
> for example prescription 2002
> -5.50-1.00x150
> -5.50-0.50x20 Both calculator suggest -5.50 for both eyes. I was
> prescribed -5.50 both
>
> next prescription: 2003
> -5.50-0.5x150
> -5.50 OS? Eyedock calculator both -5.50 / Optometic Toolbox
> suggests -5.50/-5.25. Optomotrist kept prescription at -5.50 both
>
> next prescription: Sep04 (1)
> -5.50-1.25x155
> -5.75-1.00x15 Eyedock calculator suggests -5.75 left, -5.50 Right.
> Optometric Toolbox suggests -5.75 both eyes
>
> prescription: Sep04 (2)
> -5.50-0.75x150
> -6.00-0.50x20 Both calculators suggest -5.50 left, -5.75 right.
>
> Interestly both the last prescriptions were peformed by different
> optomotrists on consecative days, so I am suprised by the variance of the
> results. Both optomotrists seemed to take a similar time to get to there
> prescription. The only notable difference to me what hat the second
> optomotrist used a autorefractor (hot air ballon) before the exam, whilst
> the first did not. Does this make the second Rx more accurate?
>
> Both of the calculators show a Right -5.50 and Left -5.75 on the second
> prescription. However of the first prescription the result are inverse. The
> eyedock shows a Right -5.75 and Left -5.50 whilst the Optometric toolbox
> shows -5.75 for both eyes.
>
> Is there any specific reason why two seperate eye exams should have two
> different Rx, and the use of two different calculators suggest lenses the
> opposite way round? Are there different methods for calculating, or is this
> just an error in the calculator?
>
> Is there any reason to assume that one Rx is more accurate than the other?
> As contact lenses are spherical is the Rx with a lower cyl the better want
> to start with?
>
> Based on your experience, and the info given from two eye test what lens
> power would you start with (I assume you with then overrefract to confirm
> the power). Out of interest I have two -5.50 lens at the moment and have
> very similar vision in both eyes, either I cannot distingish a difference or
> what I think is the slightly clearer eye changes. If I has to guess, I would
> say that both eyes are very slighly shortsighted with my current lenses (i.e
> maybe a slight halo on a light)
>
> Am I right in thinking that Right -5.75 would be a good place to start (Both
> calculators suggest on second Rx, Optometric tool box suggests on first Rx)
> with another -5.75 in left as both calculators suggest -5.75 in the left
> based on the first Rx and keep both lenses the same power?
>
> Can the prescription results be averaged to reduce any error? Is so should
> this be done after a CL power is calculated using the actual values rather
> the rounded to 0.25D (as this will take into account the cyl correction that
> is being corrected by a different in sphere). If the results from both
> calculators are averaged, we get a result of Left -5.755 and Right -5.5975.
> Thus by using a -5.75 in both eyes you are 0.1525D over.
>
> Any thoughts?
>
>

Clive to quote myself from another recent post: the human eye is made of
soft tissue, flesh & blood, not metal. It is flexible and its vision is
variable. It relies on a surface tear film or variable quality and
quantity, and also on interpretation by a brain that itself is "only
human". Therefore,it is normal for the vision not to be exactly the same
all the time. The differences you are discussing are too small to worry
about. Many (most?) people wouldn't notice a difference between -550 and
-575 at all.

The only general advice I would give:
1. In general (as already mentioned), go with lesser cyl correction
rather than more.
2. For contacts, err towards less minus if you are in your 40's and/or
do a lot of reading. Otherwise, it probably doesn't matter too much.
3. Don't worry too much about the detail (which is what you seem to be
doing now) - just go with the optometrist you felt good about.

Dom

> I have compared by prescription using an "online calculator" at
> http://www.eyedock.com/calcs/cl_calcs.htm and
> http://www.thomson-software-solutions.com/
>
> I understand that the actual power lens (soft spherical) required can vary
> on various factor such as fit, however on previous prescriptions the lens
> required matched exactly that suggested by the eyedock and Optometric Tool
> box software.

Just looking at this on the example given. Does lens choice affect the
calculation?

i.e -6.00-0.50x180 and -5.50-1.00x20 given using the Eyedock
calculator -5.75/-5.50. Using the ciba online calculator
(http://164.109.68.185/index.jsp?calc=lens) you get -6.00/-6.00 when using
Ciba Night and Day or Dallies but -5.75/-5.50 with ciba monthlies

Why does the theoretical lens power change with lens choice as the lens is
the same distance from the eye in both instances?

> Just looking at this on the example given. Does lens choice affect the
> calculation?
>
> i.e -6.00-0.50x180 and -5.50-1.00x20 given using the Eyedock
> calculator -5.75/-5.50. Using the ciba online calculator
> (http://164.109.68.185/index.jsp?calc=lens) you get -6.00/-6.00 when using
> Ciba Night and Day or Dallies but -5.75/-5.50 with ciba monthlies
>
> Why does the theoretical lens power change with lens choice as the lens is
> the same distance from the eye in both instances?

I have posted a similar question "Soft contacts and astigmatism"

If you used the "suitability checker" at www.purevisioneurope.com and put in
the prescription you have given above it says "PureVision is not currently
available for your degree of astigmatism, we recommend Bausch & Lomb
SofLens66 Torics". Anything over -0.5D Cyl is shown as not suitable for
Purevision. In comparision to other soft lens this seems to be particuarly
low, esp as the Purevision lens is aspheric.

Having said that the Ciba calculator allows any degree of astigmatism to be
entered, however the litrature states that up to 1.50d of astimatism is
acceptable.

Does the B&L lens have a very low limit for astigmatism correction? Very
often at these low levels a spherical lens would given better correction
than a toric. Is the Ciba lens any better for astigmatism correction when
using a sperical?

> i.e -6.00-0.50x180 and -5.50-1.00x20 given using the Eyedock
> calculator -5.75/-5.50. Using the ciba online calculator
> (http://164.109.68.185/index.jsp?calc=lens) you get -6.00/-6.00 when using
> Ciba Night and Day or Dallies but -5.75/-5.50 with ciba monthlies
>
> Why does the theoretical lens power change with lens choice as the lens is
> the same distance from the eye in both instances?


Might have found the reason:
(http://www.siliconehydrogels.org/edi...s_kathryn5.asp)

Other Optical Considerations with Silicone Hydrogel Lens Wear
There have been a number of anecdotal reports of patients requiring higher
powers with Focus Night & DayT SH lenses than conventional lenses. The
apparent requirement for additional power is related to the aspheric design
of the Focus Night & DayT lenses and a resultant relative decrease in
spherical aberration compared to other spherical lens designs. This is most
noticeable in the higher minus or plus designs. For example if a patient is
wearing a -9.00 D spherical lens as their current lens, that lens is -9.00
in the centre but will have several dioptres of extra minus spherical
aberration across the optical zone. This effectively makes the average power
across the optic zone somewhat higher than -9.00 D. When the aspheric design
Focus Night & DayT lens -9.00 D is placed on the eye, there is less
spherical aberration and thus the average power is somewhat lower than its
spherical counterpoint (but in actual fact closer to labeled power). There
have been reports of -0.50 to -1.00 D "extra" power being required in some
patients.