- Good morning.
Got an email, woke up this morning, checked my email inbox
and I had an email from one of our members
asking about this particular question.
It never hurts to do another whiteboard example
of working Prentice's formula,
so let's go ahead and work through this one together
to see how we end up at the correct answer.
(powerful music)
This is one of the questions
that's in the ABO review question, test bank.
I'm sure you may be familiar with it.
This gal is not the first person
who has sent me a question to clarify
how you work through this one.
The question is I have a prescription
of plus 10 minus two at 180 in my right.
In my left, I have a plus 6.25
minus two at 90.
Someone was trying to adjust the glasses,
they squeezed the nose pads together
and it lifted the frame up six millimeters.
And this is super important.
Think about this.
If it helps, get a pair of glasses.
If my OC is normally at half my B,
right directly in front of my pupil,
if I squeeze my nose pads together,
for whatever reason that might be,
it picks the frame up,
up,
up.
It takes my OCs from here
and brings them up here.
Up.
The movement is in the 90th meridian there.
It's not this way.
This would be 180.
This would be a pupillary distance error.
It pushes them in the 90th
or the vertical meridian.
So I need to work with the power at 90.
In my left, I've got that.
In my right, I have 180.
Luckily for us, it's simple flat transposition
in order to determine the power 90 degrees away at 90.
If I have my plus 10
and I add my minus two,
I've got plus eight
at 90.
Okay, just flat transposition, nothing more to it.
I go from 180 to 90.
I've got plus eight.
I don't need any of this anymore.
I've got what I need to plug in for my right and my left.
So if we work my right first,
how much did I move,
what is the distance that I moved?
I moved six millimeters.
How much power is it 90 in the meridian that I need,
is eight.
For my left, I already had my power.
Six times eight is 48.
Six times 6.25 is 37.5.
Millimeters, the formula is in centimeters,
I need to divide by 10 to convert that to that.
48 divided by 10.
37.5 divided by 10.
P,
is equal to 4.8
and 3.75.
That is the amount of prism
being created in each eye.
The two eyes work together
and depending on the position of the prisms
that power the minus, the plus,
they either subtract from each other
or they add to each other.
And in this case,
this particular question came with a beautiful drawing
as part of it.
So what is this person looking for?
If my OCs were here,
and we moved them up here,
the person wearing that pair of glasses,
because they are plus,
is looking through base-up, base-up prism.
And my handy chart
tells me it's a canceling effect.
So base-up, base-up, I subtract the two amounts.
So I have got 4.8,
subtract 3.75.
4.80 minus 3.75
gives me a total of one diopter
of prism base-up.
My rules for base-up, base-up
tell me when it's canceling,
I do only the total amount in the eye
with the highest amount.
So my total answer in this case
would be 1.05 diopter base-up prism
in my right,
because that is the stronger of the two.
It's the stronger amount of prism present.
The correct answer for this particular question,
in this series,
is 1.05 prism diopters base-up in the right eye.
That is what the person wearing that pair of glasses
would actually experience under this entire case scenario.
I hope that helps.
By all means, if in doubt,
always, always, always draw it out.
If it helps to have a pair of glasses
sitting next to you in the desk,
putting them on, moving them around, thinking about that.
Visualization is gonna be so key
to getting these right,
particularly when you're trying to take
an exam of some kind.
So there is your correct answer.
That is how I got there.
Hopefully that helps you on that question
and don't ever hesitate to emailing me
if you have a question like this.
Thanks.