- Welcome, glad you are here.
Today we start Prentice's Formula.
(upbeat soft rock)
Okay.
No singing this morning, no dancing, I'm afraid.
In fact I'm going to put on my Mr. Meanie beanie
for a moment.
There are six videos before Prentice's Formula.
If you are new to this, if you failed the ABO,
if you don't like prism, all those excuses,
don't think you can just jump in here, no.
Go back, watch the first six videos
that lead up to this one,
and only then come back and join me
for this whiteboard lesson.
Alright, we are finally here.
I know I've been talking about it forever,
but we have finally arrived at Prentice's Formula.
If you don't know who Prentice is,
I'll throw up a couple of websites at the end of this video
so you can check him out.
He's a pretty fascinating fellow.
Prentice's Formula reads:
The prism amount P is equal to hcm times D.
Not exactly a complicated formula,
especially when we break it down.
P is equal to the amount of prism created by an error.
H is the number of centimeters
that our Optical Center has moved.
It's our numeric amount.
cm is simply centimeters.
We convert to millimeters, because that's the measurement
that we use.
And D is the power in the meridian with the error.
The three examples that we're going to work
are going to be very very basic.
We're not going to be using 30, 45, 60,
or the powers in Oblique Meridian's Formula
for the first three.
This doesn't come into play quite so much.
Wait until the next one.
So a typical Prentice's Formula problem
may break down something along very similar lines.
If I had an error in a PD along the zero 180 line
of five millimeters or .5 centimeters,
I would divide it by 10
in order to convert, and I had a power of 9.50,
I would have my .5 times my 9.50 for a total of 4.75,
rounded for a total of 4.8 diopters of error.
As you will see, the upcoming examples,
this number really tells us absolutely nothing
until we combine it with the other lens, the other eye.
But what does that really mean?
What are you looking at?
What are you thinking up?
Well, let's take super eye and super lens
and actually break it down.
Here's super lens, my lens is a plus 9.50,
very very strong lens.
I said I wanted my center lens meter dot,
my optical center right there smack dab
in the middle of my pupil.
Person would receive no prism bearer,
no prism problems, they'd see clearly,
they had great vision,
that's where I muted things.
Unfortunately, the lens got bumped
when it was getting blocked and the OC
ended up not one, not two, not three, not four,
but five millimeters away from where I needed it.
What that person will be experiencing when they're
looking out here in this very very very
far edge of this lens
is going to be this five millimeters
times the power of the lens.
And it's going to be 4.8 diopters of error.
I could of course go over to this side,
up here, down, et cetera, and that's where we're headed.
That's the concept, that's Prentice's Formula
and let's go ahead and work three examples.
This is going to be example one
of working a basic Prentice's Formula problem.
While you were away, drawing this entire thing out,
and this is the most basic, basic problem,
this probably took me about eight minutes or so,
there's something I really want to stress with you,
and that is slow down.
To go from here when it is complex
and it has cylinder and a power
in Oblique Meridian's Formula,
and you have the two eyes, and you're working this
all the way through,
there are multiple places where you can slip up.
There are multiple places where you think,
"I've been working this for five minutes,
"why am I not at my answer?"
Slow down and work the problem all the way through.
In later sessions, I'm not even going to be able
to fit all this on the board,
let alone give you a nice drawing to back it up.
Don't expect these things to just go by in a second.
When I get to the end here and I start talking
about compounding or canceling prism,
there's a chart which you will find
on the Optician Works website in course five.
I would go ahead print that out and have that handy,
so that you can follow along.
Let's look at a very very basic problem.
I have a minus five sphere on my right,
and a minus 6.50 sphere on my left,
because it is a sphere with the powers the same
over the entire surface of the lens,
I don't have to worry about position,
I don't have to worry about 30, 45.
We don't have to do flat transposition.
Everything is right there.
My human, my person, remember I can't move my eyes,
their eyes are 64 millimeters apart.
Their patient PD is 64.
The glasses, however, were made with a PD,
their optical centers, 70 millimeters apart.
They were made wider than my human.
If I take my 70 that they were made
and subtract my 64, which is my human,
I have a difference of six millimeters.
If I divide that by two, because I have two eyes,
I end up at three millimeters.
Prentice's Formula works in centimeters,
so I'm going to convert that by dividing
three by 10, which gives me .3 centimeters.
Now I can start plugging things in.
My D's times my hcm.
.3 times minus five or five would give me 1.5.
.3 times 6.50 gives me 1.95.
These two right here, right now,
tell us absolutely nothing.
I need to draw this out, figure out what's going on
as far as my base direction goes.
Only then can I tell what the overall effect
for this person's, in their brain, is getting.
My human, here's my pupil, 64.
My lens OC is 70.
I have a minus lens in both eyes,
so I'm going to draw my minus lens
along the 180 degree line meridian position,
and I'm going to draw it out with my OC
wider than my pupil in my right,
wider than my pupil in my left.
Looking directly at this,
I don't even need to concern myself with this.
What is this person looking through
base in prism, it's related to the nose.
What is this?
Base in, related to the nose,
they are within this triangle,
in, in, base in, base in.
If you look at your chart,
it's going to tell you that base in, base in
prism direction, base direction,
compounds or adds together.
It makes things worse.
If I take my 1.5 and my 1.95 and I add them together,
3.45 diopters.
Rounding that up I get 3.5.
When I write the result of my two eyes together,
when it compounds, it is written
total power for both eyes with direction.
So, the complete answer for all of this
would run into 3.5 diopters
of base in prism in the right,
3.5 diopters base in prism in the left.
That would be how it would work.
Prentice's Formula example number one.
Let's do another one.
Example number two of Prentice's Formula.
This time we're going to be doing something
with an error in the 90th,
and we're going to be talking about OC heights.
Very typical prescription, we're still sticking
with the sphere power.
No worries about power and position
in this particular case.
We have a plus seven and a plus 6.75.
Power's pretty high, put a pair of glasses on somebody
and I felt that I wanted to do an OC height,
meaning I wanted to move the lens OC up,
directly in front of their eye.
Their eye sat above half the B,
which is where it would normally be placed.
So on my job order form, I wrote an OC height of 20.
Lab ignored it and they made the OC at half the B,
which is just kind of the default,
which happens to be at 16.
My lens OCs are going to be four millimeters below
where I actually wanted them to be.
Four millimeters, four divided by 10 is .4 centimeters,
which is what we're working with
when we're working with Prentice's Formula.
So, you're going to have to account for that
somewhere along the line.
My hcm .4 times my seven gives me 2.8 diopters of error.
.4 times my 6.75 from my left gives me 2.7.
These two numbers right now tell me absolutely nothing.
When I draw this out, I draw my plus lenses
in the 90th meridian.
And it looks like I'm getting base down prism in my right,
base down prism in my left, and just a whole lot of
other way to look at that.
My chart tells me when I have base down, base down,
that those cancel each other out.
They don't make things worse.
So I can take my 2.8 and take away my 2.7
which leaves me with basically nothing, 0.1.
When we have canceling situation,
we do not write it for both eyes,
we write it for the one that is the strongest,
which in this case would be my plus seven.
So the total prismatic effect created by this error
for this wearer would be 0.1 diopters
base down in the right lens.
Seems like nothing, say hey,
I could just probably dispense these
and this is fine.
You requested an OC height of 20 for a reason.
Lot of power here.
Glasses in the past had it, glasses before that had it,
there are notes on the job.
You wanted 20.
The lab made them at 16.
Send them back, have them made right.
Example number two for Prentice's Formula.
Alright, example number three.
In our right we have a minus 7.50 minus 2.25 at 90.
In our left we have a minus six minus 1.75 at 180.
Our Patient PD is 67.
It's a big person, big wide PD,
eyes are set over here someplace,
but sadly our glasses were made with a PD of 60,
meaning the OCs are in here and my eyes are out here.
My error, because it is a PD,
runs along my zero 180, my horizontal meridian.
So I need the power in both eyes at 180.
We're very lucky in this particular case,
because all I have to do is flat transposition
to convert this from 90 to 180 degrees.
Minus 7.50 plus my minus 2.25.
9.75 at 180.
If I take away my 67, my 60,
I have seven millimeters of difference.
Two eyes, seven divided by two means that
my lenses have moved 3.5 millimeters
away from where they need to be.
3.5 divided by 10 converts
my millimeters to my centimeters,
so I've got .35.
Now I can take my .35 and multiply it
times my 9.75 at 180,
and I get 3.4.
I can take my .35 and multiply it
times my six, which gives me 2.1.
When I draw this out, remember that my human is wide,
they're out here, and my OCs are in here.
I wanted it there, it's there.
They're both moved in from where I wanted them to be.
The error created by that has this person
looking through base out prism and base out prism.
Base out base out combines, so I'm taking my 3.4
and I'm adding it to my 2.1, my right and my left,
for a total of 5.5.
When I compound, I write the entire prism amount out
for both eyes.
The total result that this person
putting this pair of glasses on would experience
would be 5.5 prism diopters base out in the right
and 5.5 prism diopters base out in the left as well.
Example number three for basic Prentice's Formula problems.
We may do more like this next time around.
I doubt it, I think we'll probably jump in
to the 30, 45, 60 rule
and powers in Oblique Meridian's Formula,
since it all works off the same idea.
See you next week.