- Are you ready for Prentice's Formula Part Three?
(mysterious music)
(gentle music)
As the saying goes, the show must go on.
I am, in fact, not wearing half of my breakfast,
I had a dermatology appointment on Monday.
Next time I promise my face will look at least
a little bit better than it does right now.
What's important here is that we wrap up Prentice's Formula.
Prentice's Formula Part Three example one.
Before we do that, we have a very brief bit of housekeeping
and consider it a public service announcement.
Remember, always wear your sunscreen.
(clears throat) Onward.
Very typical prescription, minus 375, minus 275
at 66.
Minus 425, minus 325 at 137.
We wanted a PD, person's pretty big of 34/35.
For some reason, nobody entered a PD of any kind.
It went to a default and the glasses were made
at a PD of 64 or 32/32.
My PD, by now to you, should be ringing some bells
and whistles and you should be saying, "PD, I need the power
"at 180."
In order to get there, this does not fold neatly
into our 30/45/60 rule or any percent rule for that matter.
If I do flat transposition, it certainly doesn't get me
to 180 degrees, so we're gonna use
the power in oblique meridians formula.
I've drawn it out, or written it out, however you wanna
think about it, and if you followed along on a calculator,
it would come up like this.
I have 180, I need 66, that's the difference of 114 degrees.
The sine of 114 degrees squared is .83
and in your calculator, that'll run out to four more digits.
I multiplied that times my cylinder value
and that gives me a minus 2.29.
That's how much of my cylinder is in play at 114 degrees.
I take that amount and I add it to my sphere value
for a total power of minus 6.05 at 180 degrees.
That's the first thing I need.
Where's my blue marker here?
This one of my Ds here.
Power in oblique meridians formula, I need 137.
I'm sorry, I need 180, I have 137.
That's a difference of 43 degrees.
The sine of 43 degrees squared is 0.46.
0.46 multiplied times my cylinder value
gives me minus 1.51, which I then add to my sphere value,
which gives me a minus 5.76 at 180 degrees.
P is equal to HCM times D, there's my D for my right.
There's the D for my left.
Now I need to find out my HCM, the other part
of the formula.
I wanted 34/35, I got 32/32.
34 minus 32 is two, 35 minus 32 is three.
What are these millimeters, not centimeters.
Remember, Prentice's Formula, HCM.
To convert my millimeters to centimeters,
I divide it by 10, so I end up with .2 and .3 centimeters.
Distance the lens moved.
If I take .2 and my .3 and I multiplied it times
my total power at 180, I end up at 1.21 and 1.72.
Rounded, prism we only go to one decimal place.
We've got a 1.2 and a 1.7.
This, as you know by now, tells us nothing.
Now I need to go ahead and draw this out
to find out whether or not these two amounts cancel
or compound, and then figure out what that person's
actually experiencing in their brain when they put
this pair of glasses on.
Looks like a pretty good place to start.
What do we have so far?
We have our power at 180, our power at 180,
we have the error, the error, and we have our amount
of prism error created by the movement in this meridian
in this power.
Now let's figure out if this compounds or if this cancels.
I wanted 34 and they gave us 32,
so I wanted my OC here, but it ended up here.
Let's give ourselves a reference point there.
I wanted 35 and I got 32, so I wanted it here,
but I got it here.
So let's give ourselves a reference point there.
Now let's go ahead and draw this out.
Minus power.
Looking through base out, looking through base out.
You can see that we're not concerned with this,
we're concerned with what prismatic area
they're actually looking through.
Base out, base out compounds.
I take my 1.2 and I add it together with my 1.7
for a total of 2.9.
Not done.
The final answer, the one that you would get correct
when you check the box on the test, for this would be
2.9 diopters of prism, base out in your right eye.
2.9 prism diopters, base out in the left.
This is your total answer to everything
that we just worked through.
Two lessons to take away from this.
One is, it takes a little time and effort
to reach this point.
It's a multi step process, which I hope you're starting
to learn and two would be always wear a hat and sunscreen
so you don't look like this when you're my age.
Let's do example number two.
Example number two, pretty basic one.
Gentleman comes into the store with
this rather high prescription, asks to have
his glasses adjusted.
New optician running the show, they get
a little carried away and they take the guy's glasses
and they grab the newest pads and they crank them
way, way in so what happens is instead of that frame
sitting here, all of a sudden it's sitting way up here.
They've shoved the glasses up, the OCs
instead of being right there in front of their pupil
are now raised up above them.
We have an adjustment error.
The pads are squeezed too close together.
They've been raised up away from the center of the pupil,
so we have an error in the 90th.
Our prescriptions already written for the power at 90
so we don't any powers in oblique meridians formula
or conversion or anything there.
What we're trying to really figure out is
just how much of an error was created.
Would the person even notice?
Would they stumble out the door?
Would they fall off the curb?
Would they just put them on and say, "hey, that's actually
"the best they've been" and wander away?
Who knows?
Let's find out how much of an error was actually created.
How much were the glasses pushed up?
Six millimeters.
Prentice's Formula calls for centimeters,
so we divide our six by 10, which gives us 0.6, 0.6.
Multiplied times our power at 90, and our power at 90
gives us a total prism error of 3,6 and 3,9
for each of the individual lenses.
By now you know that that tells us nothing.
It is only after we do the next step
that we can determine the actual total amount
of prism error created.
Let's just go ahead and do that next.
Give me a minute.
Alright, let's finish this up.
We've got a plus six in our right with an error of 3.6.
We've got a plus 650 in our left with an error of 3.9.
Where did the error come from?
We wanted our OC here, it's now here.
We wanted our OC here, it now up here.
What do I have, I have a plus lens and a plus lens.
I'm drawing it in the 90th.
Where's the person looking?
Base up, base up.
My chart tells me that base up, base up
cancels itself out.
It reduces the overall effect.
I can take my 3.9 and I can subtract my 3.6
for a total prismatic error in this pair, .3
and I would write that out for a total answer
of this person experiencing .3 prism diopters,
base up in my strongest lens, which would be my left.
This be the total error, the total answer
for what happened when that optician squeezed
those nose pads together and pushed it up.
We'd be probably right on the edge of tolerance.
We'll get to that in a couple lessons from now.
(mysterious music)
- [Announcer] On January 22nd, 2017,
the International Olympic Committee received
a formal complaint of unsportsmanlike like behavior.
The complaint states on or about August 13th of 2016
during an open training session,
an unidentified North Korean archer did enter
the team locker room, break into a locker,
and switched a complete set of eyeglasses for another.
The supreme leader and coach of all North Korean
Olympic athletes, Kim Jong-Un, vehemently denies
the claim and dismisses it as quote, western propaganda.
The Canadian archer, name withheld, to whom
the eyeglasses belonged was one round away
from a gold medal in 300 meter competition.
Under further investigation and a careful review
of video surveillance footage of the locker room,
it was determined that such a switch did, in fact,
take place.
A search of the North Korean's dormitory was conducted
and the archer's original pair was found.
The two pairs of eyeglasses compare like this.
- Are you ready to solve the case of the meddling archer?
Good.
Let's figure out exactly how we would present our findings
to the Olympic Committee.
The glasses that were discovered in the locker
had an OC error.
Instead of being level, one was up, one was down.
That would give us an error in the 90th.
Our prescription, as required in both pairs, was 77 and 79,
but I need the power at 90.
In order to determine that, I'm gonna have to use
the powers in oblique meridians formula.
The sine of 13 squared is 0.5.
The since of 11 squared is 0.4.
0.04, excuse me.
Multiply times my cylinder value.
Added to my sphere and I have my total power, my Ds
for my power at 90.
My Canadian archer required a PD, I'm sorry, an OC
of 21/21.
That's where they saw perfect, that's where they had
no prismatic error, that's where the OCs would've been
directly in front of their eye.
The pair that he was found to actually be wearing
had one OC height at 18 and one at 24, a difference
of three millimeters in each direction.
My millimeters converted to centimeters,
because that's how Prentice's Formula is designed,
is three divided by 10, which gives me .3.
My .3 times my total power at 90 gives me
a total prismatic error of 1.0 and 1.2 for each eye.
As you know now, that tells us nothing.
What we're gonna do is we're gonna take these two numbers
and we're gonna finish drawing this out
and see what it comes up with when it either compounds
or cancels and find out if there was enough error there
to really throw that archer off.
Let's solve the problem.
Let's wrap this case up.
At 90 degrees in the right eye, we have a plus 345.
In the left we have plus 3.95.
Kinda makes sense, far sighted person would be
a good archer.
There a super lens, there a super lens.
The OC here is three millimeters low.
I wanted it here, the glasses that were found,
it's down here, 18 instead of 21.
In this pair, it's three high.
The OC is up here, I wanted it here.
Plus lenses, let's draw a couple of plus lenses
in that profile, in the 90th.
And we have got base down here.
And we have got base up here.
One we have, base down and base up.
Those two compound or add together.
So I have got 1.0 plus 1.2 for a total prismatic error
of 2.2.
The total answer of what this person would've experienced,
why they ended up shooting somebody in the stands
instead of the target, because of those glasses
being switched.
2.2 diopters, (sings)
base down in the right.
2.2 diopters, base up in the left.
Yes, this person has got some serious problems
and tolerance, way off the charts.
We're talking six millimeters where you have a tolerance
of about one, so not a good thing and yes,
I think the North Korean archers tried to pull
a fast one on this guy and I think he caught it
and I think that he should be given his medal back
or something.
Alright, that wraps up our three part series
of Prentice's Formula.
We're gonna take a little side step,
we're gonna start doing some Learn the Lens meter videos.
We're also working on some edging videos.
Let's learn to edge lenses in house.
It's gonna be a couple of weeks, may actually be
a month or more before we get back to Prentice's Formula.
When we start working it in reverse,
where we start deciding, and it's kind of more important,
how far we would need to move a lens in order to create
prism that was desired in a prescription.
So until then, have a great time and I will see you
next week with another installment.
Thanks.