- Hello, I know you're not gonna believe it,
but yes, this is actually the final installment
in this preliminary series on prism.
Next week, I promise we will actually
start doing Prentis's Formula.
The first installment of this piece
is gonna be talking about the OC versus the MRP.
But what I want you to take away most
from the next couple minutes,
which is all this segment is,
is to always remember that eyes don't move.
I cannot take my little, narrow, beady, 58 PD
and make it a 64.
I can't move one eye up here and one eye down here.
My eyes, your eyes, your customer's eyes are fixed.
That tells you that lenses move around
in front of the eye.
Put this heart when you're doing your drawings.
If you remember this and draw this,
you'll get your questions right.
You'll even be able to answer stuff
you're not really sure about
if you draw it out that way.
If you try to move your eye behind the lens,
you're gonna get stuff wrong,
so put that in the back of your mind.
Let's say I have my customer's eye,
and their prescription's a minus 8.50 OU.
A ton of power there.
Lot of power.
If I move stuff around,
it's gonna get ugly pretty quick.
And for those of you who have not met our friend yet,
my little friend.
This is Super Lens, and Super Lens goes with Super Eye.
And let's say I've got my minus 8.50 sphere.
That's it, that's how the script was written.
Here's my 8.50 lens.
Take it to the lens meter, dot it up,
I've got my three dots.
Here's my optical center.
I fill the prescription right,
I do my finishing work right,
and my optical center ends up right there
in front of the customer's pupil, visual axes.
Everything's good, you fill the script spot-on,
everybody is happy.
Optical center.
If the person came in with a script of 8.50 sphere
with one diopter base up, base down,
whatever it might be.
We're gonna get to that next, so don't
get yourself wound up quite yet.
And I deliberately move my optical center here,
here, here, or here,
base up, base down, base in, base out, whatever.
Then, this point becomes my MRP,
or my major reference point.
Those are the two terms you're gonna need to know.
You may already be familiar with them.
There are a couple other pretty good examples of this
with drawings in both plus or minus
on the Optician Works website.
Just wanted to set those
in the back of your mind as well.
They are pretty important,
and you will see them pop up in other places.
Next we're gonna talk about where that
direction comes from.
The way I have this drawn out is
to help you understand basic 180, 90 errors,
which are the kinds of things you're gonna see
on your ABO test,
any kind of practical exams.
Basic prism theory.
There are gonna be some people
that have their panties in a bunch.
Just hold on, we'll get to quadrants,
and we'll get to resultant
four or five lessons for now.
Not right here, not right this minute.
We're gonna work with four basic shapes.
We're gonna have our lenses drawn
for the 180 horizontal,
we're gonna have our lenses drawn
for the 90th, or the vertical,
and we're gonna think in very simple terms of right/left.
The base in, base up, base down
is a relationship to your nose, your schnauz, your beak.
Out, towards the temples.
In, towards the nose.
Up, away from the nose.
Down, away from the nose.
I would ask you to, so many people draw it this way,
everybody thinks about it this way,
everybody says, "Oh, it's in relationship to your nose."
For what we're doing, I would far rather
have you think about it in relationship
to the center of the pupil.
If I take my plus lens,
and I draw it out, my eye doesn't move,
and I put a plus lens here,
in relationship to the perfect center of that pupil,
the visual axis, I can look and see that my base
is out from that point,
therefore I have base out prism in this particular eye.
So, I would strongly urge you
to think about it in that way,
not so much to the nose.
Even when we get to quadrants,
I still think it's a little bit
better way to think about it.
Nose is kind of a big picture concept.
This will help you, I think,
visualize a little bit more
the subtler nuances, shall we say,
of prism and direction.
Where we're headed with the quadrants and the resultants
is it's not always this simple.
I mean, usually prism doesn't just happen to
fall along 90 or 180.
It can be anywhere, and it doesn't have to be
just one of these.
It can be up and out, it can be down and out,
it can be down and in,
and I can have a certain amount for this way
and a certain amount for this way.
When I combine those, then I have
what we would call a resultant prism,
and that's down the line a little bit.
That's the really important stuff, actually.
Two more terms to go.
Then we can wrap up Prism Part Two.
One of them is compound, the other is cancel.
We have two eyes.
When there's a prescription involved,
prism, lenses, power, our brain,
it's all about what the two eyes
are seeing together.
It's not about one eye.
So, the amount of prism, wanted, unwanted,
whatever it might be,
is always combined.
We always have to figure out what the combined effect
of the power in both eyes
is actually resulting in
what we are seeing in our brain.
So, we have two other ideas.
Resultant is down the road.
We have compound and we have canceling.
If we look at this particular set of eyes,
here's my pupil, my visual axis here.
I put these plus lenses in front of the eye,
they're decentered incorrectly,
and I have base in here,
I have base in here.
If I'm looking through base in prism here,
remember that our object is shifted
towards the apex.
If I'm viewing my hydrant, our old friend here,
the hydrant's gonna appear over here.
This eye, because the base is in,
apex is out here,
my hydrant's gonna appear over here.
It's splitting, it's making things worse.
One eyes is pushing this way,
one eye is pushing this way.
Base in, base in prism compounds the problem.
You add the amounts together.
That's where that concept comes from.
If I switch my lens from base in to base out,
now my object is gonna be shifted towards
the apex of this, I'm looking at my fire hydrant,
and it's going to appear to shift
more towards this.
So, these two are working together,
pushing things together in the same direction.
So, the powers of the two prisms combined
would cancel each other out.
The effect of the two prism directions is reduced.
So you would lower the overall effect
as perceived in the brain.
That is where compound cancel comes from,
and we'll do some more of those.
There's a chart I'll show you.
I'll be putting it up so you can work with it
as we go along.
Okay, Prism Part Two is now complete.
Next week, we finally start working
on our Prentis's Formula problems
for errors in the 180 and the 90.
Guess I better get to work on next week.
See you then.