- Welcome back to our ongoing series,
let's learn how to use the lensmeter.
Today is kit number six.
When we answer the age old question,
is it vertical imbalance, or is it prism?
(upbeat triumphant music)
Go ahead and dig around in your kit
until you find bag number six.
And when you do take out your frame,
it is a mounted pair of single vision plus lenses,
and make sure you've got the etch number six on there
to be sure that you've got the right frame in the right bag.
Let's start just as we have been doing
with our job order form turned to lensmeter kit six.
And we're gonna go ahead and look at our right lens.
Right lens, and as always I'm gonna aim for the middle.
I can turn my lensmeter on, I can rotate my power drum
to plus
2.50.
I can rotate my axis wheel around
to one hundred and sixty five.
Really important on this one,
it's always important but I do want you to double check
that your PCD or prism compensation device,
is at zero and at ninety.
Everything is well lined up.
Then I can start doing my lensmeter waltz here
and get my
sphere lines
in perfect focus.
My chrome knurled sleeve, get everything lined up.
Can rotate my power drum away from me.
75
or .75.
We'll end up at 1.75.
I've got good cylinder lines.
Okay, everything looks wonderful there.
Go ahead and mark that one.
And before we move to the left,
you're gonna recall that I've been saying all along
and we don't move the spectacle table.
Here is why.
I've been tossing the word verification
around for six lessons.
What is it exactly that we're verifying?
Well, one of the things we're verifying
is that the person who made the glasses
quite literally laid the lenses out
so that they could be cut to fit an individual frame,
laid the mount in such a way that lens OC,
or in the case of a progressive of fitting cross,
aligns properly with the person who's wearing the glasses.
In other words if my person, they're wearing them,
their eye is here...
That my OC
ends up
in front of their pupil or in front of their visual axis.
And they do that with using the bossing system
and the grid and the blocker.
That part is not really what we're worried about here.
In a single vision lens,
unless an OC height is specified on the job order form,
all layout work assumes that you're going to place
the lens OC
at half of the B.
This way we're going to decenter
for the patient's pupillary distance, or PD.
But unless an OC height is specified,
finishing work blocking is always done at half the B.
So you can see why it's important
that if my right lens OC is supposed to be half my B,
that my left lens OC also be placed at half the B.
That's why we don't move the spectacle table
when we move from our right to our left lens.
If...
One of them was blocked here,
and due to an error,
one of them was blocked here, or here,
that is vertical imbalance.
And that's what I'm gonna show you in just a minute
when we go back to the lensmeter.
Vertical imbalance is not a good thing.
Now,
if you have prism requested in a prescription like this,
and your right has none,
and your left has one and a half up,
and you look in the lensmeter when you move to your left,
and you've got your OC displaced one and a half up
in those reticle rings...
Hey, great!
Then you're spot on, you got what you need.
You got the amount of prism that there is called for.
But 99% of the time,
you don't want that.
You want the OC's to be on the same plane.
In a progressive lens,
you're going to use your fitting cross as a reference point.
And for line bifocals, if you have a segment in here,
the OC is generally ground behind it
and it is accounted for for distance PD
and an OC height if it's called for.
If it's not, then they do it usually five millimeters
above the segment height.
So that is what we're verifying,
that's where the vertical imbalance concept comes from.
That's why we do not move the spectacle table
between our right or left lenses.
Let's head back to the lensmeter
and take a look at what that looks like.
Let's go ahead and move from our right to our left.
Obviously without moving the spectacle table.
And we can set our power drum
to 50.
My axis wheel
153...
And I'm centered this way.
Go ahead and take a look inside the lensmeter.
What you see should be very close
to what you see on screen.
And that is vertical imbalance.
And that is what I want you to get out of kit number six.
I want you to be able to recognize
a problem when you see it.
This is not about step by step verification.
This is not about parentheses formula.
This is not about applying standards.
This is just about learning how to use the lensmeter.
What would you do
if you're faced with this?
The answer might surprise you.
You would send the job back to the lab.
The glasses were made wrong.
You're gonna have all kinds of questions on tests
about parentheses formula, and standards,
and applying standards, and verification.
And they're gonna make a big deal of it
in your practical exams.
There are gonna be 20 questions about it on your ABO.
You know what?
A pair of glasses that is made wrong
get sent back to the lab to be made right.
You're not gonna go around
calculating your powers into bleak meridians formula
for the power at 90, working parentheses formula
to decide whether it cancels or it compounds,
and whether somebody could wear this glasses.
No, they were made wrong.
Single vision, you have vertical imbalance,
send them back to the lab.
Straight top line bifocals, line multifocals,
vertical imbalance, send it back to the lab.
Progressive lenses have vertical imbalance
as part of their design.
So don't be in a big rush to return those.
Again, this isn't the time replaced to get into it
but if you have a lot of vertical imbalance
in a progressive,
raise the lens up to the prism reference point,
and it should be equal in both.
It may be severe and it may be both up up, down down,
whatever it might be...
But, don't reject a progressive lens
just because you have vertical imbalance.
You might wanna check with the lab.
Make sure that everything is okay on their end
and it make sense, before you go sending them back
and delaying especially a progressive resurface job,
which would run into a couple of days.
That's really what I wanted to convey with you
for lensmeter kit six.
Keeping in mind, if your job order form
does not have prism requested and you have it,
the job was made wrong.
If your prism value shows 1.5 up,
and you're verifying this and sure enough you have 1.5 up,
then you're good to go.
Lensmeter kit number six, next time we jump into
single vision lenses again, and we do some layout work
to kinda reinforce what I was just talking about
on the white board.
I will see you next week.
(machine whirring)