- Hello and welcome back to our ongoing series.
Let's learn how to use the Lensmeter.
Today is kit number four.
Kit number four is a complete pair
of mounted lined bifocals.
We're going to use the Lensmeter to read both the distance
and the near portion of those lenses.
(upbeat music)
Really brief side note for today,
Lensmeters are notoriously ignored.
Most places buy the Lensmeter when they open
and the thing is never touched, calibrated,
or serviced from that day forward.
Chances are excellent that where you work,
you're gonna end up working with a Lensmeter
that's 20, 25, 30 years old.
If it is time for recalibration or refurbishment,
there are places online or drop me an email
and I'll hook you up with somebody.
Kit number four is another mounted pair.
May as well go and find number four, double check
your etched mark to make sure you have the right frame,
right lenses in the right bag.
And you will notice when you look at it
that it is a lined bifocal.
We're gonna build on what we've already been doing
and we're gonna add in a couple of new steps.
One, we're going to take a near PD measurement
or verification.
We're going to check our segment height
and we're going to learn how to use the Lensmeter
in order to read the bifocal power.
Which is kinda weird so, hang on we'll get there.
In the mean time, we're not gonna do anything different.
Just what we've been doing.
Our right is supposed to be a minus 2.75,
minus one 1.25 at 68.
Could turn my Lensmeter on, I could turn my power drum
to minus 2.75
and I could turn my axis wheel to 68.
Right, right, right.
And what I want you to pay attention to up against
the lens stop is that the top of the line is below
the opening of the lens stop.
If you don't do that and you catch the line anywhere
above that, you're gonna get a little bit
of distortion that you don't want.
So, just double check that and then you can lower
your lens holder and just as always when you look inside,
you should have good, crisp sphere lines.
And they should be closed.
Use your spectacle table.
Get this lined up all nice.
Move your chrome knurled sleeve around.
Rotate your power drum away from you til you have
good, crisp cylinder lines.
Work everything so everything's lined up.
You are at 2.75, you move to 1.25 you should be
at the power drum at minus four.
And that lens is looking good.
We can put our three dots on there.
Without moving the spectacle tables, we'll go from
our right to our left.
And this time it says I've got a plus two,
at 111.
And let's see what we got there, we've go some nice crisp
sphere lines, those look good.
And then I can move my chrome knurled sleeve around
a little bit, help me get everything lined up.
And then it just says I got a little tiny bit of seal
just at minus 50, so I'm gonna rotate that drum away
from me just a little tiny bit.
And sure enough, I've got cylinder lines at plus 1.50.
It's good.
And I can mark that lens up.
As I mentioned, there'd be two additional
things we're going to do.
First, obviously we're going to the distance between
our two center white dots and that gives us
our distance PD.
And I have got a beautiful, perfect
64 there.
Then I'm going to measure my near PD.
When your PD is that second number,
my distance is supposed to be 64.
My near is supposed to be 60.
And I do that by measuring
from inside edge
to outside edge
of my segment,
as you seen shown on the screen there.
And it is supposed to be a 60
and I've got about 60 and a half.
So that's certainly well within tolerance
and what you would expect.
So we're good there.
And last thing I would need to check is my height,
which is supposed to be 12 in each eye.
And heights are measured from the deepest
point in the frame.
And I have got about 11 and a half.
And about 11 and a half,
which is perfect.
No problems there.
Now, we're going to make sure that our bifocal power
is the correct one and this is where things
get a little bit weird like I said.
Why do I say that?
I say that because bifocal segment areas, the add power
on a lens is often read with the frame turned away from you,
as if you were wearing it with the segment portion
against the lens stop.
Let's just talk about that for a few minutes alright?
If you go online and you look into this a little bit.
You're gonna see there's five, six different ways
of doing what I'm about to show you.
Just find one that works for you.
Another thing you're gonna find online, there is general
consensus that it doesn't really matter if you turn
the pair around, until you get up into higher powers.
The higher the distance portion of the script,
the higher the strength the add power is,
the more important that it becomes that you turn it around.
Another piece of this is that if you go for a practical exam
and it doesn't matter what the lens power is,
if you don't turn that pair around when you're checking
your bifocal segment, you're gonna fail.
Those exam proctors are looking for that.
Practice it.
Get used to that turning it around to read the segment.
Experiment, find one method that gives you the consistent
results that you need.
And just do it that way.
Right lens, right lens, right lens easy enough.
I'm going to start with my distance portion
in the Lensmeter against the lens stop,
just as if I had the glasses facing the other way.
I am going to rotate my power drum around until I have good
sphere lines
in focus, in minus cylinder form,
should just about the prescription
that I was, but it may very well be an eighth
or a quarter difference.
So just get the sphere lines, and the axis doesn't matter
it's going to move.
So, just spin your axis wheel until you get sphere lines,
it doesn't matter what number it's at.
Alright I've got those.
Now, I'm gonna take my cheaters
and I'm gonna read my power drum
and I have got a minus three.
And I'm actually gonna write that down.
Now I'm gonna take my right lens
and I'm gonna move it up into my segment
and I'm gonna rotate that power drum in the plus direction
until I have sphere lines in focus.
Now, what you're gonna see in here is that the cross hairs
where the cylinders and spheres cross over each other
is way, way down at the bottom.
That's normal, that's expected.
It's down around three.
Just leave things alone.
Move that power drum until you have good, beautiful,
crisp sphere lines and you're good.
Let's see where we ended up.
We ended up at minus 1.50.
So that is a plus 1.50 difference and sure enough my add
power in my right is supposed to be a plus 1.50.
Great.
So far so good.
Let's do that now with the left.
Here's my left, my left in the distance.
Rotate my power drum as much as I need to.
Rotate my axis wheel as much as I need to,
make sure I'm minus cylinder form.
I am at plus 1.75.
Now I'm gonna bring myself up into my segment
and I'm gonna rotate that drum.
This time I'm up around the two ring
and I am rotating.
I need to tweek my axis wheel a little bit,
til I get good sphere lines.
That looks pretty darn good right there.
Set those to the side and I am now at plus 3.25
just difference of 1.50.
I am good.
My add power is a 1.50 in both eyes.
The other and really far simpler way of checking
a standard lined bifocal power is using your good
old fashioned lens clock.
And what you wanna do is my right lens, right lens
is clock your front base curve, then you put your pin
right on the white dot.
Hold it, read it and I've got a beautiful plus six.
Then bring your clock down onto your segment,
aim right for the middle of it.
Read the lens clock and I've got exactly a plus 7.50.
Low and behold, I've got my plus 1.50.
Gonna do the same for my left lens.
I'm gonna put my center pin right on that white dot.
I have got a plus seven there.
And if I clock my segment right up near the line,
I'm probably about millimeter below the line there.
I have got 8.50 a difference of 1.50.
My add in these lenses is a 1.50.
By all means if they have a lens clock
during your practical.
Just to be sure you might want to do both steps.
If you're checking jobs in,
again guys bifocals,
line bifocals are just becoming kind of dinosaurs
you may or may not even see them anymore in your lifetime.
But you do need to kinda know this stuff.
When they do come along, just practice.
Practice, practice.
Again, it's not gonna be perfect.
This isn't gonna be beautiful, crisp on target stuff.
Get used to the steps involved
and when you get to any practical exam,
you're gonna be fine.
That is lens kit number four, a lined bifocal
and next week we hit progressives.
So, I will see you then.