(grunting softly and thoughtfully)
(shuffling)
(grunting in satisfaction)
(intrepid music)
- Hello and as always I would like to welcome you
to the Laramy-K OpticianWorks training center,
where today we are going to talk about
the nominal lens formula.
Much like the optical cross,
we can take the nominal lens formula
that you just saw in the opening sequence, and draw it.
So that's what we're gonna do.
We're gonna take the nominal lens formula,
and we're gonna take each of its individual parts,
and we're gonna draw them
so that we can see the relationship of the front curves
front curve
and the back curve or curves necessary to create lens power.
Because of the way lenses are designed today,
particularly freeform,
the nominal lens formula is just a concept,
or theory, or an idea,
but it is a really, really important, crucial concept
or idea for you to have,
so let's hit the whiteboard and start digging into it.
Can't remember where the line comes from,
but somewhere they were saying like the beginning
the beginning is a good place to start.
Is it like Mary Poppins or something, I think?
We're gonna do first things first.
If you have not already done so,
by all means you need to have watched
the very first video that I ever did,
called The Optical Cross.
We get very up close and personal with this particular lens.
It is a minus five, minus two,
in fact there's the package that it came out of.
This is actually the lens.
It is actually a minus five, minus two,
and really briefly in that video
I talk about the fact
that that lens will have a curve on the backside
that will create the minus five power.
If I did flat transposition,
the other point on that lens is gonna be a minus seven,
and there's a curve on the backside that creates that,
but you need to watch that video
and then come back here and pick it up from there.
The nominal lens formula is this,
and it tells us that the power of a lens, DL,
is equal to the front base curve,
the curve that is on the front of the lens,
added to the back curve,
in the case of a spherical lens,
or the back curve combined with a second curve
for something like this, which is a spherocylinder lens,
used to correct for astigmatism.
It's gonna have two curves on the back.
Everything we're gonna be doing here
is working off of the nominal lens formula.
We can draw the nominal lens formula like a lens:
we can make this the front, and we can make this the back.
That is known as toric transposition,
however I tossed that word around a little bit
on social media lately,
and nobody really seems to know what it means anymore,
so we're just gonna say
we're working with the nominal lens formula.
We're gonna dissect this.
We're gonna solve for that when we have that,
that when we have that and that,
that when we have that and that (laughs),
okay, so that's where we're headed with this:
we're gonna have four installments.
If my front base curve it is always going to be a plus,
it'll be either plano for a super-high minus,
or going up, two five, 50, 75, one,
and the plus, it's gonna have that convex shape,
and then, depending on the powers required
for the lens that has been ordered,
I'm going to have to grind some curves on the back
to create them.
The more shallow curve on the back side of a lens
is called the toric curve,
and you definitely wanna know all this,
so pay attention here,
and that creates the spherical portion of the lens design.
The steeper, stronger, more powerful curve
is called the cross curve,
and that creates the portion that creates the cylinder,
and the astigmatism correction.
Those will always be minus
in relationship to the plus front base curve.
(whooshing)
Before we can do any drawings
with the front curves and back curves,
we need to determine
which curve we are going to put on the front.
That is the base curve in the concept, or theory,
the base curve,
which in modern lens design is a positive curve,
is the curve from with all other curves are measured.
We must know our front curve
before we can decide which curves we need to put on the back
to get the powers that we need.
We can do that in a couple of ways.
These again are concepts.
We can use Vogel's Rule, which is what we're gonna do,
that says that you always want to get the base curve
as close to plus six as possible.
Again, back in the day, glass lenses,
simple, two-axis surfacing, that was a great rule.
You can also use Tscherning's ellipse.
That's how you spell that.
That's a fellow's name, I believe.
And of course there are base curve charts as well.
Conceptual, certainly going to want to know this.
I've got a lot of this stuff on the OpticianWorks website,
so if you're taking any exams
I would certainly know this stuff.
There is Vogel's Rule.
Notice that it is different for plus and minus powers.
So let's go through it.
For the prescription of minus five, minus two,
that first one that we have,
to reach the base curve we take our spherical equivalent,
which is our minus five, plus or minus one,
gives us minus six.
We take half of the spherical equivalent,
which is minus three, and we add six to it.
We end up with a base curve of plus three.
That is gonna be the number on top
that we start working with in the next video.
Still minus, spherical equivalent,
minus one, half of that is minus 50,
add it to that, gives us minus 2.25.
Because it's a minus prescription,
we take half of that value, minus 1.125,
and we add six to it.
That gives us a base curve of plus 4.87.
For simplicity's sake, since this is all conceptual,
we're gonna round that up to five
so that the math stays easy.
For a plus prescription, we take the spherical equivalent,
which is our plus 0.875,
we do not take half of that amount,
we simply add it to our six
for a base curve of plus 6.87, or seven.
So I will see you next week.
We'll pick up right where we left off.
We'll take our base curves.
We'll put it on our conceptual drawing,
and figure out what curves we need to put on the back
to work backwards and reach that minus five, minus two,
minus 1.75, minus one,
and our plus 1.50, minus 1.25.
(whooshing)
Thank you so much for watching.
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Thanks.