- Flat appears to be my position
and the subject of today's exhibition.
(Upbeat music)
Hello, I would like to welcome you
to the Laramy-K Optician Works training center.
Where, today, we are going to cover
the steps involved in doing flat transposition.
Flat transposition. The tool used for refraction
is the phoropter, there's a picture of it,
you've seen that before.
And strictly by the practitioner's choice, that tool
can be ordered to read prescriptions, or lens powers,
in either a plus or minus cylinder form.
It's really just a matter of preference these days,
and flat transposition allows us to switch
from a minus cylinder form to a plus cylinder form.
Now, why do you need to do this?
Well, some people are going to write it this way
and some people are going to write it that,
and you need to be consistent so if everything that happens
in your particular store is plus cylinder then you're going
to need to convert all those minus cylinder forms coming in
and, of course, vice versa.
You wouldn't want to call in 300, 400 jobs to the lab
in plus cylinder form then one day give them one
in minus and then try to figure out
why things didn't work out so well.
You do need to know how to do this.
Let's hit the bench for a second
and let me go over where the big picture concept
comes from of where those two are.
The big picture concept, of flat transposition
is very, very much the same as the optical cross.
So, you may recognize this lens from that video
this is a minus five, minus two.
Meaning that it has minus five
and a total power of minus seven.
The minus seven is going to be placed at my thickest point
on the lens that is also going to have
the steepest curve anywhere found on the lens.
My minus five is going to be the thinnest point
on the lens and slightly shallower curve
than that of the minus seven.
This lens will never be more powerful than the minus seven
or less powerful than the minus five.
You can see, you can feel, the difference
in between these two points which will always be
90 degrees away from each other.
Here's my seven it is the thickets point.
I can take my finger quite literally run it down you can
visualize it, feel it, see it, to where my minus five is.
From that minus five I would again start building up
in power and thickness reducing the radius
of curvature till I reached my minus seven.
If I have a minus five minus two
and I write the prescription as a minus five minus two at 90
that would mean that I have my lens oriented like this.
Minus five at 90 minus seven over here
two diopters difference that's
where my cylinder would come from.
So if I started with my minus five at 90
ended up with my seven at 180.
My five to here would be minus cylinder,
minus five, minus six, minus seven.
Simply the exactly same lens, same curves,
everything else; it just depends on which way
I'm going to write those numbers.
If I decided to write it as minus seven at 180,
then I would be adding a plus cylinder lens
going from seven, to six, to five.
Five being less minus or more plus than minus seven is.
That's kind of where the big picture concept comes from
same lens, same powers, same materials, same everything;
it just depends on where you choose
to start writing where your powers are oriented.
If you haven't already watched that video
on the optical cross I would strongly urge you to do so.
It was the very first video that I ever made
and, for some reason, I seem to be wearing the same shirt.
Now, to switch forms that's the rule,
I'm going to leave that up there for just a second,
you can hit pause if you want
to write it down or work with it.
And we're gonna through three examples
now, I think I have 15 practice examples
on the website that are scored under a practice test.
I think it's in the eye and lens design
but don't hold me to that.
But you've got tons of this, practice, practice,
practice, you've got to get this down.
You can't make mistakes when you're doing this
you're going to create a set of garbage lenses,
have a very unhappy customer, and a very unhappy employer.
To convert a minus cylinder form to a plus cylinder form,
we're gonna follow the rules.
We're gonna take our sphere and add it to our cylinder.
One point seven five, using a calculator
with a plus/minus sign, plus point 75 minus.
Gives me a minus two 50.
This plus this, is this, that is my new sphere power.
Now, the next step, according to the rule
is to take your cylinder amount
leave it the value exactly the same
and change the sign from minus to plus.
Now, we need to change our axis by 90 degrees.
That distance that that travels
the lens from the highest point to the lowest point.
Most power, least power within the lens
that I just showed you.
And in this case I can take my 88 and I can add 90
and I end up at 178 for new axis.
178 does fall between zero and 180 degrees
so we're good to go.
My minus cylinder form written as
plus cylinder form is a minus two 50 plus 75 at 178.
If I take this lens order written in minus cylinder form
and I want to make it plus I'm going to follow the rules.
I'm going to take my minus two 25 and I'm going
to add one point two five and I end up at minus three 50.
This is my new sphere power.
Take my cylinder amount, keep it the same,
change my minus to a plus.
This is my new cylinder sign.
To keep my axis, going from 70, I have to add 90 degrees.
70 plus 90, it's a new one, axis of 160
this is between zero and 180 degrees, I'm good to go.
My rewritten, it's not new, it's just written
in a different way. Minus two 25, minus one 25
at 70 becomes minus three 50 plus one 25 at 160 degrees.
To go from a plus two minus one to the same exact lens order
written in plus cylinder form.
I'm going to take my two, I'm going to add minus one
and I end up at plus one.
This is my new sphere.
I take my cylinder value, keep it exactly the same
and change my minus to a plus.
Now here's where things get a little bit tricky.
If I go 110 plus 90, I end up at 200.
200 does not fall between zero and 180 degrees.
That's a big problem because we write
our lens orders in those amounts zero, 180, right?
So instead of adding, I need to subtract 110 minus 90
keeps me within zero/180, brings me to 20 degrees
that is my new axis, this rewritten in plus cylinder form
becomes a plus one plus one at axis 20.
Practice, practice, practice. Ugh... and use a calculator.
And one last one because I know somebody out there
is gonna say "hey those were all minus cylinder
what happens if you have plus cylinder
and I have to go the other way?"
Well, nothing changes; it is the exact same rule,
the exact same process.
If I had a minus three 50 plus one 75 at 30.
I would do the exact same thing.
I would take my calculator and I would take
my minus three 50 and add one 75.
I end up at minus one 75, that is my new sphere value.
My plus one 75 becomes minus one 75, my new cylinder sign.
My 30, I need to move my axis by 90 degrees,
I end up at 120 degrees for my new axis or new position.
In other words, where I am writing these lens powers from.
My minus three 50 plus one 75 at 30
becomes minus one 75 minus one 75 at 120
and of course, you can work that
backwards, forwards, double check yourself,
make sure everything makes sense and you are good to go.
Plenty of other examples and practice
on the Optician Works website that is flat transposition.
Thank you so much for watching
I do hope you found that helpful.
If you did, please hit the like button,
please share this with others, leave me a comment,
and by all means make sure that every uncut lens
in your life comes from Laramy-K.
I'll see you again next week.