studying for ABOC and need help

[LauraH]

This is my first post on Optiboard. I joined awhile ago and have just been a "lurker" for awhile. Sorry about that.
Well, I am finally posting my first thread. YEAH!
I am studying to take my ABOC in May and am having some trouble with toric transposition. Every time I think I have the concept and go back and try to explain it to the other people in my study group I come out with the wrong answers and get all confused again.

When finding the total power of a len:
front surface + back surface = total power

yes???

Well, I am using that and still getting confused. I think I have it tonight but, tomorrow night when I go to try and tell everyone else what I got and how I will get the wrong answers.

for example, I will work one and could someone please, please tell me if I am doing it right.

Rx: -6.00 sph B.C. +3.50
so, I want to find the toric and cross curves of the lens.

I set up my problem:
-6.00 X180=+3.50 + X (X being the curve I am trying to find)
and, -6.00X090=+3.50 + X
so, I would get -9.50X180 and -9.50X090 correct???

I would do something similar if I wanted to find the B.C. and only had the Rx and T.C./C.C..

example:Rx:-1.00 -1.00 X090 T.C:-7.50X090 C.C:-8.50 X180
I would find total power at both axis and set up the equations again
-1.00X090=-7.50X090 + X=+6.50 AND,
-2.00X180=-8.50X180 + X=+6.50 SO, the B.C. would be +6.50

finally, to come up with the Rx from the other 2 I would do this:

B.C:+8.00 T.C.-6.00X090 C.C.-8.25X180

SO, X=+8.00-6.00X090 X=total power @ 090
and X=+8.00-8.25X180 X=TOTAL POWER @180
THAT GIVES ME +2.00X090 AND -0.25X180 right?
then I make that into an Rx and get:
+2.00 -2.25 X090 or -0.25 +2.25 X180

I think I understand it, and I think these examples are right but every time I sit down with the other 2 people taking the test with me I get the answers totally wrong and am not sure what I am doing wrong.

Please help
Thanks

[Pete Hanlin]

Laura,
Welcome to OptiBoard. It certainly seems like you have a good grasp of the concept!

Basically, front curve + back curve = Rx

I started out in the lab (and I'm convinced that working on the lab side of things really does help with optical concepts). With this particular concept, it really helps to think about the problem in terms of what tool (aka, "lap") you would need to grind a lens of a certain power.

For example, if I wanted to grind a -2.00 sph lens, I had a number of options (assuming I disregarded the helpful little sheets on the wall which identified the ideal base curve selection for each Rx)...

On a plano (0.00) base, I would put a -2.00 curve on the back side of the lens.
+2.00 base required a -4.00 backside curve
+4.00 base required a -6.00 backside curve
and so on as long as the total of the two sides equalled -2.00 sph total.

Now, if I needed a -2.00 -1.00 x180 rx in a lens, and I used a +4.00 base, I would use backside curves of -6.00x180 and -7.00x090... If you have a surfacing lab at your location, go back and grab some of the surfacing sheets (after the lab is done with them, of course!). Look at the power requested, the base curve and the tool curves generated by the computer. You should begin to "see" the concept as you view several jobs, until you can do the computations in your head as fast as you're reading them off the sheet.

Its also helpful to remember that, when constructing a compound lens, your backside curve is its flattest at the axis of the cylinder. Using the front curve and the Rx, simply do the algebra (which you seem to be doing flawlessly) to figure out your backside sphere curve. For the cylinder (cross) curve, simply combine in the cylinder power.

I have no idea if this is at all useful to you. Like I said, you seem to have the concept down pat, so I'm just trying to give you the way it always made most sense to me. If you are studying this hard for the exam, I am supremely confident you will do exceptionally well.

Good luck, and keep posting!

[Darris Chambless]

Hello Laura,

Pete is absolutely right as are you. To help a little bit more and to maybe simplify a little further all you need to do is remember basic math and what you get when you combine a = and a - number. Your formulas are correct but if you think of it in terms of:

If I have a +3.50 and I need to get a -6.00 as the answer what would I have to add or subtract to get it? As your equation shows if you subtract 9.50 you end up with a -6.00. Basic mathematics my dear Laura.

+6.00FBC -3.00BBC = +3.00 sph
+2.00FBC -6.00BBC = -4.00 sph
+1.50FBC -9.50BBC = -8.00 sph
and so on so forth and Pete has the best explanation of cross curve and the easiest to remember.

It can actually make a little more sense if you have someone that can show it to you on an optical cross.

Just be thankful you're not having to figure out total power at a particular axis or meridian ;)

You'll do well I'm sure.

Darris C.

[LauraH]

Thanks for the help with the studying. Sorry I haven't written back sooner--computer crashed. But, I think I did OK on the exam. We'll see in about 6 weeks.

The comparison to laps really helped. I used to work in a lab at my last job and it really tied the concept together.

Thanks again,
LauraH

[stephanie]

Good luck on the test I am sure you did fine. Let us know we are always eager to congratulate another optician on a job well done!!



Steph

[chip anderson]

Laura:

Basicly what everyone said here is true. But remember this for thin lenses. If a good deal of thickness is required, some compensation must be made to one curve or the other (usually the anterior) for thickness. This is usually 1/3 of the center thickness added to the radius of the front curve.


If you want a good way to get his concept. Draw a straight line. Set a compass (the circle drawing kind) on the line and draw and arc. Now draw arcs of the same radius at differing distance along this line. You will see a plano does not have the same curve on the front as the back, to be a plano the front must be longer in radius than the posterior by a ratio of aproximately 1/3rd of the thickness, if not it will be a plus lens.

If the curve of the anterior is longer than the posterior you can still have a plus lens if the thickness is great enough.

Sorry to add confusion to an otherwise simple answer.

Chip