ABO test question

**adairk** · 05-15-2009, 02:29 PM

I'm studying for the ABO exam and am having trouble with the following practice question. I have the correct answer in the book but need to know to arrive at the answer. I would appreciate help with this, and the simpler the better. Thanks!

What prismatic effect would a patient experience looking 4mm below the optical center of a pair of +4.00 -1.00 x 030 lenses?

a) 1.30 diopters of prism B. U.
b) 1.30 diopters of prism B. D.
c) 1.60 diopters of prism B. U.
d) 1.60 diopters of prism B. D.

**Now I See** · 05-15-2009, 02:51 PM

You'll have to apply prentice's rule:

P = d x D/10

d = the distance from the optical center in mm
D = the dioptric power of the lens

Using your RX

1st you have to find the total power of the lens at 90. *find this using the oblique meridian formula*

+4.00 + (-0.75) = +3.25

next up will plug that number into prentice's rule:

P = 4 x (3.25)/10

P = 13.00/10

P = 1.3

*Watch out on some of these questions, sometimes they will ask you the same type of question using cm instead of mm! Remember the rule works only if you have (d) in mm.*

Hope that helps a little.

**adairk** · 05-15-2009, 03:02 PM

Thank you so much! Wish me lots of luck!

**braheem24** · 05-15-2009, 03:04 PM

Always visualize where you eye is in relation to the thick part of the lens.

On a +rx looking down causes the thick part to be above your eye and therefore base up.

**Now I See** · 05-15-2009, 03:09 PM

Good Luck!

Just remember, READ the questions...don't let the thoughts of time get in your way...Breathe, Read, Breathe, Read..oh, yeah, don't forget to answer!...LOL! :D

I don't know what books you have in your stash, but this one I have is called Optical Formulas Tutorial by Ellen D. Stoner and Patricia Perkins. It's a pretty good book, I keep it here for reference and use it quite a bit.

**disclaimer, I do not work for the author or book distributer or its affiliates...just a happy optician, that can't keep formulas in my head! :)

**adairk** · 05-15-2009, 03:34 PM

Yikes! I'm trying to help my daughter with this but am lost. We googled "oblique meridian formula" and came up with a whole new page of complicated sounding stuff. Heather, where did you come up with .75 in your first explanation? Is the oblique meridian formula a simple formula or what? Millie

**braheem24** · 05-15-2009, 03:52 PM

0 degrees away from the axis of the Rx, there is 0 times the cylinder power.
30 degrees away from the axis of the Rx there is .25 times the cylinder power.
45 degrees away from the axis of the Rx there is .50 times the cylinder power.
60 degrees away from the axis of the Rx there is .75 times the cylinder power.
90 degrees away from the axis of the Rx there is 1 times the cylinder power.

Or the opposite depending if it's plus or minus Cyl, I have not seen an ABO question asked with any other meridians.

**Now I See** · 05-15-2009, 05:06 PM

The chart that braheem posted is the one that I used....I agree with you, adairk, these formulas get pretty confusing! :) Keep us posted on how your ABO goes! :)

**adairk** · 05-15-2009, 05:07 PM

Okay - our learning disability has officially kicked in! One more question and we would appreciate the steps you go through arriving at the answer. Thank you in advance!

What prismatic effect would a patient experience looking 5mm above the optical center of a pair of -5.00 -2.00 x 150 lenses?

a) 2.50 diopters of prism B. U.
b) 2.50 diopters of prism B. D.
c) 3.25 diopters of prism B. U.
d) 3.25 diopters of prism B. D.

**chip anderson** · 05-15-2009, 05:27 PM

c 3.25 u

**sharonm516** · 05-15-2009, 05:47 PM

Originally Posted by adairk

Okay - our learning disability has officially kicked in! One more question and we would appreciate the steps you go through arriving at the answer. Thank you in advance!

What prismatic effect would a patient experience looking 5mm above the optical center of a pair of -5.00 -2.00 x 150 lenses?

a) 2.50 diopters of prism B. U.
b) 2.50 diopters of prism B. D.
c) 3.25 diopters of prism B. U.
d) 3.25 diopters of prism B. D.

Again apply Prentices Rule......

Im taking the ABO too....there is a ton of Prentices on it....

**GAgal** · 05-15-2009, 08:43 PM

Originally Posted by adairk

Thank you so much! Wish me lots of luck!

:cheers: Good luck on your ABO! :cheers:

**adairk** · 05-15-2009, 09:08 PM

I seem to be missing a step. I get Prentices Rule but when I look at the problem that I posted -5.00 -2.00X150 with 5mm
I would take 5x5=25/10. I would get a or b answer but it C. So what am I missing?

**GAgal** · 05-15-2009, 09:19 PM

Originally Posted by adairk

I seem to be missing a step. I get Prentices Rule but when I look at the problem that I posted -5.00 -2.00X150 with 5mm
I would take 5x5=25/10. I would get a or b answer but it C. So what am I missing?

It is (6.5 X 5)/10, because you are working on the 90 meridian. It is 60 degrees away so 75% of the cylinder would be in effect.

**braheem24** · 05-15-2009, 09:21 PM

Originally Posted by adairk

-5.00 -2.00X150 with 5mm

First you figure out how far you are from the meridian in the question. In this situation the question is asking for 5mm above.

Since we're dealing with up and down prism we need to figure out the power at 90.

150 is 60 degrees away from 90 therefore use the 60 power in the chart above which is 75% of the power. Take the -2.00 and multiply it by 75% gives you -1.50. Add the -1.50 and the -5.00 together and you have -6.50 at the 90.

Next, you take the -6.50 and apply the prentice rule -6.50*5mm/10= 3.25

Lastly, we know that we are 5 mm above the -6.50 so we know we will be closer to the thick part of the lens that is on the top of the frame. If the closest thickness/base is above our eye we know that gives us BU prism.

3.25 is the power and up is the direction :cheers:

**adairk** · 05-15-2009, 11:16 PM

It finally clicked Thank you very much

**adairk** · 05-16-2009, 06:29 PM

I'm taking the test tomorrow so I won't be asking for help much longer!

What is the effective power of a pair of -9.00 D lenses prescribed at a refracted vertex distance of 21mm if the lenses are worn at a vertex of 8mm?

What is the total dioptric power for near vision with the following prescription?
OD: -3.25 -1.00 x 180
OS: -3.00 -1.25 x 175
Add: +1.75, OU

I have the answers - just want help in how to solve the problems. If you use a formula that has a name, it would be helpful to mention that. Thanks!

**braheem24** · 05-16-2009, 06:55 PM

This should help, it's called vertex distance...

http://www.google.com/search?hl=en&r...stance+formula

Dont confuse yourself with more plus less minus or whatever the sayings are, Always remember the lens always becomes more plus the further you move it away from the eye, minus or plus does not matter same effect.

**GAgal** · 05-16-2009, 07:18 PM

Originally Posted by braheem24

Dont confuse yourself with more plus less minus or whatever the sayings are, Always remember the lens always becomes more plus the further you move it away from the eye, minus or plus does not matter same effect.

And remember it is a multiple choice test, usually two of the answers are going to be in the wrong direction, i.e a two answers less minus the farther away a plus lens moves away from the eye, so you can eliminate them right off the bat. And another thing that Mike Disanto told me

You will only "know" about 70% of the questions on the test (and some of them will be wrong).
About 20% you can get it narrowed down to one or two, so just pick one and you have a 50/50 shot
The remaining 10%, just pick your favorite letter and use that for all of them.

So, take your time, breathe, don't over think the answer or take too much time on a question, and you should be fine :D

**sharonm516** · 05-17-2009, 03:29 PM

Originally Posted by GAgal

It is (6.5 X 5)/10, because you are working on the 90 meridian. It is 60 degrees away so 75% of the cylinder would be in effect.

Smarty!!! :idea::bbg:

**Uncle Fester** · 05-17-2009, 09:59 PM

Originally Posted by GAgal

And remember it is a multiple choice test, usually two of the answers are going to be in the wrong direction, i.e a two answers less minus the farther away a plus lens moves away from the eye, so you can eliminate them right off the bat. And another thing that Mike Disanto told me

You will only "know" about 70% of the questions on the test (and some of them will be wrong).
About 20% you can get it narrowed down to one or two, so just pick one and you have a 50/50 shot
The remaining 10%, just pick your favorite letter and use that for all of them.

So, take your time, breathe, don't over think the answer or take too much time on a question, and you should be fine :D

I luv ya Georgia girl but this is what's wrong with opticianry in the States right now imho.:o

==Upon further reflection this is easy to say from an old flatulent noise:)

**braheem24** · 05-18-2009, 08:38 AM

GAgal is ABO-AC,

I'm sure she started guessing at a few when she took her ABO. Whereas myself, I didnt study for the ABO with 1 year experience and finished the ABO in 20 minutes and scored a 94%.

Where did it get me? I'm now I'm still only ABO from my arrogance of still not wanting to study.

People that want to succeed should be given the chance to slowly better themselves. I believe ABO-AC truly confirms you know your optics whrereas an ABO is a good steeping stone that leads to that certification.

**chip anderson** · 05-18-2009, 09:19 AM

I am a person that can probably pass a multiple choice test on anything with no preparation. Having said this I am not at all sure that without preparation I could pass a basic optical test without some review.

I do not think think multiple choice tests should be used for certification in anything as I am sure there are many like me that can find the most logical answer given four choices to most things.

I would even go so far as to say that most tests for certification should include a good deal of hands-on work under observation.

Comments anyone?

Chip

**gmc** · 05-18-2009, 10:29 AM

Hi all,

I'm licensed in Florida and took the exam for the 2nd time yesterday. The first time was about 20 years ago. (long story)

It was very easy as it should be for me. I only spent a couple of hours reviewing tolerances and formulas.

I've also prepared dozens of people for the exam and the pass/fail rate is very closely related to the amount of prep time put in.

I've found that each time the test is given, a different content area seems to be emphasized. On this test, there were several questions about nominal power calculations.

Such as: Given a front curve of x and a back curve of y, what would the lens power be?

Or: Given a front curve of x and a desired lens power of y, what should the back curve be?

If someone didn't have a good handle on this concept, they probably didn't score very well.

**finefocus** · 05-18-2009, 10:49 AM

[QUOTE=adairk;296586]
What prismatic effect would a patient experience looking 4mm below the optical center of a pair of +4.00 -1.00 x 030 lenses?

And, the prismatic imbalance is zero, since both lenses have identical amounts of base up prism. What we have here isn't a prism correction, just badly placed OC's. Extra credit question: how much panto or retro is indicated by this OC placement?