Help with prism question.

[ekays1809]

I'm currently studying for my ABO (which I'll be taking in November, fingers crossed) and I'm having some trouble with this prism question:

• Right eye Rx is a -4.00 Sphere
• Left eye Rx is +6.00 Sphere
• Someone squeezed the nose pads together a little too much and raised the OC heights on these glasses 5mm
• How much prism and in what direction will the wearer now experience?

• 2.0 BD R
  3.0 BU L
• 5.0 BD R
  5.0 BU L
• 5.0 BU R
  5.0 BD L
• 1.0 BU L

When I do this question I get 2 BD OD and 3 BU OS. When I check my answers the quiz says the correct answer is 5 BD OD and 5 BU OS. What am I doing wrong? Any help is appreciated. Thank you.

[Paul Smith LDO]

Don't sweat it, it's a misprint. At least they got the prism direction correct.

[Robert Martellaro]

The vertical imbalance is five prism diopters base down right, or five prism diopters base up left.

http://www.2020mag.com/ce/TTViewTest...essonId=109555

[PRECISIONLAB]

This is an example of how you can be quick to answer. Please read the question again as it is asking how much prism and in what direction will the wearer experience. Refer to the rules of prism...same base direction cancel prism affect and opposite base direction compounds prism affect. So yes -4.00x 5 = 2.0 BD and +6.00x5=3.00 BU and since base direction is opposite you add prism amount for each eye hence 5.0

[Paul Smith LDO]

This is an example of how you can be quick to answer. Please read the question again as it is asking how much prism and in what direction will the wearer experience. Refer to the rules of prism...same base direction cancel prism affect and opposite base direction compounds prism affect. So yes -4.00x 5 = 2.0 BD and +6.00x5=3.00 BU and since base direction is opposite you add prism amount for each eye hence 5.0

Yes, but not in each eye. There is a total of 5.00 D of vertical prism. None of the answers given list 5.0 BD OD or 5.0 BU OS. 2) and 3) imply 10.0 D of vertical prism.

[Uncle Fester]

ekays 1809- Are your answers exactly as written on the test? Without an "or" between the second, third and fourth prism and base numbers it seems like a trick question to me.

Now the pads are 20mm apart. How many millimeters were the nose pads squeezed together???

[Robert Martellaro]

Please read the question again as it is asking how much prism and in what direction will the wearer experience.

Right. Calculate the amount and direction of prism induced in each eye. You need this information to determine the degree and direction of the resultant vertical imbalance.

ekays 1809- Are your answers exactly as written on the test? Without an "or" between the second, third and fourth prism and base numbers it seems like a trick question to me.

It really doesn't matter- it can be one or the other or both, although we wouldn't use both values at the same time to designate the vertical imbalance. The trick is that we understand how to determine the correct value for vertical imbalance, and its subsequent effect on the eyeglass wearers vision and comfort.

[Robert Martellaro]

Ekays,

The good news is that it's unlikely you'll see this practice question on the basic ABO test. You will see it on the ABO-AC, probably with oblique axes.

If you have any more questions on vertical imbalance, or optics in general, post 'em, and someone will help sort them out.

Best regards,

Robert Martellaro

[Uncle Fester]

It really doesn't matter- it can be one or the other or both, although we wouldn't use both values at the same time to designate the vertical imbalance.

But yous guyz who are so much smarter than me. I see that answer differently. Of course I'm not on the downhill side of the IQ bell curve. And I'll hasten to add some, like you Robert, are on the end of that side of the bell!!!