Vertical Imbalance question!! ABO EXAM

[ellenakimmy]

Hello Optiboard!!, I need help with vertical Imbalance questions!
The first question is " A patient's Rx is OD: -2.00 +1.00 x 090 and OS: -1.00 +3.00 x180 Add +2.00. At a reading level of 10mm, for a slab off, which lens should be used and how much imbalance is there?
What confuses me is that each eye has a different refractive power(opposite sign) at 90 degrees and the book is telling me that the difference is 4 diopter?

[wmcdonald]

Transpose the OS to minus cylinder, and you then have +2.00 - 3.00 x 090, correct? You can then clearly see that the power at 090 in each eye is as follows: -2.00 x 090 (OD), and +2.00 x 090 (OS)......a 4D difference. Using Prentice's Rule : P= hD, .10cm(4), Slab Off OD 4 prism diopters if a traditional slab. Always place the traditional slab on the strongest minus or weakest plus, in this case the OD.

[Diane]

This is why it's always a good idea to put the power on an optical cross. As I've looked at Warrens explanation,
He is spot on. Visualization on the lens cross would help.