ABO Exam question

[britopt]

Help!!

Q. RX OD -6.50 +1.50 X90 combined with 3.0 prism base in.
Where is the major reference point located in relation to the optical center of the lens?

I am usually fine with Prentices rule etc but I cannot for the life of me get the same answer as my workbook...perhaps I've overdone the revision and it's time for a break?? Or perhaps I'm getting my UK and US terminology mixed up and confusing myself. Am I just having a blank moment? What answer do you guys get?

Thanks!!

[lensgrinder]

The optical center is where light is not refracted, the MRP is where you will spot the lens in the focimeter. You have a total of -5.00D in the 180 meridian. Using Prentice's rule you will divide:
3.0/5=0.6cm or 6.0mm in. You have to spot the lens 6mm in or towards the nose from the O.C.

[britopt]

Thankyou!! The lightbulb just went on above my head. As I suspected I was having a dumb moment....can't add -6.50 and +1.50. Doh!!

Think I'll take a break now....:hammer: