Extra Questions for Module 11
          Vertex Distance Compensation
Each worksheet is different, because each one was originally written by a different person.  Use the directions at the beginning of each worksheet for that worksheet.

Vertex Power Worksheet


Nominal Power Front Vertex Power Back Vertex Power
1.  If a lens has a front surface power of +3.00, a back surface power of -8.50, a thickness of 2.2 mm, and is made of CR39, what are the nominal, front vertex, and back vertex powers?      
2.  If a lens has a front surface power of +9.50, a back surface power of -3.25, a thickness of 5.7 mm, and is made of CR39, what are the three different powers?      
3.  The same lens in number 2 is now made of aspheric curves in a 1.66 material.  It still has an actual front surface power of +9.50,  a back surface power of -3.25, and a center thickness of 3.3 mm.  Now what are the three different powers?      
4.  A bifocal lens made from polycarbonate has a back surface power of -4.75, a front surface power of +8.75, and the bifocal portion has a surface power of +10.50. 
  • If the center thickness of the distance portion is 4.1 mm  what are the three powers for the distance section? 
  • If the thickest part of the segment is 5.2 mm, what are the three powers for the reading portin of the lens?
  • What ADD do you get if you use the back vertex powers that you see in the focimeter?  What ADD do you get if you use the front vertex powers from the focimeter?
      
     
     


The answers are here.  Please do them yourself before you go to the answers.  If you understand it all and you don't need the practice then it will only take you a few minutes to do them.  If you are struggling with the formulas do one or two, check them, do a few more, check them, . . . just make sure that you do not look ahead of the ones that you did and are checking.

 Press the BACK button at the top left of the screen to return to the assignment page.