DISP - Introduction and Hand Neutralisation - Week 1

Question 1

Describe how to convert between + form and - form for sphero-cyl lenses. What is this called?

Answer 1

New sphere = old sphere + cyl
New cyl - old cyl, but opposite sign
New axis - Old axis + 90 degrees
This is transposition

Question 2

How can the power sign of a lens be assessed by thickness?

Answer 2

If the centre is thickest - positive
If the edges are thickest - negative

Question 3

Do positive lens magnify or minify? What about negative lens?

Answer 3

Positive - magnify
Negative - minify

Question 4

Briefly decribe how to hand neutralise a lens.

Answer 4

Hold the lens at arm length, and aim at a crosshair.
Rotate the lens to see if scissoring occurs. If so, it is a sphero-cyl lens.
If so, align one of the lines, and move in that axis, and make note of the direction to check its power sign.
An against movement is positive, a with movement is negative.
Add trial lenses to neutralise the movement, in both axes.