Prism decentration

[Whitwoo]

A +4.50D lens needs to be decentred 4mm inwards to line up with the pupil. If prism is ordered 3 base in, what would be the total decentration of the lens?

Can someone help? :)

Whitney

[chip anderson]

Zero if it lines up with the pupil in distant gaze.

As ususal Chip didn't read the entire post before shooting out an answer. Once again, my appologies.

[mullo]

Since prism = c * F

c = prism/F = 3/4.50 = 0.67cm = 6.7mm

A plus lens decentered inwards creates base in prism. Therefore the lens would have to be decentered in an additional 6.7mm or 10.7mm total (4mm + 6.7mm) to create the 3 prism dioptres BI.

[Whitwoo]

woohoo i was right :D
Thanks guys!