Prism Confusion – Please Help!
When verifying the prescribed Prism, I’m confused with plus and minus lens. I hope that someone can help me out on this. Here is the example.
EXAMPLE:
A right lens prescription reads: +3.00 -1.25 x 135, 2^ Base Out
How much the lens be centered before it is spotted?
SOLUTION:
To verify this prescription, the lens must be placed in the lensmeter and moved until the illuminated target is located 2^ units to the left (temporal) of the crosshair origin (Target center location always corresponds to prism base direction. This is true whether the lens is plus or minus in power)
Here is my problem:
Based on my understanding, the right lens will be placed on the lensmeter as normal (with the back side of the lens against the lensmeter stop, the power being measured is called the back vertex power). And this is a plus lens, so that its prism bases are base to base at the center (OC). Therefore, I need to move the lens to the left 2^ units to find the right spot for this case. Am I right or wrong on this?
But if the example above changes to minus lens like: -3.00 -1.25 x 135, 2^ Base Out for the right eye too. I think the lens will moves to the right instead so that it will have the Base Out?
I’m very confused. Can anyone explant it to me with a picture for plus and minus lens for BO, BI, BU, BD.
Thanks,
HVU.
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