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00065_Field_65.txt
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1997-10-28
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LENS POWER
BVP versus FVP
Another consideration in specifying lens power is
the difference between "front vertex power" and
"back vertex power". Clinically we usually specify
back vertex power (BVP), but occasionally the
laboratory will specify front vertex power (FVP).
With a contact lens of significant power (over 6
diopters) there is often a significant difference
between the two. This means if you specify or
measure the power as BVP and the lab makes the
lens as FVP you will not get the lens you want.
The BVP is read with the back surface against the
stop of the lensometer-the focal length is
measured from the back surface.
The FVP is read with the front surface against the
stop of the lensometer-the focal length measured
from the front surface.
When ordering a high powered lens (+ or -) be sure
to specify BVP.