since the front surface of a high minus lens may e
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Vertex Distance, Effective Power, and
Compensated Power - NAO ch. 2 sub. 2
Questions And Answers With Verified Solutions |
Latest Update 2025
vertex distance definition - ANSWER "the straight- line measurement in
millimeters (z axis) from the back or the ocular surface vertex of a mounted
spectacle lens to the apex of the cornea."
since the front surface of a high minus lens may essentially be plano, the center
thickness about 2.0 mm, and the bevel moved towards the front of the lens, the
back vertex will probably lie: - ANSWER in the plane of the eyewire groove or
slightly behind it.
true or false:
to measure vertex distance before the actual lenses are in place, you can measure
to either the front or back of the eyewire and adjust the measurement by the
distance of the groove. - ANSWER true
how is the vertex distance measured when the lenses are in place? - ANSWER
with a distometer.
if the doctor doesn't specify a vertex distance, what is the standardized
measurement you should use? - ANSWER 12mm
when a difference exists between the doctor's specified vertex distance and the
spectacle vertex distance, what needs to happen? - ANSWER the compensated
prescription must be determined.
, how to find the compensated prescription in a compound prescription? -
ANSWER 2 computations must be made, one for the power in each of the 2
principal meridians. the difference between the 2 compensated meridional
powers yields the power of the compensated cylinder.
what formula would you use to find the power change when the vertex distance
has changed? - ANSWER f = 1000/D (focal length in mm)
-add the change in vertex distance if the vertex distance is increased, or subtract
the change if vertex distance is decreased.
-then change the resultant amount into power (D = 1000/f)
true or false:when fitting lenses of relatively low power, the distance at which the
patient was refracted and the distance at which the lenses are fitted is of little
significance. - ANSWER true
true or false
as the power increases, the distance hat which the lenses are fitted has an effect
on the way a patient sees out of the lenses. - ANSWER true
what happens to the power in a plus lens when it moves closer to your face? -
ANSWER the effect of the power decreases.
what happens to the power in a plus lens when it moves farther from the face? -
ANSWER it increases the effect of the power.
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