Measurement of lens power by
Lensometer
Faculty
Aravind School of Optometry
ARAVIND EYE CARE SYSTEM
Aravind Eye Hospital
& Postgraduate Institute of Ophthalmology
Madurai, India
Corrective lenses
Lens type
Lens form
Corrective use
Sphere
Convex ( + )
Concave (--)
Hypermetropia
Myopia
Cylinder
&
Sphero-cylinder
Convex ( + )
Concave (--)
Hyperopic Astigmatism
Myopic Astigmatism
Prism
 Correcting squinting eyes
 Relieving eye strain
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Measuring Lens Power
 Hand neutralisation by trial lens method.
 By Lensometer.
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Trial lens method
 View cross target at distance
 Hold lens on visual axis, close to eye.
 Align lens such that cross target is continuous.
 Move lens vertically along line of vertical limb of target
 Determine direction of horizontal limb movement
 Place trial lens flush with ‘unknown’ lens
 No movement = neutral
 Repeat the same for line of horizontal limb of target.
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Hand Neutralisation
Neutralization of lens power using trial lenses
Lens Movements
Cross Target
“With”
“Against”
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Hand Neutralisation of Toric Lenses
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Lensometer
 Measures the corrective lens power
 Sphere
 Cylinder and its axis
 Prism.
 Optic center
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Components of a Focimeter
Eyepiece
Spring Clip
Table
Lens Stop
Axis Dial
Optic Centre Marker
Power Wheel
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Constructing a Focimeter
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

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
Zero Position
Target at first principal focus
Telescope
With Negative Power Lens
Unknown lens at 2nd principal focus
Distance between standard lens &
target is increased
Lens holder for
unknown lens
Standard lens



With Positive Power Lens
Unknown lens at 2nd principal focus
Distance between standard lens &
target is reduced
Target
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Constructing a Focimeter
x
Light Source
& Moveable Target
fo
Standard lens
f ’o
x’
Lens holder for unknown lens
(holding negative lens in figure)
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Telescope
Calculation
 Movement of the target per dioptre
Fo = +25.00D
Target travel (mm) per dioptre:
x=1000 / Fo² = 1000 / 25² = 1.6mm per Dioptre
So, for a focimeter required to measure  20D
the total required travel of target = 40 x 1.6 = 64mm
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Target system
 American optical system – crossed line
target.
 European optical system – Ring dots
system
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Focimeter Preparation
 Focus the eyepiece
 Ensure that all the readings at Zero.
 Calibration
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Focimeter – crossed line target system
 Insert the spectacle.
Reticule scale
 Determine the lens power.
 Mark the optical centre.
 Measure the power of the
second lens.
1
3
5
Line Target
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Prism dioptres
delineation
Focimeter - Ring of Dots Targets
Reticule scale
90
180
0
Prism dioptre
delineation
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Procedures
 Focus the eyepiece by focusing the hair-line black reticule.
 Place the back vertex (the ocular side) of the unknown lens against the lens
stop of the lensometer.
 Make sure both eye wires of frame, right & left, are touching the stage.
 Move the lens side to side to align with center the target that places the optical
center of the lens at the stop.
 Notice that the target lines are a cross composed of two sets of lines oriented
90 apart.
 Try focusing the target lines.
 If all lines are in focus simultaneously, the lens is spherical.
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How to measure the
cylindrical power and axis?
 If only one set is in focus (the other set 90 away is blurred), the lens is cylindrical.
 If this is the case, focus one set of lines with the power drum, while simultaneously
rotating the axis wheel so that the lines are not only clear, but they are also unbroken.
 Note the power on the drum.
 If the unknown lens is cylindrical, the set of lines 90 away will be out of focus.
 The power of the first set of clear lines is called the sphere reading.
 Now focus the other set of lines (90 away from the first set) so that they are clear and
unbroken.
 Note the new power on the drum & also note the axis on the outside wheel.
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How to measure the
cylindrical power and axis?
 If the 2nd set of lines was focused at a lower + (more --) power, you have measured in
‘—’ cylinder form.
 Since our convention is ‘—’ cylinder form, get used to making the less ‘—’
measurement in 1st measurement. That would mean that the 2nd measurement would
be more +, thus allowing to record the reading in ‘—’ cylinder form.
 In this case (minus cylinder form), the first measurement is the sphere, and the
increase, or change in minus power you had to travel to the second, (higher minus
power) focus is the cylinder power.
 Record the values.
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Example - Line Targets
Power wheel
Sphere setting
+1.00 / -2.00 x 120
Power wheel
Cylinder setting
Axis
- +2.00
- 0.00
-
-
- +1.00
- -1.00
-
-
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Example - Ring of Dots Targets
+1.00 / -2.00 x 120
Power wheel
Sphere setting
Power wheel
Cylinder setting
Axis
90
180
90
0
180
- +2.00
-
- +1.00
-
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0
- 0.00
-
- -1.00
-
Focus of cylindrical lens
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Measure the bifocal add
 Lift the front of the lens against the lens stop.
 Take a reading of one set of unbroken, focused lines through
the distance portion, then move the stage up until the bifocal
add is against the stop.
 Re-focus the same lines again, but in segment.
 The difference between the 1st & 2nd reading is the add power.
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Vertical Prism
2.0 Base Up
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Horizontal Prism
1.5  Base In
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Oblique Prism
3.0 up @ 150
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Focimeter Use - Sources of Error
 Failure to focus the eyepiece
 Zero setting & axis alignment
 Centration of the reticule & target
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Projection & Automatic Focimeters
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