• A lensmeter or lensometer, also known as a
focimeter is an ophthalmic instrument used for
measuring lenes power.
• It is mainly used by optometrists and opticians to
verify the power of spectacle lenses, to properly
orient and mark uncut lenses, and to confirm the
correct mounting of lenses in spectacle frames.
• Lensmeters can also verify the power of hard
contact lenses, if a special lens holder is used.
Types of Lensmeters
• There are Auto-lensometers , electronic fully
objective that work very well and are very
• A manual lensometer must always be kept on
hand in case the auto-lensometer is not
• Every optometrist and technician must know
how to use a manual lensometer to read a
pair of glasses.
Optics of manual lensometers
• The lensometer measures the vertex power of
the lens through the back focal length.
• A lensometer like an optical bench consisting
• A. An illuminated moveable target
• B. A powerful fixed lens
• C. A telescopic eyepiece focused at infinity.
Parameters of a lensmeter
• The parameters of a lensmeter are the values
specified on the patient's prescription:
sphere, cylinder, axis, and in some cases,
• There are some variations in the design and
operation of lensmeters.
• some have the diopter scale on the diopter
wheel, and others have a scale inside the
How to read glasses
• To read glasses on the manual lensmeter you must
understand the different scales and parameters.
• The Diopteric number line used starts at zero and
extends in the plus direction for about 20 diopters,
and in the minus direction for about 20 diopters.
• Each diopter is divided into half, quarter, and eighth
• As in the case with many ophthalmic instruments,
positive numbers are in black, and negative
numbers are in red.
Adjust the eye piece
• The first step in lensometry is to adjust the eyepiece.
• This step is necessary to insure the accuracy of
measurement. The eyepiece is adjusted for small
refractive error and to account for any
• Turn the eyepiece toward plus (counterclockwise)
until the reticule (cross lines) becomes blurr.
• Turn the eyepiece back in the minus (clockwise)
direction, slowly, until the reticule just comes into
focus, then stop. This is the correct position for to
Measuring a spherical, single vision
• A single vision lens should be inserted and ensure
the optical center is adjusted so that the mires at
center in the reticule.
• The mires are brought into sharp clear focus by
adjusting the power wheel. You would then read
the lens power from the power drum.
• In spherical lenses there is no change in shap and
size of mires (target).
• Usually the single line and the triple line both will
come into focus at the same time( often 180 &90
Measuring cylindrical lens
• Using the power wheel and the axis wheel to put the
mires into focus.
• By rotating the axis wheel put the single line mire in
focus, in this state the diopter power on the power
wheel is at zero.
• Now rotate the power wheel, the triple line becomes in
focus and note the power reading, this is cylinder
power and direction of triple lines is the axis of
• The plus direction reads in plus cylinder notation. The
minus direction reads in minus cylinder notation.
Measuring sphero-cylindrical lens
• When the power wheel rotated one of mired
appeared to be clear. So the axis wheel should be
rotated until the single mire becomes clear and
adjust the power wheel to the sharp focus( e.g.
+2.0D this would be the sphere power) suppose
the single line at 90.
• However, the triple line were in 180 and rotate
the power wheel again until the lines in sharp
focus let e.g. power is +2.5D.
• Then (+2.5 – 2.0) = 0.5 axis 180
• The final power is +2.0/+0.5CX180