Lensometry CH5

Question 1

The prescription of a lens is written in the following order:
a) cylinder power, sphere power, sphere axis
b) sphere power, cylinder axis, cylinder power
c) sphere power, cylinder power, cylinder axis
d) sphere power, sphere axis, cylinder axis

Answer 1

c) By convention, a lens prescription is written sphere power, cylinder power, cylinder axis.
(By the way, there is no such thing as sphere axis!)

Question 2

n a glasses prescription reading +1.25 – 3.75 × 082, the –3.75 refers to:
a) sphere power
b) cylinder power
c) cylinder axis
d) add power

Answer 2

b) See answer 1.

ANSWER 1: By convention, a lens prescription is written sphere power, cylinder power, cylinder axis.
(By the way, there is no such thing as sphere axis!)

Question 3

In a glasses prescription reading –2.25 + 1.50 × 173, which of the following is true?

a) The prescription is written in minus cylinder.
b) The sphere power is “plus.”
c) The cylinder power is written as “plus.”
d) The cylinder axis is a multiplier.

Answer 3

c) The second number in the prescription (1.50) is the cylinder power, and it is written as
plus.

Question 4

In a glasses prescription reading +1.00 + 1.00 × 180 | 1.25/2.50, the number 1.25 refers to:
a) the trifocal power
b) the bifocal power
c) the total add power
d) the cylinder power

Answer 4

a) The prescription is written as a trifocal. The 1.25 will be the trifocal (intermediate distance), and the 2.50 will be the bifocal (closest distance). In the case of a trifocal, one might
speak of the add as being 1.25/2.50, but not refer to the 1.25 alone as being the add.

Question 5

Which of the following glasses prescriptions would definitely be questioned by an
optician?
a) +1.50 + 2.25 × 181
b) Plano – 9.25 × 072
c) +11.75 – 1.75 × 175
d) –2.50 + 6.50 × 018

Answer 5

a) While any of the prescriptions might earn a phone call from the optician to verify the
numbers, the first one would undoubtedly earn a phone query because of the axis. There is no axis at 181; the axis designation goes from 001 to 180 and not beyond.

Question 6

When using an automated lensometer, it is important to designate:
a) lens material
b) lens manufacturer
c) desired cylinder type
d) desired base curve type

Answer 6

c) An automated lensometer can be set to read in plus or minus cylinder

Question 7

An advantage of the automated lensometer is the ease with which it:
a) detects Fresnel prism
b) reads progressive add lenses
c) identifies lens material
d) identifies polarized lenses

Answer 7

b) The most modern automated lensometers make reading a progressive add lens a snap.
Fresnel prism is obvious without a lensometer because of the ridges on the lens. Lens material and polarized lenses are not detected with any lensometer

Question 8

An automated lensometer may be disadvantageous in identifying:
a) optical centers
b) add powers
c) prisms
d) warped lenses

Answer 8

d) The automated lensometer is not very useful in identifying a warped lens. Characteristics of the lens itself (aside from the numerical measurements) are better identified with a manual lensometer.

Question 9

A key advantage of the automated lensometer is that it:
a) eliminates math errors
b) converts the glasses prescription to a contact lens prescription
c) takes vertex distance into account
d) identifies photosensitive lenses

Answer 9

a) Because you can set the automatic lensometer for plus or minus cylinder, any math errors in transposition are eliminated, as are errors in reading the numbers off the wheel of a manual lensometer and figuring the cylinder power, axis, and add power. No lensometer can do the tasks in answers b, c, and d

Question 10

All of the following are true regarding automated lensometers except:

a) they should be set on an antistatic mat
b) they should not be placed and used in direct sunlight
c) the internal lenses and mirrors can be cleaned by removing the instrument housing
d) the computer components are sensitive to dust

Answer 10

c) The housing should not be removed by anyone except qualified service personnel. To
reduce dust exposure, cover the instrument when not in use

Question 11

11. Label the parts of the lensmeter1
    (Figure 5-1):
    *axis wheel
    *lens holder
    *frame stage
    *eye piece
    *lens stop
    *power wheel
    *table control lever

Answer 11

E: axis wheel
A: eye piece
C: lens stop
F: table control lever
D: frame stage
B: lens holder
G: power wheel

Question 12

The first step in reading a pair of glasses with the manual lensometer is to:
a) position the glasses in the holder with the temples away from you
b) position the glasses in the holder with the temples toward you
c) clean the lenses before reading
d) adjust the eye piece of the instrument

Answer 12

d) Before you can read glasses accurately, you must first adjust the eye piece.

Question 13

When adjusting the lensometer eye piece:

a) you should wear your own habitual correction
b) the lensometer’s power dial should be set to your prescription
c) the lensometer’s axis indicator should be set to 180
d) the lensometer’s axis indicator should be set to match your own astigmatism

Answer 13

a) If you wear glasses or contact lenses during the work day, you should have them on (or
in) when you adjust the eye piece.

Question 14

Before adjusting the lensometer eye piece:

a) turn the eye piece to the most minus position
b) turn the eye piece to the most plus position
c) turn the eye piece to an axis of 180
d) turn the eye piece to the axis of your astigmatism

Answer 14

b) The eye piece should be rotated to its most plus position. (Minus will trigger your
accommodative reflex.)

Question 15

When adjusting the lensometer eye piece, the eye piece is slowly rotated until:
a) the target just begins to blur
b) the target is first seen to be clear
c) the target has turned 90 degrees
d) the target clears then begins to blur again

Answer 15

b) Stop turning the eye piece when the target first becomes clear. If you continue to turn it,
you will add unneeded minus, and your own accommodation will interfere

Question 16

Failure to properly adjust the lensometer eye piece before reading lenses may result
in:
a) missing ground-in prism
b) a misaligned cylinder axis
c) reading in plus instead of minus cylinder
d) an inaccurate reading

Answer 16

d) If you do not adjust the eye piece, you may get inaccurate readings.

Question 17

A lens can be read in a lensometer in plus or minus form:
a) by changing the axis 180 degrees
b) by changing the axis 90 degrees
c) by changing the axis 360 degrees
d) by changing the axis 45 degrees

Answer 17

b) You can read any lens in plus or minus form by rotating the axis by 90 degrees.

Question 18

The proper position of glasses on the manual lensometer is:
a) bottom edge of frame against the frame stage, temples facing away
b) top edge of frame against the frame stage, temples facing away
c) bottom edge of frame against the frame stage, temples facing toward you
d) top edge of frame against the frame stage, temples facing toward you

Answer 18

a) Glasses are placed on the frame table with the bottom toward you, and against the frame
stage, and the temples pointing away

Question 19

When beginning to read the right lens, the first step is to:
a) try to center the target by moving the stage
b) move the stage so the target is in the uppermost part of the field
c) move the stage so the target is in the lower part of the field
d) change the eye piece again to refocus

Answer 19

a) The first step in reading lenses is to attempt to center the target

Question 20

Which target is analyzed first?
a) the instrument makes this selection
b) the circular mires
c) the wide/triple lines
d) the narrow/single lines

Answer 20

d) The narrow/single lines are cleared first. Note: While different brands of lensmeters may have different targets, this text is using narrow/single and wide/triple target lines.

Question 21

If all the lines of the lensometer target clear at the same time, the lens is:
a) monocentric
b) spherical
c) cylindrical
d) spherocylindrical

Answer 21

b) In a spherical lens, the target lines will all clear together

Question 22

If the narrow/single line and wide/triple lines of the lensometer target do not clear at
the same time, the lens is:
a) bicentric
b) spherical
c) plano
d) spherocylindrical

Answer 22

d) In a spherocylindrical lens, the narrow/single and wide/triple lines do not clear together

Question 23

If the lens is determined to be spherocylindrical, one can deduce that the patient has:
a) myopia
b) hyperopia
c) astigmatism
d) presbyopia

Answer 23

c) Cylinder is used to correct astigmatism. (Spherocylindrical means the lens has sphere
combined with cylinder.)

Question 24

You are reading the right lens, and the narrow lines appear broken. This means you
need to adjust the:
a) eye piece
b) lens table
c) axis wheel
d) power wheel

Answer 24

c) If the lines appear broken, the axis needs to be aligned. Turn the axis wheel until the lines
are solid.

Question 25

If the narrow/single lines are clear and you wish to read the lens in minus cylinder
form, you should:
a) turn the power-focusing wheel toward yourself
b) turn the power-focusing wheel away from yourself
c) change the axis 90 degrees
d) focus the triple lines first instead

Answer 25

b) If you want the reading in minus power, you should turn the wheel away from yourself
after clearing the narrow/single lines, in order to clear the wide/triple lines

Question 26

The narrow/single lines are clear. You wish to read the glasses in plus cylinder. Turning the power wheel toward yourself does not bring the wide/triple lines into focus.
What should you do?

a) read the lenses in minus cylinder instead
b) subtract the reading for the narrow/single lines from the reading for the wide/triple lines
c) add the reading for the narrow/single lines to the reading for the wide/triple lines
d) rotate the axis 90 degrees

Answer 26

d) If you want to read a lens in plus cylinder, after clearing the narrow/single lines, you need
to be able to clear the triple/wide lines by turning the power wheel toward yourself. If you
cannot do this, rotate the axis by 90 degrees and start again by clearing the narrow/single
lines.

Question 27

You have taken the following lensometer reading: The narrow/single line clears at
–2.00. The wide/triple lines clear at –3.00. Axis is on 083. The prescription is:
a) –2.00 – 3.00 × 083
b) –2.00 – 1.00 × 083
c) –2.00 – 1.00 × 173
d) –3.00 + 1.00 × 083

Answer 27

b) The single line clears at –2.00, so the spherical power is –2.00. There is a –1.00 step
between –2.00 and –3.00, so the cylinder power is –1.00. The axis is given as 083. Thus,
the prescription is –2.00 – 1.00 × 083

Question 28

You have taken the following lensometer reading: The narrow/single line clears at
+6.25. The wide/triple lines clear at +8.00. Axis is on 132. The prescription is:
a) +8.00 – 6.28 × 042
b) +6.25 + 8.00 × 132
c) +6.25 + 1.75 × 042
d) +6.25 + 1.75 × 132

Answer 28

d) The single line clears at +6.25, which is your sphere power. There is a +1.75 step
between +6.25 and + 8.00 so the cylinder power is +1.75. The axis is given as 132. The
prescription is +6.25 + 1.75 × 132.

Question 29

You have taken the following lensometer reading: The narrow/single line clears at
–1.50. The wide/triple lines clear at +1.25. The axis is on 097. The prescription is:
a) –1.50 + 1.25 × 097
b) –1.50 + 2.75 × 007
c) –1.50 + 2.75 × 097
d) –0.25 – 1.25 × 097

Answer 29

c) The single line clears at –1.50, which is your spherical power. There is a +2.75 step from
–1.50 to +1.25 so the cylinder power is +2.75. The axis is given as 097. The prescription is
–1.50 + 2.75 × 097.

Question 30

You have taken the following lensometer reading: The narrow/single line clears at
+2.25. The wide/triple lines clear at –0.25. The axis is on 178. The prescription is:
a) +2.25 – 0.25 × 178
b) +2.00 – 0.25 × 178
c) –0.25 + 2.50 × 088
d) +2.25 – 2.50 × 088

Answer 30

c) The single line clears at +2.25, which is your spherical power. There is a –2.50 step from
+2.25 to –0.25, making this the cylinder power. The axis is given as 178. The prescription
is +2.25 – 2.50 × 178. However, this is not given as an answer, which is your cue to transpose.2 Add the sphere and cylinder powers together algebraically: +2.25 + (–2.50) = –0.25,
your spherical power. Change the sign on the cylinder power: + 2.50. Rotate the axis by
90 degrees (178 – 90): 088. The answer is –0.25 + 2.50 × 088.

Question 31

You have centered and read the distant portion of the right lens of a pair of bifocals.
What is the next step?

a) move the stage up to read the bifocal segment
b) switch to the left lens, center, and read its distant portion
c) switch to the left lens and note the position of the target
d) turn the glasses over and read the front power of the right lens

Answer 31

c) It is very important to go from distant, right lens, to distant, left lens, without moving the
stage. Otherwise, you could miss induced vertical prism.

Question 32

To read the power of bifocal or trifocal segments, you should:
a) move the frame table upward until the segment mires appear in the center
b) slide the glasses down until the segment mires appear in the center
c) slide the glasses up and hold them there by hand
d) always turn the glasses around to read the back of the lens

Answer 32

a) To read the segments, move the frame table up. This will stabilize the lenses in proper
alignment (versus moving the glasses up without using the frame table). Sliding the glasses up and holding them there can introduce error.

Question 33

The power of a bifocal will be the difference between:

a) the axes of the distant and bifocal portions of the lens
b) the point where the wide lines clear on the distant part of the lens and where the thin
lines clear on the bifocal
c) the point where the narrow/single lines clear on the distant part of the lens and where
they clear on the bifocal
d) the point where the narrow/single lines clear on the distant part of the lens and where
the wide lines clear on the bifocal

Answer 33

c) The bifocal power is the difference between the reading where the narrow/single lines
clear on the lens’ distant portion and where they clear on the lens’ bifocal (or bottom segment of a trifocal). Alternately, it is also the difference between the reading where the wide/ triple lines clear on the lens’ distant portion and where they clear on the lens’ bifocal (or bottom segment of a trifocal). This works because the proportions are the same in both the distant and near parts of the lens. The main thing is to use the same lines (eg, both narrow or both wide) when moving from the distant to the near part of the lens.3 For example, in the distant lens reading, narrow lines clear at +1.00 and wide lines clear at +2.00 × 180. In the bifocal reading, if the narrow lines clear at +3.00, the wide lines will clear at +4.00, keeping that 1.00 diopter of cylinder.

Question 34

The distance portion of the lens prescription is –1.25 sphere. You are now reading the
bifocal add, and the lensometer dial reads +1.50. The add should be recorded as:
a) +1.50
b) +0.25
c) +0.75
d) +2.75

Answer 34

d) It may be helpful to remember that the power wheel is actually a number line. Each mark on the wheel represents 0.25 D. Ask yourself: how many lines/units did I move to get from–1.25 to +1.50?
The answer is 11 lines; 11 × 0.25 = +2.75

Question 35

The distant portion of the lens prescription is +1.25 (narrow/single lines) – 1.25 (wide/
triple lines) × 180. You are now reading the bifocal add, and the narrow lines clear at
+3.00. The add should be recorded as:
a) +3.00
b) plano
c) +1.75
d) +4.25

Answer 35

c) The difference between 3.00 and 1.25 is 1.75. (How many lines from +1.25 to +3.00?
The answer is 7 lines; 7 × 0.25 = +1.75.)

Question 36

The distance portion of the lens is –2.00 + 2.00 × 072. You are now reading the bifocal
add, and the wide lines clear at +3.00. The add is:
a) plano
b) +1.00
c) +2.00
d) +3.00

Answer 36

d) This time you were asked to analyze the add by using the wide lines. For the distance
portion of the lens, the wide lines would have cleared at plano. (You started at –2.00 with
the thin lines clear, then turned the wheel toward you; the wide lines cleared at plano, which gave you the cylinder power of +2.00.) When reading the bifocal, the thin lines would clear at +1.00 and the wide lines at +3.00. Because you are using the wide lines, the add would be the difference between plano and +3.00, which is +3.00.

Question 37

The best method for reading the add on a no-line progressive lens using a manual
lensometer is to:
a) mark the lens first using the template from that manufacturer
b) use the least plus reading for the distance and the most plus reading for the add
c) take the distant lensometry reading at a point between the laser marks on the lens
d) center the target in the lensometer

Answer 37

a) Most companies who manufacture no-line progressives have a template for their lens.
The lens is laid on the template, and a wax pencil is used to trace the markings onto the
lens. A circle indicates where the reading should be taken

Question 38

To read the bifocal portion of an aphakic spectacle lens:

a) the distance and bifocal spherical power should be read with the temples toward the
technician
b) the distance and bifocal spherical power should be read with the temples away from the technician
c) one need only read the distance spherical power, because all aphakic lenses have a
+3.50 add
d) one must adjust the eye piece to compensate for the high amount of plus power

Answer 38

a) To accurately read the add on an aphakic lens, you should read the distant and add sphere power on the back of the lens. (Of course, when recording the distant portion, you should read the lens front, as usual; you only turn the lens over when calculating the add power.)
Some authors recommend reading the add power on every lens by reading the back of the
lens (ie, temples toward you). Others apply this technique only to lenses over +3.00 in the
distant portion.

Question 39

Regarding the power of the midlevel trifocal lens segment:
a) it is impossible to read the power on the lensometer
b) it is always half the power of the bifocal
c) it is always twice the power of the bifocal
d) it can be read using the lensometer

Answer 39

d) Even though the middle portion of most trifocals is half the add power, it’s not always
so. The lens can be read on a lensometer. (Note the mention of a segment, which lets you know that this is not a progressive add lens.)

Question 40

Lenses that are not correctly centered on the visual axis result in:
a) off-axis prescriptions
b) extra edge thickness
c) unwanted prism
d) accurate prescriptions

Answer 40

c) Unwanted prism results when lenses are not aligned with the visual axis. (Note: The
visual axis is the “line” from the fovea, through the pupil, to the object of regard, not to be
confused with the axis of astigmatism.)

Question 41

Prism diopters are measured using a lensometer by:
a) the rings in the reticle
b) the number of lines in the target
c) the position of the target
d) the position of the glasses on the stage

Answer 41

a) Prism diopters are indicated by the rings in the reticle of the lensometer, with each ring
representing one prism diopter. The target position tells you the direction of the base. (Yes, that was rather tricky.)

Question 42

Prism direction is indicated in the lensometer by:
a) the rings in the reticle
b) the number of lines in the target
c) the position of the target
d) the position of the glasses on the stage

Answer 42

c) See answer 41.

ANSWER 41: Prism diopters are indicated by the rings in the reticle of the lensometer, with each ring
representing one prism diopter. The target position tells you the direction of the base. (Yes, that was rather tricky.)

Question 43

If the target can be centered in the lensometer, how does one determine whether or not there is induced prism due to decentration?
a) by marking the centers
b) by marking the centers and measuring the distance between them
c) by marking the centers and observing the glasses on the patient
d) decentered lenses cannot be centered in the lensometer

Answer 43

c) Looking at the marked lenses on the patient will show if there is induced prism because
the centers of the lenses will not line up with the patient’s visual axis. Or, measure the
distance between the lens centers and then compare this number to the patient’s pupillary
distance (not given as an option on this question). A decentered lens can often be centered in the lensometer. (A lens with ground-in prism cannot.)

Question 44

The target of the right lens is displaced to the right, and the lines cross at the third
ring. The prism power and direction is:

a) 3 prism diopters base-out
b) 3 prism diopters base-in
c) 1.5 prism diopters base-out
d) 1.5 prism diopters base-in

Answer 44

b) Because the lines cross at the third ring, you know there are 3 prism diopters. An image
displaced to the right in the right lens is displaced nasally, so the base is in.

Question 45

The target of the left lens is displaced to the right, and the lines cross between the first
and second rings. The prism power and direction is:
a) 1 prism diopter base-in
b) 2 prism diopters base-out
c) 1.5 prism diopters base-in
d) 1.5 prism diopters base-out

Answer 45

d) Because the lines cross between the first and second rings, there are 1.5 prism diopters.
An image displaced to the right in the left lens is displaced temporally, so the base is out.

Question 46

Base-out prism is induced when:

a) the optical center separation is greater than the pupillary distance with minus lenses
b) the optical center separation is less than the pupillary distance with minus lenses
c) the optical center separation is less than the pupillary distance with plus lenses
d) the optical separation is offset vertically with plus or minus lenses

Answer 46

b) This one is tough. Remember that a minus lens is made up of prisms aligned apex-toapex. If the pupillary distance is wider than the optical center separation, this would place
the visual axis temporal to the centers (Figure 5-2). Hence, base-out

Question 47

To check a manual lensometer for accuracy:

a) adjust the eye piece then read a trial lens
b) set the target lines for your refractive error, then see if they are clear
c) set the eye piece at zero, then read a trial lens
d) set the eye piece at zero, and see if the target lines clear at plano

Answer 47

a) An accurate lensometer should read a trial lens exactly. Be sure to adjust the eye piece
first, however.

Question 48

All of the following pertain to maintenance of a manual lensometer except:

a) reset the eye piece to zero after each use
b) turn off the instrument when not being used
c) cover the instrument when not in use
d) do not put too much ink on the ink pad

Answer 48

a) It might be convenient for the next user if you set the eye piece to zero, but it makes no
difference in the maintenance of the instrument. Be sure to read all questions carefully!