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22
MD
()
=
()
J D
22
()
=−
()
×
()
θ
J D
45
22
()
=−
()
×
()
.
θ
Cy
()+()
JJ
A
=×
()
½/.JJ
45 0
https://t.me/med1917
S. R. Bharadwaj
have non-zero values only for the J0 component,
while those with oblique astigmatism will have
non-zero values for both, the J0 and J45 components. Effectively, an astigmatic vector along any
axis is now referenced to two standard crosscylinder axes, enabling them to be compared
meaningfully. For the two patient visits noted
above with oblique astigmatisms, the J0 and J45
terms will be −0.38 D and +0.32 D, respectively,
for the rst visit and +0.50 D and +0.86 D,
respectively, for the second visit. It is now meaningful to surmise that astigmatism has indeed
increased in the second visit of the patient relative to the rst visit, even while the spherical
equivalent of refraction has remained unaltered.
spherical component
cylindrical component /2
+
0
The reverse, i.e., reconstructing the magnitude
and axis of astigmatism from the J0 and J45 power
vector terms, is easily possible by applying the
formulae shown in Eqs. (2.4) and (2.5) [24].
Cylindrical component /cos
Cylindrical component /sin
(2.1)
(2.2)
(2.3)
exception, use infrared light to estimate the
refractive error of the eye. This is in contrast to
retinoscopy, which uses visible light. Given the
ability of infrared light to penetrate deeper into
tissues vis-à-vis visible light [27], the plane of
reection of the input light source is likely to be
from the retino-choroidal junction for infrared
light, while it may be from the vitreoretinal junction for visible light. Assuming that the average
thickness of the retina in humans is ~250μm, the
difference in the plane of reectance between
visible and infrared light will translate into
approximately 0.5 D of variation in the refractive
power of the eye [28]. This value scales inversely
with the eye’s axial length, resulting in up to 2 D
of error in refractive power estimation for infants
[28]. This error, popularly known as the “small
eye artifact,” will result in autorefractors overestimating myopia by ~0.5 D and ~2.0 D in adults
and infants, respectively, relative to retinoscopy
[28]. While the quantum of over/underestimation
of refractive error will obviously depend on the
thickness of the retina, the axial length of the eye,
and the wavelength of infrared light used, clinicians should appreciate the existence of the small
eye artifact and factor this into the subjective
refraction techniques they pursue to nalize the
refractive error correction prescribed to the
patient.
linder component =− ×√
xis of astigmatismatan
2.3.3 Impact ofMeasurement
Wavelength ontheAccuracy
ofAutorefraction
While the measurement technique may vary
across autorefractors, one common factor is the
wavelength of light used for estimating the
refractive error. All autorefractors, without
2
0
2
45
2
(2.4)
2.4 Technique
2.4.1 General Guidelines
forObtaining Measurements
(2.5)
Most autorefractors are straightforward “point
and shoot” devices that can be operated by minimally trained technicians following a quick
orientation session. The general steps involved in
obtaining measurements using these devices
involve: (1) aligning the patient to the optical axis
of the measurement device, which is easier when
the device has a built-in chin/forehead rest than
handheld devices; (2) ensure clear focus of the
pupil; (3) offer clear instructions to the patient to
Using Autorefractors
abc
2 Autorefraction: Objective Estimation ofRefractive Error
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23
stably xating on the visual target, avoid blinking
during the measurements, and relax their focus as
much as possible; and (4) obtain multiple measurements of the eye’s refractive error to ensure
reliable results.
In this general context, the following points
may be considered to optimize the obtained measurements. Most monocular measurement
devices involve the patient looking through a narrow turret and xate on a target projected inside
this turret. Once the patient achieves this, the
examiner must align the measurement graticule
along the horizontal and vertical axes to the
patient’s pupil and avoid spurious measurements
from misaligned optics. The measurement system may also be moved back and forth from the
patient to ensure appropriate focus; optimal focus
may be determined automatically by the instrument (e.g., the measurement graticule turning
from red to green), or it may be qualitatively
judged by the examiner noticing the sharpest
focus of the rst Purkinje image and/or the surrounding iris tissue. The examiner must take
additional care when using binocular measurement devices like the open-eld autorefractor or
the photorefractor, for the chances of device misalignment are greater with these devices vis-à-vis
their monocular counterparts. For the open-eld
autorefractor, even while viewing is binocular,
the measurement is made monocularly, and all
aforementioned cautionary steps should be followed for optimal measurements. For the hand-
held photorefractor, it is important that the device
is placed on an axis with the patient’s face and
centered around the mid-point between the two
eyes. Care must be exercised to ensure that the
images of both eyes are visible in the designated
measurement window of the photorefractor and
that each eye is equidistant from the edges of this
measurement window.
Recent studies on the spasm of near reex, a
binocular vision dysfunction characterized primarily by pseudo-myopic refraction [29, 30],
have demonstrated large short-term variations in
the refractive error status of the eye (Fig.2.4a–c)
[15, 17]. Such uctuations have been deemed a
sensitive non-cycloplegic marker for this dysfunction [15, 17]. While these were identied in
the research laboratory using a dynamic photorefractor with a high temporal sampling rate
(50Hz) [15, 17], they could also be identied in
routine clinics by taking repeated measurements
of refraction using a standard autorefractor. Many
autorefractors default to taking multiple measurements, the average of which is deemed as the
sphero-cylindrical refractive error of the patient.
In addition to noting this mean value, the examiner should also pay attention to the variations in
the refractive power over repeated measurements
to identify conditions like the spasm of the near
reex. This dysfunction can then be cross- veried
through a vacillating reex in retinoscopy, and a
nal diagnosis may be made through cycloplegic
refraction.
Fig. 2.4 Raw data of the accommodative responses during steady xation plotted as a function of time in a representative emmetropic and myopic control subject (Panels
a, b) and a patient with the accommodation variant of the
spasm of near reex (SNR-A, Panel c). The accommodative responses of the controls show minor uctuations in
the refractive error of the eye arising from microuctua-
tions of accommodation [31]. In contrast, the uctuations
of accommodation are signicant in a patient with a
spasm of near reex. Note that the y-axis scales are different in panels (a–c): each tick mark represents 0.5 D of
change in refractive error in Panels (a) and (b), while it
represents 1 D of change in refractive error in Panel (c).
(Figure adapted from Bharadwaj etal. [17])
24
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S. R. Bharadwaj
2.5 Can Autorefractors Replace
Retinoscopy inClinical
Practice?
With the evolution of autorefractor technology
combined with articial intelligence and machine
learning models in predicting the eye’s spherocylindrical refractive error [32], is it time to bid
adieu to retinoscopy as an objective refraction
technique? After all, the accuracy and precision
of refractive error estimation using retinoscopy is
an art in itself, potentially requiring several years
of practice [21, 22]. The current evidence on the
performance of autorefractors, independent of
their design and operating principle, is that they
work with good accuracy and precision for regular sphero-cylindrical refractive errors. Their performance in hyperopes and children with strong
accommodative tendencies becomes sub- optimal,
and their performance in individuals with complicated optics has not been evaluated yet. In all
these cases, retinoscopy remains the preferred
technique for estimating the eye’s spherocylindrical refractive error. Therefore, eye care
institutions/hospitals and training programs
should continue to invest in experienced retinoscopists and training in retinoscopy, even while
autorefractors may replace/supplement retinoscopy for estimating the uncorrected refractive
error in routine cases.
ability than other autorefractors, it may be used
for mass screening due to the portability of the
instrument and its ability to measure both eyes
simultaneously. Open-eld autorefractor designs
offer better control over the eye’s accommodative
status and may be useful for estimating the
peripheral refraction of the eye. Refractive errors
calculated using autorefractors may be slightly
biased in the myopic direction relative to retinoscopy, owing to the infrared light used for the estimation. Such a bias may be larger in children
than adults. Astigmatism is a vectorial quantity
and their magnitudes cannot be compared without considering the axes. Vector decompensation
techniques must be adopted for legitimate comparison of astigmatic values. Finally, while
autorefractors may replace retinoscopy for
refractive error estimation in routine cases, the
latter remains the gold standard for patients with
strong accommodative tendencies (e.g., the pediatric population), complicated optics, certain disease conditions and media opacities where
autorefractions may be out of bounds or produce
erroneous results.
Funding Hyderabad Eye Research Foundation.
Disclosure None.
References
2.6 Conclusions
Autorefractors vary signicantly in their measurement principles and instrument design, and
they continue to evolve in their technology and
measurement accuracy/repeatability. Automated
refraction shows minimal bias and good shortterm repeatability, relative to retinoscopy.
However, the inter-instrument variability can be
as large as ±2.0 D for spherical error and ±1.0 D
for cylindrical error in select cases. Autorefractors
based on Scheiner disc, best focus, or ray deection principles are well-suited for routine objective refraction in the clinic. They are less suited
for eye screenings owing to their bulky designs.
Although photorefraction shows poorer repeat-
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global prevalence of blindness and distance and near
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2. Hashemi H, Fotouhi A, Yekta A, et al. Global
and regional estimates of prevalence of refractive
errors: systematic review and meta-analysis. J Curr
Ophthalmol. 2018;30(1):3–22.
3. Coles-Brennan C, Sulley A, Young G.Management
of digital eye strain. Clin Exp Optom.
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4. Heus P, Verbeek JH, Tikka C.Optical correction of
refractive error for preventing and treating eye symptoms in computer users. Cochrane Database Syst Rev.
2018;4(4):CD009877.
5. Kidd Man RE, Fenwicick EK, Sabanayagam C, etal.
Prevalence, correlates, and impact of uncorrected
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6. Tahhan N, Papas E, Fricke TR, et al. Utility and
uncorrected refractive error. Ophthalmology.
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7. Padhy D, Bharadwaj SR, Nayak S, etal. Does the
accuracy and repeatability of refractive error estimates depend on the measurement principle of autorefractors? Transl Vis Sci Technol. 2021;10(1):2.
8. Venkataraman AP, Brautaset R, Dominguez-Vicent
A.Effect of six different autorefractor designs on the
precision and accuracy of refractive error measurement. PLoS One. 2022;17(11):e0278269.
9. Thibos LN, Applegate RA, Schwiegerling JT, et al.
Standards for reporting the optical aberrations of eyes.
J Refract Surg. 2002;18(5):S652–60.
10. Bruce AS, Catania LJ. Clinical applications of wavefront refraction. Optom Vis Sci.
2014;91(10):1278–86.
11. Ferreira A, Vieira R, Maia S, et al. Photoscreening
for amblyopia risk factors assessment in young children: a systematic review with meta-analysis. Eur J
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12. Bharadwaj SR, Sravani NG, Little JA, etal. Empirical
variability in the calibration of slope-based eccentric
photorefraction. J Opt Soc Am A Opt Image Sci Vis.
2013;30(5):923–31.
13. Sravani NG, Nilagiri VK, Bharadwaj
SR. Photorefraction estimates of refractive power
varies with the ethnic origin of human eyes. Sci Rep.
2015;5:7976.
14. Bharadwaj SR, Candy TR. Cues for the control of
ocular accommodation and vergence during postnatal
human development. J Vis. 2008;8(16):14–6.
15. Bharadwaj SR, Roy S, Satgunam P. Spasm of near
reex: objective assessment of the near-triad. Invest
Ophthalmol Vis Sci. 2020;61(8):18.
16. Patel AM, Kumar P, Vaddavalli PK, et al.
The value of eccentric infrared photorefraction in evaluating keratoconus. Optom Vis Sci.
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17. Bharadwaj SR, Ravisankar C, Roy S, Satgunam
P. Fluctuations of steady-state accommodation is a
marker for screening spasm of near reex. Transl Vis
Sci Technol. 2021;10(11):9.
18. Anderson HA, Glasser A, Stuebing KK, Manny
RE. Minus lens stimulated accommodative lag as a
function of age. Optom Vis Sci. 2009;86(6):685–94.
19. Fedtke C, Ehrmann K, Holden BA. A review of
peripheral refraction techniques. Optom Vis Sci.
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20. Erdinest N, London N, Lavy I, etal. Peripheral defocus and myopia management: a mini-review. Korean
J Ophthalmol. 2023;37(1):70–81.
21. Bharadwaj SR, Malavita M, Jayaraj J. A psychophysical technique for estimating the accuracy
and precision of retinoscopy. Clin Exp Optom.
2014;97(2):164–70.
22. Goss DA, Grosvenor T. Reliability of refraction—a literature review. J Am Optom Assoc.
1996;67(10):619–30.
23. Major E, Dutson T, Moshirfar M. Cycloplegia in
children: an optometrist’s perspective. Clin Optom
(Auckl). 2020;12:129–33.
24. Thibos LN, Wheeler W, Horner D.Power vectors: an
application of Fourier analysis to the description and
statistical analysis of refractive error. Optom Vis Sci.
1997;74(6):367–75.
25. Alpins NA.A new method of analyzing vectors for
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1993;19(4):524–33.
26. Harris WF, Evans T, van Gool RD.Quantitative analysis of eyes and other optical systems in linear optics.
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27. Delori FC, Pibsen KP. Spectral reectance of the human ocular fundus. Appl Opt.
1989;28(6):1061–77.
28. Glickstein M, Millodot M.Retinoscopy and eye size.
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29. Roy S, Bharadwaj SR, Patil-Chhablani P, Satgunam
PN.Spasm of near reex: a comprehensive management protocol and treatment outcomes. J AAPOS.
2021;25(3):162 e1–6.
30. Hyndman J. Spasm of the near reex: literature
review and proposed management strategy. J Binocul
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31. Charman WN, Heron G. Microuctuations in
accommodation: an update on their characteristics and possible role. Ophthalmic Physiol Opt.
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2022;15(Suppl 1):S22–31.
Retinoscopy
https://t.me/med1917
DeepakKumarBagga
andJ.MargaretWoodhouse
3
3.1 Introduction
Retinoscopy is a manual, objective method of
evaluating the refractive status of an individual’s
eye. Accurate, objective refraction can signicantly reduce the time taken for subjective refraction. It is an essential eye testing procedure for
young children, people with disabilities, dementia, etc., and who may not be able to co-operate
for autorefraction and may be unable to respond
to subjective refraction. Retinoscopy is useful in
many ways as, in addition to refraction, it also
provides information about optical aberrations
and media opacity. Retinoscopy is performed
using a retinoscope, which emits a line or “streak”
of light into a patient’s eye; the reection of this
light is used to gauge refraction objectively. Spot
retinoscopy, which employs a circle of light, is
less commonly used. This chapter describes the
fundamental techniques of streak retinoscopy.
3.2 History
In the mid-nineteenth century, several attempts
were made to measure the refractive status of
the eye by observing the shadows cast within a
person’s eye; these were called “shadow tests.”
The development of retinoscopy is illustrated
in Table3.1. For further details on historical
perspectives, readers can refer to published
texts [1, 2].
D. K. Bagga (*)
Institute for Vision Rehabilitation, Kallam Anji Reddy
Campus, L V Prasad Eye Institute, Hyderabad, India
e-mail: dkbagga@lvpei.org
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_3
J. M. Woodhouse
School of Optometry and Vision Sciences, Cardiff
University, Cardiff, UK
e-mail: Woodhouse@cardiff.ac.uk
27
28
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Table 3.1 Evolution of retinoscopy
Year Contributors Evolution of retinoscopy
1859 Sir William
Bowman
1873 Ferdinand
1880 Henry parent Used lenses to determine refractive errors and coined the term “retinoscopie”;
1903 Alexander
1920 Jack
a
Ferdinand Cuignet is acknowledged as the “father” of retinoscopy
b
Jack C.Copeland as the “father” of streak retinoscopy
a
Cuignet
Duane
C.Copeland
Described an objective technique for detecting astigmatism in keratoconus using an
ophthalmoscope for the rst time
Used a simple mirror ophthalmoscope to assess refractive errors qualitatively by
observing the movement of reex and described it as “keratoscopie”
however, later changed it to “skiascopie” (skia means shadow)
Introduced the use of cylindrical lenses for retinoscopy in astigmatic errors
Devised a bulb that produced linear beams of light and additional features for a
b
rotating bulb, changing the direction of light beam—The streak retinoscope
D. K. Bagga and J. M. Woodhouse
3.3 Equipment
3.3.1 Retinoscope
Historically, the early retinoscope designs had a
plane/concave mirror with a peephole in the center for the examiner to view the movement of
light reected from the patient’s retina, known as
the “reex.” While performing retinoscopy, a
candle, or later an incandescent bulb, was used as
a light source. The light source would be positioned above the patient’s head, and the retinoscope’s mirror would direct light into the patient’s
eye. The examiner observed the shape, intensity,
and direction of movement of the reex in the
patient’s pupil to estimate the refractive status.
Later, spot retinoscopes were developed with an
inbuilt light source, increasing the convenience
and portability of retinoscopes. These selfilluminated retinoscopes opened the opportunity
for community eye health programs as they could
be carried to locations without electricity.
Jack C. Copeland developed the rst streak
retinoscope by accident. While Copeland was
using a spot retinoscope, it fell to the oor, damaging its bulb lament; the damaged lament
produced a linear reex and was the rst streak
retinoscope. Later, Copeland improved the
instrument and described various techniques for
using the streak retinoscope. Given Copeland’s
contribution, he is considered the “father” of
streak retinoscopy.
3.3.2 Description ofaStreak
Retinoscope (Fig.3.1)
Peephole: The rst component in the observation
system of a retinoscope is a peephole. The examiner observes the characteristics of the reex in a
patient’s pupil through the peephole in the
mirror.
Sleeve/collar: Up/downshifts of the sleeve
allow changes in the vergence of the light rays
leaving the retinoscope, and its rotation changes
the orientation of the light beam to observe the
reex in different meridians. In most retinoscopes, the sleeve at the bottom position provides
divergent rays, and when above, the para-stop
provides convergent rays (Fig.3.2a, c).
Para-stop: This is a mechanical guide for the
sleeve position, wherein the emitted rays from
3 Retinoscopy
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Para-stop
Sleeve
Handle,
includes battery.
Fig. 3.1 Streak retinoscope
29
Peephole
Head, includes
bulb, convex
lens, mirror
Rheostat
the retinoscope are parallel. Some clinicians prefer using a para-stop to ensure that they use the
plane mirror effect during the retinoscopy
(Fig.3.2b).
Rheostat: This is a variable resistor that is
used to control the brightness level and to turn on
the instrument.
Groove/magnetic cards: Target cards are
accessories for dynamic retinoscopy xation and
can be attached through the groove in the Heine
Beta 200 retinoscope (HEINE Optotechnik
GmbH & Co. KG, Germany). However, in the
Welch Allyn retinoscope (Welch Allyn Inc.
Corporate Headquarters, NewYork), the head’s
magnetic logo allows placing these dynamic retinoscopy cards.
Battery: The power system for a retinoscope
includes rechargeable or replaceable batteries.
There are also retinoscopes with power cables
that can be directly plugged into an electric power
supply for turning on their illumination system.
3.3.3 Trial Lens Set/Phoropter
In addition to a retinoscope, a phoropter or, a
range of trial lenses, and a trial frame are needed
to perform retinoscopy. For high refractive errors
(>5 D), it is important to note the vertex distance.
Therefore, a trial frame that allows measuring the
vertex distance of lenses must be preferred. Key
features of the phoropter include adjustable interpupillary distance, pantoscopic tilt, and highcontrast labeling of the axis on the outermost
cell. For refraction in infants, clinicians may prefer using lens racks.
30
a
Sleeveabove the para-stopposition
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D. K. Bagga and J. M. Woodhouse
Fig. 3.2 (a) The sleeve
is at the bottom position,
and the light source
(bulb) is positioned at a
shorter distance than the
principal focus of the
lens, producing
divergent rays. (b) The
sleeve is at the para-stop
position, and the light
source is positioned at
the principal focus point
of the convex lens,
causing emergent rays to
run parallel. This is the
plane mirror effect. (c)
The sleeve-up position is
where the distance
between the light source
(bulb) is more than the
focal length of the
convex lens, causing
convergent emerging
rays. If the emerging
convergent rays focus at
a point before entering
the patient’s eye, it is
considered the concave
mirror effect. In this
effect, the observed
movement of reex will
be reversed from those
observed during
retinoscopy with the
plane mirror effect
Peephole
Convex lens
b
Peephole
Convex lens
c
Mirror
Bulb
Sleevebelow the para-stop position
Mirror
Bulb
Sleeve at the para-stopposition
Mirror
Peephole
Convex lens
Bulb
Far point at infinity
ab
ab
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3.4 Basic Principles
During retinoscopy, the examiner uses a combination of trial lenses to bring the far point of the
patient’s eye to their test distance. Since the
examiner works quite close to the patient, the
patient is rendered articially myopic. The examiner can then derive the refractive error by adjusting the lens power to account for the induced
myopia. This means adding minus power (or subtracting plus power) equivalent to the dioptric
value of the examiner’s working distance (see
examples).
Example 1
In a person with emmetropia, the strength
of the convex lens corresponds to the work-
ing distance that will bring the person’s far
point from innity to the test distance
(Fig.3.3a, b) [1].
Example 2
In a person with myopia, the far point is
located at a nite distance (Fig.3.4a) [1]. If
a −2.50 D concave lens brings the far point
31
to 66.67cm (Fig.3.4b), and the net refractive error will be obtained by adding the
working distance correction (−1.50 D).
Therefore, this person has −4.00 D myopia
{−2.5 D + (−1.50 D)}.
Example 3
In another case, the examiner observes the
end point of retinoscopy with a +3.50 D
spherical convex lens at 66.67 cm
(Fig.3.5a, b) [1]. This case will have hypermetropia of +2.00 D {+3.50 D −1.50 D
(compensation for working distance)}.
Some practitioners prefer to begin retinoscopy
by putting an appropriate convex lens (in the
above examples, this would be +1.50 D) in the
trial frame before checking the reex. In the
end, the lens is simply removed to give the nal
correction. This lens, known as the working
distance lens, adds extra weight and reections
and takes up room in the trial frame, so it is best
avoided, and a simple calculation, as above, can
be carried out.
+1.50 D
66.67 cm
Fig. 3.3 (a) In an emmetropic eye, the retina and innity are conjugate foci. (b) Placing a +1.50 D sphere lens in trial
frame brings the far point to 66.67cm (100/1.5=66.67cm)
-2.50 D
25 cm 66.67 cm
Fig. 3.4 (a) In a myopic eye, the far point is within a nite distance. (b) Placing a −2.50 D spherical concave lens in a
trial frame brings the far point to 66.67cm
32
ab
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D. K. Bagga and J. M. Woodhouse
+3.5 D
66.67 cms
Fig. 3.5 (a) In hypermetropic eyes, the far point is behind
the eye. (b) A convex lens is used to obtain the end point
of retinoscopy in cases with hypermetropia. The magni-
A working distance lens also restricts the
examiner to remain at a xed distance during
retinoscopy. Once close to the endpoint, experienced practitioners sometimes nd it useful to
vary the working distance to nd the neutral
reex rather than make minor changes to the
lenses and then subtract the appropriate sphere
power for the nal working distance. This speeds
up the process but does require the practitioner to
have a precise perception of distances.
tude of hypermetropia is determined by subtracting the
power corresponding to the test distance
3. The movement of the reex in relation to the
intercept—“with” or “against” (Figs.3.7 and
3.8).
4. Speed of reex: the speed of the reex motion
in relation to the movement of the streak indicates the magnitude of the refractive error. A
“slow” speed indicates high refractive error,
and a “fast” speed indicates low refractive
error. Therefore, near the endpoint of retinoscopy, the relative speed of the reex increases
signicantly [3].
3.5 Essentials ofRetinoscopy
3.5.2 Endpoint ofRetinoscopy
3.5.1 Reex
The endpoint of retinoscopy is a “neutral reex,”
When performing retinoscopy, the examiner
moves the streak light beam across the patient’s
eye perpendicular to the streak, with the sleeve/
collar at its lowest position (divergent beam).
This light beam creates an illuminated patch on
the patient’s retina, and this light reects and
emerges after passing through the optical system.
During the retinoscopy, the examiner observes
the image of this illuminated patch in the pupillary area, referred to as the reex (Fig.3.6).
The beam of light seen outside the pupillary
area on the patient’s face is referred to as the
intercept. During retinoscopy, the examiner
observes the following characteristics of the
reex to estimate the refractive error:
1. Thickness in relation to the intercept.
2. Brightness: a brighter reex suggests low
refractive error, and a dull reex suggests high
refractive error.
that is, a reversal of the reex’s movement. The
endpoint can be quickly conrmed by moving
the retinoscope slightly further away from the
patient to produce an “against” movement and
closer to produce a “with” movement. The endpoint is, therefore, a transition from one to the
other.
3.5.3 Astigmatism
The lenses that neutralize the movement in astigmatism will differ in the two principal meridians.
The examiner turns the sleeve/collar to align the
reex with the intercept in one meridian and neutralizes the movement. Then, the examiner turns
the streak 90° to the opposite meridian and uses
cylindrical lenses aligned along the streak to neutralize the movement. If the examiner chooses to
work with a plus cylinder, the least plus/most
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