subjective refraction

1. 4THCOFFEE
D R R E S H M A P E T E R
SUBJECTIVE
CORRECTION OF
REFRACTION

2. 4THCOFFEESubjective refraction
• Relies on cooperation of patient as well as his responses to determine refractive
correction
• Consists of 3 distinct phases
1. To correct spherical element of Ref Error in a way as to facilitate accurate determination
of any astigmatic element
2. Determination of a) astigmatic error
b) astigmatic axis
3. Balancing and / or modification of refractive correction to ensure optimal visual
performance and patient comfort

3. 4THCOFFEE
UNCORRECTED MYOPIA
Symptoms
• Blurred distance vision
• Headaches from screwing up the eyes to try obtain clearer vision by the pinhole effect
• Clear near vision
Signs
• Poor distance vision on a letter chart
• Good near vision on a near test chart.

4. 4THCOFFEEUNCORRECTED HYPERMETROPIA
Symptoms
• Eyestrain, especially for close work, caused by accommodative effort to form a clear image
• Blurred vision with medium-to-high amounts of hypermetropia and in advancing age
(blurred vision is not usually a problem with low amounts of hypermetropia).
Signs
• usually no signs in low hypermetropia
• screwing up the eyes and wrinkling of the brow may be seen in high amounts of
uncorrected hypermetropia
• a nasalward deviation (esotropia) of one eye in high amounts of uncorrected
hypermetropia.

5. 4THCOFFEE
Manifest refraction (dry refraction)
Refraction w/out cycloplegic drops
Cycloplegic refraction (wet refraction )
Refraction done w/ cycloplegic drops given to dilate the pupils and prevent
accommodation.
Post-cycloplegic refraction( Dry refraction performed at least several days after wet
refraction)
Purpose is to see how much of the full cycloplegic refraction found on the previous
visit can be tolerated.

6. 4THCOFFEE
• Ideally, Ref Error measured w/ accommodation relaxed.
• Amount of habitual accommodative tone varies from person to person, and even within
an individual it varies at times and with age.
A practical approach to satisfactory refraction is
1.Careful noncycloplegic (or manifest) refraction, ensuring relaxed accommodation with
fogging or other nonpharmacologic techniques.
2.If the results are inconsistent or variable  a cycloplegic refraction
3.If the findings of these 2 refractions are similar, the prescription may be based on the
manifest refraction.
4.If there is a disparity  a post cycloplegic evaluation
• Most children require cycloplegic refraction because of their high amplitude of
accommodation.
Cycloplegic and Noncycloplegic Refraction

7. 4THCOFFEE
In subjective refraction,
1. Measure Interpupillary Diameter (IPD)
2. Make the correction for your working distance
• add –1.50 DS for 2/3 metre or remove the 'working distance lens’ if one was used
3. Occlude the fellow eye
• unless nystagmus is present in which case use a fogging technique
4. Since patients usually do not tolerate the full cylindrical correction, it helps to reduce the cylinder by
approx ¼th of its value before starting the subjective examination.

8. 4THCOFFEE
5.Using the distance test type, 1st verify the sphere
• offer small plus and minus additions until no further improvement can be made.
• Patients w/ good VA can appreciate a diff of 0.25 DS
• Patients w/ poor VA may only appreciate larger increments, e.g. 0.50 DS.
6.Next verify the axis of the cylinder before adjusting its power.
• If a large change is found in the cylinder, it is better to recheck the sphere.
7.If the patient is myopic, Duochrome test should be done monocularly and binocularly
8. An adjustment to the spherical power must be made for every half-diopter change in
cylinder power
For every +0.50 diopter of cylinder power added, remove +0.25 diopter from the sphere.

9. 4THCOFFEE8. Record the prescription and acuity for each eye, and the binocular acuity.
9. Measure back vertex distance (BVD) if Spherical lens power > 5D
10. Use the Maddox rod to check the muscle balance for distance.
• Occlude each eye in turn to ascertain that the spot and line are visible to the appropriate eye uniocularly,
and then uncover both eyes and see if the patient is able to perceive them simultaneously.
• If the patient sees them one at a time uniocularly but not simultaneously with both eyes uncovered, his
binocular vision must be defective or absent.

10. 4THCOFFEE
10. In cases where the Maddox rod test reveals a significant extraocular muscle imbalance,
especially a vertical one, check to see if the addition of the appropriate prism improves the
patient's binocular comfort and acuity.
Full value of prism as found by the Maddox rod test is rarely required.
11. If the patient is presbyopic, calculate the likely reading addition and add this to the
distance lenses in the trial frame.
In practice Reading addition is estimated by rule-of-thumb from the patient's age: Beware
of prescribing too great a reading addition.
• The most frequent reason that patients seek a retest is that too strong a near addition has been prescribed
• In normal circumstances not more than +2.50 DS addition should be given.
• However, pseudophakic patients often prefer a +3.00 DS addition.
• Record the near acuity for each eye alone and binocularly.

11. 4THCOFFEE
12. Use the Maddox wing to check the near muscle balance.
13. Orthophoria for distance but a large exophoria for near indicates convergence
insufficiency.
• This may cause such symptoms as headache or eye-strain after close work, or it may be asymptomatic.
• If the patient has symptoms, the convergence should be strengthened by means of convergence exercises.
• If base-in prisms are prescribed, the convergence may become weaker still, and progressively stronger
prisms will be required.
• Vertical muscle imbalances for near may require prismatic correction, but again the full value of prism is
rarely required or tolerated.

12. 4THCOFFEE
Anatomical IPD measurement Method
1. A mm rule is rested across bridge of the patient's nose
2. Patient asked to look at the examiner’s LE.
3. Zero of ruler is aligned w/ nasal limbus of patient’s RE
4. The patient is then instructed to look at the examiner’s RE and the position of the temporal
limbus of the patient’s LE is noted, giving the anatomical IPD
Measurements are taken from limbus to limbus to exclude inaccuracy due to differences
or changes in pupil size.
Alternatively,
1. A fixation light may be held in front of each of the examiner’s eyes in turn
2. A similar procedure is followed
3. The distance between the corneal light reflexes on the patient's eyes being measured.
Measurement of Interpupillary Distance

13. 4THCOFFEE• Important measurement in making spectacles is  distance b/w visual axes for distance
vision (approx Anatomical IPD - 1mm)
Unsuitable spectacle frame  Decentration  unwanted prismatic effect
• important in babies and small children, esp in high power lenses
• common cause of intolerance of aphakic glasses.

14. 4THCOFFEE
In best sphere technique without the aid of retinoscopy,
Find the max amount of positive power or the min amount of negative power that can
be tolerated by the eye, without causing blurring of the retinal image.
1. Occlude left eye
2. Measure the unaided vision gives estimate of the magnitude of any uncorrected
myopia or manifest hypermetropia
3. Estimate ametropia
these estimates are of no use if the patient is accommodating to self correct any
hypermetropia.
Best vision sphere

15. 4THCOFFEE
• Lens must be held in the plane of the trial frame and along the visual axis to avoid
inducing off-axis aberrations.
• It should also be moved quickly and precisely, allowing enough time in each position for
the patient to make a decision.
• May be needed to repeat this process a number of times to confirm the result
• Plus lenses either
blur the retinal image indicate max amount of positive power is already in place, or
relax accommodation(where it is in use).

16. 4THCOFFEE
• When a plus lens is held before the eye, the question to the patient should be
Is the target better w/ or w/out this lens, or is there no real difference
If ‘no difference’,better 
4. add positive power until no more can be tolerated.
5. From blur point, reduce by +0.25 DS.
Initial plus lens may be in the region of +1.00 DS. Later in the procedure, a +0.50 DS may
be used.
7. When total power is close to the endpoint; add spherical lens power in •+.25 DS steps.
8. Continue to add positive power until the addition of an extra +0.25 DS results in
blurring.
9. This final +0.25 DS is discarded and the remaining lens is the BEST VISION SPHERE.

17. 4THCOFFEE
The question required when adding negative power should be altered to:
Is the target clearer or just darker with this lens?
• When minus lenses are required care must be taken not to over-minus the patient,
which results in a stimulation of the patient’s accommodation.
• Should the target appear darker but not clearer, or smaller but not clearer ,extra minus
power should not be added because this just stimulates accommodation.
• A negative lens should be added to the trial frame only if the patient can resolve a
greater number of letters on the letter chart.

18. 4THCOFFEE
If a good retinoscopy has been performed,
1. Start with the right eye, the left being occluded. This is called a monocular
refraction.
2. Add -1.50 DS for 2/3 metre or remove the 'working distance lens’ if one was used
3. The procedure is repeated on the left eye with the right occluded.
• However, it is possible and often preferable to refract under binocular conditions.
• In both binocular and monocular refraction ,Control accommodation in the person
with pre-presbyopia, so a ‘fogging’ technique is employed
• FOGGING 
• Purpose is to avoid extra minus spherical correction by starting the refraction from a
position of extra plus spherical power.
• Plus power is then removed, stopping as soon as the letters on the acuity chart are
read correctly to find the final spherical power
Determination of the best vision sphere

19. 4THCOFFEE
• Results are often rechecked and confirmed throughout the test using the same or a
different technique, e.g. best sphere and duochrome because patient’s answers are
frequently inconsistent
• The endpoint is the max plus or min minus that the patient will tolerate without
causing blurring of the retinal image.

20. 4THCOFFEE
• 1. Pin hole
• 2.Duochrome Method
• 3. Simultan technique((using plus and minus Freeman twirls)
• 4. Scheiner disk
Refinement of the BVS after retinoscopy

21. 4THCOFFEE
• An opaque disc with a central circular aperture about 1 mm in diameter.
• Aperture < 1 mm  diffraction effects and reduction in retinal illumination result in
dim, unfocused image.
• Aperture > 2 mm approaches the size of some human pupils and so might not
significantly reduce the blur circle produced by an uncorrected RE
PINHOLE DISC

22. 4THCOFFEE
• In uncorrected ametropia, a distance point source of light produces a blurred image on
the retina composed of a series of blurred discs.
• The dimensions of a single blurred disc depend on
 degree of ametropia present
diameter of the individual’s pupil
distance of the point source from the eye
• A pinhole may be employed to reduce the diameter of these blurred discs and thus
improve the VA.
• A diameter of 1.32 mm was recommended by Lebesohn (1950) as optimally balancing
the opposing demands of reduction of the blur circle and diffraction, although the
aperture most commonly seen is rather smaller (1 mm).

23. 4THCOFFEE• If the pinhole is placed before an uncorrected ametropic eye, the VA should increase.
• Normally RE correction should improve the VA by at least as much as that produced by
the pinhole.
• Pinhole disc can th be used to estimate the max VA that the eye would obtain if RE were
to be corrected.
• If acuity does not improve through the pinhole, pathology is suspected, eg amblyopia,
macular disease and central media opacities.
• However, in an irregular cornea or peripheral media opacities ,the pinhole may give a
better result than can be achieved by refraction.
• If the pinhole fails to improve VA, the reason for the reduced acuity is unlikely to be
purely refractive.
• In practice, the pinhole disc test can prove very useful, especially if subjective
techniques are unsuccessful and VA does not improve with the addition of lenses.

24. 4THCOFFEE
Red-green test or bichrome test
• To verify the spherical endpoint
• Makes use of the chromatic aberration of the eye
• Of particular use in the refraction of myopic
• Cycloplegia may be necessary
• Test is sensitive to an alteration in refraction of 0.25 D or less.
• Not used with patients whose VA < 6/9, because the 0.50 D difference between the 2
sides is too small to distinguish.
• As this test is based on chromatic aberration and not on color discrimination, it is used
even with color-blind patients (although it may be necessary to identify the sides of the
chart as left and right rather than red and green).
Duochrome Test

25. 4THCOFFEE
• RI of various optical components vary with the wavelength of incident light,
light of longer wavelength (i.e. towards the red end of the spectrum)  longer
focal lengths than shorter wavelength light.
• Total amt of ocular chromatic aberration present -approx 2.50 D although
placing an achromatic doublet before the eye does not appear to improve VA
significantly.
• Chromatic aberration appears to be slightly reduced (about 0.30 D) with smaller
pupils and rather more (about 1.00 D) with accommodation
• Rabbetts (1998) estimated that yellow light with a wavelength of 570 nm was
preferred by the eye.
• If this wavelength is used as a reference point, green light (wavelength of 535
nm) focuses 0.25 D in front of the retina and red light (wavelength 620 nm)
focuses 0.25 D behind it.
• So, by using appropriate filters a test may be constructed that, by comparing the
clarity of targets presented on red and green backgrounds, allows the
practitioner to focus the yellow reference wavelength accurately on the retina
and achieve maximum acuity. Such a test is known as the duochrome test

26. 4THCOFFEE
• Because of the chromatic aberration of the eye, the shorter (green)
wavelengths are focused in front of the longer (red) wavelengths.
• The eye typically focuses near the midpoint of the spectrum, between the red
and green wavelengths.
Myopic eye sees the red letters more clearly than the green
Hypermetropic eye sees the green letters more distinctly
With optimal spherical correction, Letters on red and green halves appear equally sharp.

27. 4THCOFFEE
• 2 ranks of black Snellen letters, silhouetted against illuminated coloured glass.
• A split red-green filter makes the background of VA chart appear vertically divided into a
red half and a green half.
• • The filters used are Courtoid Red 15 and Green 16 and are 0.25 mm thick
• Red and green are used because their wavelength foci straddle the yellow–green by equal
amounts (approx 0.40 D on either side).
• Commercial filters used in test produce a chromatic interval of approx 0.50 D b/w red and
green wavelengths.
• When the image is clearly focused in white light, the eye is 0.25 D myopic for the green
letters and 0.25 D hyperopic for the red letters.

28. 4THCOFFEE
1.Each eye is tested separately for the duochrome test
2.Eye slightly fogged (by 0.50 D) to relax accommodation.
3.Patient views the letters by means of red and green light respectively, and can easily tell which
appear clearer.
4.Letters on red side should appear sharper; Clinician should add minus sphere until the 2 sides
appear the same.
5. If the patient responds that the letters on the green side are sharper, the patient is
overminused, and more plus power should be added.
RAM-GAP mnemonic—“red add minus; green add plus’.

29. 4THCOFFEE
An eye with overactive accommodation may still require too much minus sphere in
order to balance the red and green.
• If red letters are only marginally clearer than the green when viewed monocularly,
Green letters may be clearer when seen binocularly.
• A small reduction, e.g. +0.25 DS, to one or both eyes may be needed to make the red
letters clearer binocularly ensure that the patient will be comfortable, and not
accommodating, when wearing the prescription.

30. 4THCOFFEE
• Sequential presentation of plus and minus spheres, mounted together in a ‘twirl’ with a
handle, although individual trial lenses may be used.
• Lenses presented are normally•}0.25 D
• However, if VA < 6/9, it is unlikely that patient is able to differentiate reliably between
these low-powered lenses, so •}0.50, 0.75 or 1.00 D twirls may be required.
1. Plus lens must be presented for at least 1 sec to relax accommodation.
2. Minus lens should not be held for > 1 sec, which is reaction time plus response time for
accommodation. If this time is exceeded, it is likely that the patient will accommodate.
Simultan technique (using plus and minus Freeman twirls)

31. 4THCOFFEE
3. Patient is asked if letters are clearer with the 1st or 2nd lens or are they both the same?
If 1st lens clearer or both are same,
4. +0.25 DS is added to the trial frame.
5. Additional +0.25 DS lenses are added until the VA blurs.
6. End-point is the most plus or least minus that does not blur the VA.
If the second lens is clearer (as opposed to just smaller and darker),
7. 0.25 DS is added.
8. If acuity improves, add further minus lenses in 0.25 D steps only for as long as the
acuity continues to improve.

32. 4THCOFFEE
9.Ask: ‘Do the letters actually appear clearer or just smaller and blacker?’
10. If the letters look smaller and blacker but not clearer, or if patient reports no change or
a drop in acuity do not add the −0.25 DS,.
End-point is the most plus or least minus that does not blur the VA.
• The rapidity with which the minus lens must be withdrawn can cause problems when a
patient is slow to react.
• For this reason, many practitioners have modified the Simultan technique to eliminate
the minus lens completely.

33. 4THCOFFEE
• Adding plus only
• After initially determining that the sphere is a little (not more than 0.50 DS) under-
plussed by the duochrome or Simultan techniques, +0.25 DS is introduced
•
If vision clearer or identical with the plus lens,
• +0.25 DS is added to the sphere in the trial frame and the sequence is repeated by
presenting the +0.25 DS lens once more.
• If the patient rejects the plus lens, −0.25 DS is added to the sphere in the trial frame
and again the sequence is repeated.
• Accommodation may be induced when minus power is added but we are always
adding plus, and therefore relaxing accommodation, immediately before comparison
is made.
• Both this technique and the unmodified Simultan method are repeated until the
patient accepts no more plus without losing clarity.

34. 4THCOFFEE
• described by Christopher Scheiner in 1619
• opaque disc w/ 2 small circular apertures each 0.75 mm in diameter, 2–3 mm apart
and equidistant from the centre in opposite directions along a common meridian
• These dimensions allow light through both holes to enter the eye’s pupil.
USES
1. a subjective optometer used to detect and measure spherical ametropia
2. To demonstrate existence of accommodation
3. As the focusing mechanism in the one-position keratometer and some
autorefractors
The Scheiner disc

35. 4THCOFFEE
To detect and measure spherical ametropia,
1. Scheiner disc is placed before 1 eye only, the other eye being occluded.
2. If a point source of light at 6 m is viewed using the disc, image is formed through 2
different portions of the pupil.
3. If the viewing eye is emmetropic, the 2 images thus formed are coincident and
focused.
4. The individual sees a single spot of light

36. 4THCOFFEE
• Simple myopia causes the target to be imaged in crossed diplopia
• Hypermetropia without accommodation  uncrossed diplopia
• The separation of the two retinal images depends upon the degree of ametropia
present and the type of ametropia may be found by covering one of the two pinholes.

37. 4THCOFFEE
• if the upper pinhole is occluded, the lower retinal image disappears in a myopic eye.
• As a result of retinal inversion, this is perceived by the patient as the disappearance of
the upper image, whereas someone with hypermetropia reports the absence of the
lower one.

38. 4THCOFFEE
• Scheiner also described a version of the disc with 3 pinholes distributed in an equilateral
triangle.
• Type of RE could be determined by whether the observer saw an erect or inverted triangle
of images.
• Spherical lenses may be interposed b/w target and eye to bring diplopic images to
coincidence and thereby determine the ametropia of the eye along the selected meridian.
• may be useful with those patients unable to cope w/ more commonly employed methods.

39. 4THCOFFEE
Problems encountered when using the Scheiner disc include the following:
1. The disc is difficult to centre correctly.
2. The central part of the eye’s optical system is not used during the test.
3. The measurement of astigmatism is difficult.
4. The patient’s accommodation may not be relaxed during the test.

40. 4THCOFFEE
• A type of sphero-cylindrical lens used during refraction
• Popularised by Edward Jackson
USES
• To check the axis of the cylinder prescribed and then its
power.
• To verify that no cylindrical correction is necessary for the patient if no cylinder was
detected on retinoscopy.
The Cross-Cylinder

41. 4THCOFFEE
Power of the cylinder is twice the power of the sphere and of the opposite sign.
• Net result is the same as superimposing 2 cylindrical lenses of equal power but
opposite sign with their axes at right angles.
• Lens is mounted on a handle which is placed at 45° to the axes of the cylinders.
• Cross-cylinders are named by the power of the cylinder, and this is marked on the
handle

42. 4THCOFFEE
• The axes marked on the lens are the axes of no power of the individual cylinders.
• The power of each cylinder lies at 90° to the marked axis and coincides with the marked
axis (of no power) of the other cylinder (of opposite sign)
• Crosscylinders are available in two powers, 0.50 and 1.00 D.
1 D cross-cylinder  to check axis of trial cylinder, and power in patients w/ poor VA.
0.50 D cross-cylinder  to check power of trial cylinder in patient w/ good VA.

43. 4THCOFFEE
• Orientation of the trial cylinder checked by superimposing another cylinder with its axis
lying obliquely to the axis of the trial cylinder.
• Power of cylinder can be checked by superimposing further cylinders of varying power
and sign in the same axis as the trial cylinder.
Cross-Cylinder Technique
1. Adjust the sphere to yield the best VA with accommodation relaxed.
2. Place the prescription the patient is wearing into a trial frame
3. Fog the eye to be examined with plus sphere while the patient views a VA chart; then
decrease the fog until the best VA is obtained.
4. Then use test figures that are 1–2 lines larger than the patient’s best VA. (the cross-
cylinder blurs the vision and larger letters are used to make discrimination between
the positions of the cross-cylinder easier for the patient)
5. Introduce the cross cylinder,
first for refinement of cylinder axis
then for refinement of cylinder power

44. 4THCOFFEE
To check the axis,
• the cross-cylinder is held before the eye with its handle in line with the axis of
the trial cylinder.
• The cross-cylinder is turned over and the patient asked which position gives a
better visual result.
• The cross-cylinder is held in the preferred position and the axis of the trial
cylinder rotated slightly towards the axis of the same sign on the cross-cylinder.
• The process is repeated until the trial cylinder is in the correct axis for the eye,
at which time rotation of the cross-cylinder will offer equally unacceptable
visual alterations to the patient.

45. 4THCOFFEE
To check the power of the trial cylinder
• If no cylindrical correction is present initially, cross cylinder may be placed at
90° and 180°, to check for the presence of astigmatism.
• If a preferred flip position is found, cylinder is added with the axis parallel to
the respective plus or minus axis of the cross cylinder until the 2 flip choices are
equal.
• If no preference is found with the cross-cylinder axes at 90° and 180°, then
check the axes at 45° and 135° before assuming that no astigmatism is present.
• Once any cylinder power is found, axis and power should be refined in the
usual manner.

46. 4THCOFFEE
• Another method of determining the presence of astigmatism is to dial 0.50 D of
cylinder into the phoropter (while preserving the spherical equivalent with a
compensatory 0.25 D change in the sphere).
• Ask the patient to slowly rotate the cylinder axis once around using the knob on the
phoropter.
• If doing so has no effect, there is no clinically significant astigmatism.
• If the patient finds a preferred position, it becomes the starting point for the cross-
cylinder refinement.

47. 4THCOFFEE
• To achieve the best results from the test it is important that the patient has the
clearest vision possible before the cross-cylinder is used.
• Ensure that the correction remains constant by changing the sphere half as
much and in the opposite direction as the cylinder power is changed.
• In other words, for every 0.50 D of cylinder power change, change the sphere
by 0.25 D in the opposite direction.
• Periodically, the sphere power should be adjusted for the best visual acuity.

48. 4THCOFFEE
The stenopeic slit is an opaque trial lens with an oblong slit whose width forms a
pinhole with respect to vergence perpendicular to the slit.
Uses –
If an examiner is unable to decipher the astigmatism by routine retinoscopy in
In irregular astigmatism
unclear media (lenticular or corneal opacities)
small pupils
Stenopeic Slit Technique

49. 4THCOFFEE
• RE neutralized with spherical lenses and the slit at various meridians to find a
spherocylindrical correction.
• This correction can then be refined subjectively.
• If the subject can accommodate, fog and unfog using plus sphere to find the most plus
power accepted.
• Then turn the slit until the subject says the image is sharpest. If, for example, –3.00 D
sphere is best there, when the slit is oriented vertically, this finding indicates –3.00 D at
90° in a power cross.
• If the best sphere with the slit oriented horizontally is –5.00 D, then the result is –3.00 –
2.00 × 90.

50. 4THCOFFEE• Astigmatic dial -a test chart with radially arranged lines
• To determine the axes of astigmatism.
• A pencil of light from a point source is imaged by
an astigmatic eye as a conoid of Sturm.
• The spokes of the astigmatic dial that are parallel to
the principal meridians of the eye’s astigmatism are seen as
sharp lines, which correspond to the focal lines of the conoid
of Sturm.
Astigmatic Dial Technique
Conoid of Sturm and retinal image of an
astigmatic dial as viewed by an eye with
compound hyperopic astigmatism
Figure shows how an eye with compound hyperopic astigmatism sees an astigmatic dial.
The vertical line of the astigmatic dial is blackest and sharpest as vertical focal line of
each conoid of Sturm is closer to the retina than the horizontal focal line is.
By accommodating, however, the patient might pull both focal lines forward, far enough
to make even the horizontal line of the astigmatic dial clear.

51. 4THCOFFEE
• To avoid accommodation, fogging is used.
• Sufficient plus sphere is placed before the eye to pull both focal lines into the
vitreous, creating compound myopic astigmatism
• Because accommodating with the eye fogged causes increased blurring of the
lines, the patient relaxes accommodation.
• The focal line closest to the retina can then be identified with certainty as the
horizontal line because it is now the blackest and sharpest line of the astigmatic
dial

52. 4THCOFFEE
• After examiner locates 1 of the principal meridians of the astigmatism, Conoid of Sturm
can be collapsed by moving the ant focal line back toward the post focal line.
• This is done by adding a minus cylinder with an axis parallel to the anterior focal line.
• In Fig the vertical focal line has been moved back to the position of the horizontal focal
line and collapsed to a point by the addition of a minus cylinder with an axis at 90°.
• Notice that the minus cylinder is placed with its axis perpendicular to the blackest
meridian on the astigmatic dial.
• Also note that as the conoid of Sturm is collapsed, the focal lines disappear into a point
focus.
Conoid of Sturm collapsed to a single point

53. 4THCOFFEE
• All of the lines of the astigmatic dial now appear equally black but still are not
in perfect focus, because the eye remains slightly fogged to control
accommodation.
• At this point, a visual acuity chart is used; plus sphere is removed until the best
visual acuity is obtained (Fig ).
Minus sphere is added (or plus
sphere subtracted) to produce a
sharp image and a visual acuity
chart is used for viewing

54. 4THCOFFEE
In summary, the following steps are used in astigmatic dial refraction:
1. Obtain the best VA using spheres only.
2. Fog the eye by adding plus sphere.
3. Ask the patient to identify the blackest and sharpest line of the astigmatic dial.
4. Add minus cylinder with the axis perpendicular to the blackest and sharpest line
until all lines appear equal. (If using a positive cylinder phoropter, add plus cylinder
with the axis parallel to the blackest and sharpest line until all lines appear equal.)
5. Reduce plus sphere (or add minus) until the best visual acuity is obtained.

55. 4THCOFFEE
• Astigmatic dial refraction can also be performed with plus cylinder equipment,
but this technique must be used in a way that simulates minus cylinder effect.
• As each 0.25 D of plus cylinder power is added, change the sphere
simultaneously by 0.25 D in the minus direction.
• Doing so simulates minus cylinder effect exactly by moving the anterior focal
line posteriorly without changing the position of the posterior focal line.

56. 4THCOFFEE
• After cylinder power and axis have been determined,final step of determining
monocular refraction is to refine the sphere.
• Endpoint - strongest plus sphere, or weakest minus sphere, that yields the best VA
• When the cross-cylinder technique has been used to determine the cylinder power
and axis, the refractive error is presumed to a single point.
• Add plus sphere in +0.25 D increments until the patient reports decreased vision.
• If no additional plus sphere is accepted, add minus sphere in –0.25 D increments
until the patient achieves the most optimal visual acuity.
Refining the Sphere

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• Using accommodation, Patient can compensate for excess minus sphere.
• Therefore, it is important to use the least minus sphere necessary to reach the best VA
• In effect, accommodation creates a reverse Galilean telescope eye generates more plus
power as minus power is added to the trial lenses before the eye.
• As this minus power increases, patient observes that the letters appear smaller and more
distant.
• The patient should be told what to look for.
• Before subtracting each 0.25 D increment, tell patient that letters may appear sharper
and brighter or smaller and darker, and ask patient to report any such change.
• Reduce the amount of plus sphere only if the patient can actually read more letters.
• If the astigmatic dial technique has been used and the astigmatism is neutralized (ie, if all
the lines on the astigmatic dial are equally sharp or equally blurred), the eye should still
be fogged; additional plus sphere only increases the blur.
• Therefore, use minus sphere to reduce the sphere power until the best visual acuity is
achieved.
• Again, the examiner should be careful not to add too much minus sphere.

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Refining sphere after dialing in extra plus spherical power to avoid over-minusing.
• Purpose is to avoid extra minus spherical correction by starting the refraction from a
position of extra plus spherical power.
• Plus power is then removed, stopping as soon as the letters on the acuity chart are read
correctly.
1. Endpoint refraction fogged using a +2.00 D sphere before each eye VA reduced to
6/60–6/30.
2. Place a –0.25 D sphere before first 1 eye and then the other, and rapidly alternate
cover; the patient should then be able to identify the eye with the –0.25 D sphere
before it as having the sharper image at the 6/30 or 6/20 level.
3. If the eyes are not in balance, sphere should be added or subtracted in 0.25
increments until balance is achieved.
In addition to testing for binocular balance, the fogging method also provides information
about appropriate sphere power.
• If either eye is overminused or underplussed, the patient should read farther down the
chart—as far as 6/20, 6/18, or even 6/12—with the +2.00 D fogging spheres in place.
• In this case, the refraction endpoints should be reconsidered.
Fogging

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• Final step of subjective refraction is to make certain that accommodation has
been relaxed equally in both eyes.
• Several methods of binocular balance are commonly used.
• Most require that the corrected visual acuity be nearly equal in both eyes.
• uses the prism dials on the phoropter to split the binocular image vertically into
separate images and fogs each eye separately to make sure neither is over-
minused.
• This separation can be achieved by using the prism dials on the phoropter.
Binocular Balance

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Most sensitive test of binocular balance is prism dissociation.
1. Refractive endpoints are fogged with +1.00 D spheres
2. vertical prisms of 4 or 5 prism diopters (Δ) are placed
before 1 eye
3. Use of the prisms causes the patient to see 2 charts, 1 above the other.
4. A single line, usually 6/12, is isolated on the chart
5. Patient sees 2 separate lines simultaneously, 1 for each eye.
6. Patient can readily identify differences between the fogged images in the 2 eyes
of as little as 0.25 D .
7. In practice, +0.25 D sphere is placed before 1 eye and then before the other.
8. In each instance, if the eyes are balanced, the patient reports that the image
corresponding to the eye with the additional +0.25 D sphere is blurrier.
9. After a balance is established between the 2 eyes, remove the prism and reduce
the fog binocularly until best VA is obtained.
Prism dissociation

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• BVD- distance from the back surface of spectacle lens to cornea w/ eyelid closed
• For any spherical correcting lens, distance from lens to its focal point - constant
• Changing the position of correcting lens relative to eye also changes relationship b/w
F2 of correcting lens and far point plane of eye.
• With high-power lenses, as in aphakia or high myopia,
a small change in lens placement considerable blurring of vision unless the lens
power altered to compensate for new lens position.
• RE > ±5.00 D, BVD significant in prescribing the power of the spectacle lens.
Measurement of Back Vertex Distance

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• Whether the lens has plus or minus power
Moving correcting lens closer to the eye reduces its effective focusing power
(the image moves posteriorly away from the fovea)
Moving lens farther from the eye increases its focusing power
(the image moves anteriorly away from the fovea).
• Such a lens should be in the back cell of the trial frame.
• BVD may be conveniently judged using a ruler held beside the arm of the trial frame
whilst viewing the patient from the side.

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• +10.00 D lens placed 10 mm in front of the cornea sharp retinal imagery.
• As focal point of correcting lens is identical to the far point plane of eye and since this lens
is placed 10 mm in front of eye, Far point plane of the eye must be 90 mm behind cornea.
• If correcting lens is moved to a new position 20 mm in front of eye and far point plane of
eye is 90 mm, then focal length of new lens must be 110 mm, requiring a +9.10 D lens for
correction.
• Thus, prescription must indicate BVD at which the refraction was performed.
• Optician must recalculate the lens power as necessary for the actual vertex distance of the
chosen spectacle– frame combination.

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Distometer ( vertexometer)
• Used to measure BVD

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• Phoropters may be used to refract the eyes of patients with highly ametropic
vision.
• Variability in BVD and other induced errors make prescribing directly from the
phoropter findings unreliable.
• These problems avoided if highly ametropic eyes are refracted over the
patients’ current glasses (overrefraction).
• If the new lenses are prescribed with the same base curve as the current lenses
and are fitted in the same frames, many potential difficulties can be
circumvented, including vertex distance error and pantoscopic tilt error, as well
as problems caused by marginal astigmatism and chromatic aberration.
• Overrefraction may be performed with loose lenses (using trial lens clips such
as Halberg trial clips), with a standard phoropter in front of the patient’s
glasses, or with some automated refracting instruments.
Over-Refraction

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• If the patient is wearing spherical lenses, new prescription is easy to calculate by combining the
current spherical correction with the spherocylindrical overrefraction.
• If the current lenses are spherocylindrical and the cylinder axis of the overrefraction is not at 0° or 90°
to the present correction, other methods previously discussed are used to determine the resultant
refraction.
• Such lens combinations were often determined with a lensometer used to read the resultant lens
power through the combinations of the old glasses and the overrefraction correction.
• This procedure is awkward and prone to error because the lenses may rotate with respect to one
another on transfer to the lensmeter.
Other uses.
1. Patient wearing a soft toric contact lens may undergo overrefraction for ordering new lenses.
2. Patients wearing rigid, gas-permeable, hard contact lenses for irregular corneal astigmatism or
corneal transplants.
3. in the retinoscopic examination of children.

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Helpful Hints to Avoid Intolerance
• Do not change the axis of the cylinder, especially in a myope, unless there are
compelling reasons for doing so.
• Do not change the lens form worn by a myope.
• Do not prescribe a large cylinder for any patient who has never worn a cylindrical
correction before. Break them in gradually. The exception to this is pseudophakic
patients who tolerate a full astigmatic correction well.
• Do not overcorrect hypermetropes: better to leave them 0.25 DS undercorrected so
they can read the bus numbers in the far distance.
• Do not fully correct myopes: better to leave them 0.25 DS undercorrected so they do
not have to use their accommodation for distance.

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• Do not give too great a reading addition, so the patient cannot read the newspaper
held at arm's length
• Do not recommend bifocal or progressive lenses without carefully considering the
needs, occupation and frailties of the patient.
• Discuss with the patient the subjective and practical points relevant to the new
prescription, e.g. warn a new bifocal wearer to be careful at steps.
• Do not alter a satisfactory prescription unless there is a very definite reason to do so.
• Do not advise patients to buy new glasses because of a minor change in prescription
that they will not be able to appreciate subjectively.

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• Move through the four steps expeditiously. This makes the process more efficient, more
precise and easier for the patient and the examiner. Comparing different lenses during
subjective refraction is a test of first impression.
• A good rule of thumb is to not wait for a delayed response, i.e., have a "no pauses" policy.
• When initially measuring a patient's visual acuity, there is no need to start with the largest
letters on the visual acuity chart, unless you have an indication that visual acuity is reduced.
The goal is to find the smallest line the patient can read, and you want to get there as
efficiently as possible.
• During subjective refraction work with the smallest line that the patient can read, going
further down as acuity is improving. The patient is able to make finer discriminations on the
smaller lines.
• Watch to make sure the patient is not squinting, as this will give an unwanted pinhole effect.
• Run through The Sequence again if there is a large change in any of the steps.

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• If retinoscopy, autorefraction, or the old glasses suggest there is a spherical refractive
error without astigmatism, it is still necessary to determine if astigmatism is present
(unless the vision is a crisp 20/20 with the spherical correction). Two methods that
can be used to make this determination are as follows:
• One method uses the Jackson cross cylinder (JCC)
• An alternative method does not use the JCC. Dial in, and then out, 0.50 diopter of
plus cylinder power at each of the 4 axes (90, 180, 45 and 135 degrees). It is not
necessary to adjust the sphere when doing this. If 1 of the 4 choices results in the
letters becoming clearer, from that starting point

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• It is important to understand that some patients can give a very precise and
repeatable end-point, and others cannot. Sometimes there is a medical explanation
for those that cannot. Cataracts, macular edema, dry eyes, age-related macular
degeneration and other conditions can cause vision to fluctuate.
• If you find the patient is not progressively moving to a correct end-point, but in fact
deviating from it, you can deviate from the normal sequence either during or after
the normal subjective refraction steps. You can make larger jumps in spherical power,
cylinder axis, and cylinder power than you would normally, and the axis dial can even
be turned freehand without use of the JCC.
• It is also important to "know when to quit." It is appropriate to end the subjective
refraction if it seems that the responses are not reliable enough to be helpful. In that
situation, it should be noted on the patient's chart that the refraction resulted in a
"poor end-point." By doing so, you will know to be cautious about prescribing from
that result.

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• 11. During subjective refraction we are giving the patient choices and asking for their
preferences. However, we as refractionists also have preferences:
• - Less minus in the sphere— to avoid over-minusing.
• - Less power in the cylinder— to make adjustment to the glasses easier for the
patient.
• - Axis at 90 or 180 degrees — to lessen the chance that the patient will have tilting of
viewed objects.
• - Oblique axes adding to 180 degrees — the axes are then symmetrical, which
suggests that they are correct.
• 12. If, during the spherical measurement, the patient keeps choosing lenses moving in
the minus direction (this can be called "the minus march") and then begins to
progressively choose less minus, continue to allow the patient to move in the plus
direction. This pattern can occur if accommodation is at first increasing, but
subsequently relaxing, as the refraction proceeds.
• 13. When there is only +0.25 diopter of cylinder power, it can be difficult to correctly
position the axis.

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• 14. When refracting a child, at the completion of the refraction, allow the patient to
view the chart binocularly. Add + 0.25 diopter of sphere to each eye and determine
whether the child's ability to read the chart is unchanged. If it is unchanged, continue
to remove minus binocularly until the visual acuity is affected. This additional step will
decrease the likelihood that a child will be over-minused as a consequence of their
very strong accommodative ability.
• 15.When prescribing aphakic glasses, the vertex distance must be specified. This is
measured with a special caliper called a distometer.
• 16. It may be helpful to occasionally use Halberg clips, supplemental cylinder power
inserts, or the 0.0 power lens in the trial lens set.
• A Halberg clip is placed over the existing lens in the patient's glasses. The clips are
made to hold trial lenses that are placed in front of the patient's glasses.
• A supplemental cylinder power insert is intended for use when the power of the
patient's cylinder exceeds 6.00 diopters.
• The 0.0 power lens can be helpful in several situations. It can be used to have
consistency of lens additivity when one eye has a purely cylindrical correction. It can
also be helpful as a "magic lens" for a patient whom you suspect of malingering or
wanting glasses only because a sibling or friend has them.

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THANK
YOU