This document discusses guidelines for prescribing astigmatic corrections and minimizing distortion caused by meridional magnification. It notes that full astigmatic corrections can cause intolerable binocular spatial distortion. To reduce distortion, it recommends using minus cylinder lenses, minimizing vertex distance, rotating the cylinder axis toward 90° or 180°, and reducing cylinder power - balancing the resulting blur with remaining distortion. The Jackson cross cylinder test can minimize residual astigmatism from altered corrections. Adaptation usually alleviates distortion within a few days for adults.

1. Cylinder prescription
Dr. Prashant.P.Patel
D.O ;D.N.B
Consultant ophthalmologist
Sudarshan Netralaya
Amreli

2. Commonly taught guidelines
1. Children accept the full astigmatic correction.
2. If an adult cannot tolerate the full astigmatic
correction, rotate the cylinder axis toward 90° or
180 and/or reduce the cylinder power to
decrease distortion.
• When reducing the cylinder power, keep the
spherical equivalent constant by appropriate
adjustment of sphere.
3. With older patients, beware of changing the
cylinder axis.

3. The Problem: Distortion
• Why have such guidelines developed? Why can
some patients not tolerate the full astigmatic
correction in the first place?
• One text on clinical refraction states that full
correction of a high astigmatic error may initially
result in considerable blurring of vision.
• Another teaching is that with the full astigmatic
correction the image is too sharp-the patient is
not used to seeing so clearly.
• Statements such as these are not only
misleading; they are incorrect.

4. The reason for intolerance of
astigmatic spectacle corrections is
• Distortion caused by meridional
magnification.
• Unequal magnification of the retinal image in
the various meridians produces monocular
distortion manifested by tilting lines or
altered shapes of objects .

5. But monocular distortion by itself is rarely
a problem; the effect is too
small.
• Maximum tilting of vertical lines (declination
error) in the retinal image will occur when the
correcting cylinder axis is at 45° or 135°.
• But even under these conditions each diopter
of correcting cylinder power produces only
about 0.4° of tilt.

8. Consider, for example,
• a patient with symmetrical oblique
astigmatism wearing a + 1.00 diopter cylinder,
axis 135° before the right eye and a + 1.00
diopter cylinder, axis 45°before the left eye.
• If the patient looks at a vertical rod 3 meters
away, the retinal images of the rod will be
tilted toward each other at the top
(declination error) approximately

9. • 0.4° each, a barely perceptible amount under
monocular conditions.
• But under binocular conditions, the vertical
rod will theoretically appear tilted toward the
patient (inclination) approximately 35°!
• Such large errors in stereoscopic spatial
localization are clearly intolerable, seemingly
out of proportion to the amount of monocular
distortion that produces them.

10. • Fortunately, errors in stereoscopic spatial
localization are usually compensated for in
most patients by experiential factors:
perspective clues, the known size and shape
of familiar objects, the knowledge of what is
level and what is perpendicular, etc
• But the fact remains that some patients
cannot or will not tolerate binocular spatial
distortion, and herein lies our problem.

11. • We have no effective means of treating
binocular spatial distortion except by altering
or eliminating the monocular distortion that
produces it.

12. Sources of Monocular Distortion
• We can identify two basic sources of
meridional magnification,
1.One involving the design of the spectacle lens.
2. The location of the spectacle lens with respect
to the entrance pupil of the eye.

13. Shape Factor of the Spectacle Lens
• All spectacle lenses having curved front surfaces
produce a magnification inherent to the lens
itself.
• The more convex the front surface and the
thicker the lens, the greater will be this "shape
factor" magnification.
• If the front surface of the lens is spherical, the
shape factor magnification will be the same in all
meridians, producing only an overall size change
in the retinal image.

14. • If the front surface of the lens is cylindrical or
toric, the shape factor magnification will vary
from one meridian to another, producing
distortion of the retinal image.
• Again, this only occurs with lenses having the
cylinder ground on the front surface of the lens,
the so-called plus cylinder form or anterior toric
spectacle lens.
• Lenses having the cylinder ground on the back
surface (minus cylinder lenses, posterior toric
lenses, "iseikonoid" lenses) do not produce
differential meridional magnification due to the
shape factor, because the front surface power is
the same in all meridians.

15. Distance of the Spectacle Lens From
the Entrance Pupil
• More important than the shape factor of the
spectacle lens in producing distortion is the
location of the spectacle lens relative to the
entrance pupil of the eye.

16. • The conventional method of calculating the
total magnification produced by a spectacle
lens is to multiply the shape factor
magnification times the "power factor"
magnification.

17. • The power factor magnification is a function of
the dioptric power of the correcting lens and the
distance of the correcting lens from the "seat of
ametropia."
• For example, consider a +4.00 D cylindrical lens
placed at a vertex distance of 12 mm from an eye
with simple hyperopic astigmatism.
• This lens will produce a power factor
magnification of 0% in the axis meridian and 5%
in the meridian perpendicular to the axis
meridian, for a differential meridional
magnification of 5%.

18. • Astigmatic refracting surfaces located away
from the entrance pupil of the eye cause
meridional magnification and distortion of the
retinal image.
• The direction of meridional magnification is
determined by the axis orientation of the
astigmatic refracting surface, and the amount
of meridional magnification increases not only
with the power of the astigmatic refracting
surface but also with increased distance of the
astigmatic refracting surface from the
entrance pupil of the eye.

19. Misconceptions
• It is almost universally taught in clinical
ophthalmology, with a rare exception, that
retinal images slant in uncorrected oblique
astigmatism and are straightened by the
proper astigmatic spectacle correction.

20. • A second misconception in clinical teaching, again with
only a rare exception, deals with the effect of residual
astigmatism on distortion.
• It is taught that the axis and power of this residual
astigmatism determine the direction and amount of
distortion that will be present.
• Primary determinants of distortion are simply the axis
and power of the spectacle lens itself not residual
astigmatism.
• For example, if a correcting cylindrical lens is rotated to
produce an axis error, the amount of distortion will
remain substantially the same, but the direction of
distortion will rotate an amount corresponding to the
rotation of the cylindrical lens.

21. Relative Contributions From Sources of
Distortion
• The various sources of meridional magnification
do not make equal contributions to the amount
of magnification produced.
• These graphs show the meridional magnification
to be expected with various degrees of
astigmatism, both in the uncorrected state and in
the corrected state, using for correction a
common form of astigmatic spectacle lens.

22. graph

23. • Note that with uncorrected astigmatism, the
meridional magnification that occurs is much less
than, and in the opposite direction to, the
meridional magnification produced by the
correcting spectacle lens.
• Also note that the shape factor of the spectacle
lens, if a plus cylinder lens is used, increases the
meridional magnification only about 2%.

24. Photographic Simulation of Distortion

25. Minimizing Monocular Distortion
• Necessary in patients who cannot tolerate their
present spectacles or in those whose new
refractions demand a change in prescription such
that we might anticipate problems with
distortion.
• How can we minimize meridional magnification?
1.Minus cylinder spectacle lenses
2. Minimize vertex distance
3. Alter the astigmatic correction by rotating the
axis or reducing the power of the correcting
cylinder.

26. Specifying Minus Cylinder (Posterior
Toric) Spectacle Lenses
• The small amount of meridional magnification
caused by plus cylinder (anterior toric)
spectacle lenses is avoided simply by
specifying minus cylinder lenses in the
prescriptions.
• In practice, this distinction is rarely necessary,
for minus cylinder lenses have become the
preferred form for routine dispensing.

27. Minimizing Vertex Distance
• There is often little room for manipulation of
vertex distance with common styles of frames.
• But when distortion is anticipated, we can at least
avoid the so-called fashionable glasses that sit at
the end of the patient's nose.
• With contact lenses. the vertex distance is
reduced to zero and distortion is practically
eliminated.
• Contact lenses should always be considered as an
alternative to spectacle correction if the patient
remains unsatisfied with other attempts to
reduce distortion.

28. Altering the Astigmatic Correction
Rotating the cylinder axis
• New astigmatic spectacle corrections in adults are
better tolerated if the axis of the cylinder is at 90" or
180" rather than in an oblique meridian.
• In fact, it has long been taught that oblique axes should
be rotated toward 90" or 180", if visual acuity does not
suffer too much, to avoid problems from oblique
distortion.
• Direction of meridional magnification is determined
principally by the axis orientation of the correcting
cylinder, whether or not the axis is correct, and vertical
or horizontal aniseikonia is known to be more
tolerable than oblique aniseikonia.

29. • It is the axis and power of the spectacle lens
itself, correct or not, that principally
determine the direction and amount of
distortion.
• With this concept understood, and on the
basis of clinical experience, we may thus state
that the direction of distortion may be made
more tolerable, if necessary, by rotating the
cylinder axis toward 90° or 180°.

30. Reducing the cylinder power
• The other method that is commonly used to
lessen distortion is reduction of the power of
the correcting cylinder.
• This makes sense, for we have seen that the
amount of meridional magnification is largely
determined by the power of the correcting
cylinder.
• The less the cylinder power, the less the
meridional magnifi cation.

31. Decreased distortion, but at the
expense of visual acuity
• Distortion may be decreased by reducing the
cylinder power or changed in direction by
rotating the cylinder axis, but either of these
manipulations causes blurring of the image.

32. photo

33. Residual astigmatism
• Whenever the cylinder power is reduced from its
correct value, or the cylinder axis is rotated away from
its correct position, residual astigmatism appears.
• The residual astigmatism does not produce distortion.
but it does produce blur of the retinal image.
• This limits the amount that the cylinder power may be
reduced or the amount that the cylinder axis may be
rotated away from its correct position.
• If we must minimize distortion, we must be careful at
the same time not to create excessive blur from
residual astigmatism.

34. It is the amount of residual astigmatism that produces
the blur, not the axis of the residual astigmatism.
• It is easy to judge the amount of residual astigmatism when
reducing the cylinder power of a spectacle correction, for
the residual astigmatism is simply equal to the amount the
cylinder power is reduced.
• It is more difficult to judge the amount of residual
astigmatism induced by rotating the cylinder axis away
from the correct position. The resulting residual
astigmatism may always be calculated using the rules for
combination of obliquely crossed cylinders.
• A simple graph may be easier to remember. Figure A-S
should provide a working knowledge of the amount of
residual astigmatism induced by rotating the cylinder axis.

35. • As indicated by the graphs in
Figure A-8, when we rotate
the cylinder axis away from
its correct position and keep
the cylinder at its full value.
• If the cylinder axis is rotated
30° from its correct position.
the residual astigmatism
becomes equal to the
original uncorrected
astigmatism.
• If the cylinder axis is rotated
90° from its correct position,
the residual astigmatism
becomes twice the value of
the original uncorrected
astigmatism!

36. • Cylinder axis is rotated away from its correct
position, the power of the cylinder should be
reduced in order to minimize residual
astigmatism.

37. Optimal value for cylinder power
• If we choose to shift the cylinder axis to make the direction
of meridional magnification more tolerable, we should also
reduce the cylinder power slightly to minimize residual
astigmatism.
• Use the Jackson cross cylinder test for cylinder power.
• It may be shown from cross cylinder theory that for any
setting, this test automatically gives us the optimal cylinder
power to result in minimum residual astigmatism.
• The axes of the cross cylinder are simply aligned with the
principal meridians of the correcting lens, and the patient is
asked, "Which is better, one or two?" as the cross cylinder
is flipped.
• From the patient's responses, the refractionist adjusts the
cylinder power until both flipped positions of the cross
cylinder appear equally clear.

38. Optimal value for sphere power
• When refining the cylinder power by cross
cylinder testing, the sphere should be
adjusted for best visual acuity during the
refinement and also as the final step.
• When empirically reducing the cylinder power,
the spherical equivalent concept provides a
useful estimate for adjusting the sphere, but it
should not be relied upon without a final
subjective check.

39. Adaptation to Distortion
• Children adapt readily to induced distortion from
astigmatic spectacle corrections.
• Adults adapt less readily. Experiments in adults with
induced aniseikonia for periods of up to two weeks
have suggested that adaptation to spatial distortion is
primarily an interpretive process rather than a
physiological process.
• In these adult subjects, the sense of distortion usually
disappeared completely in several days.
• A physiological component was measured as being
responsible for 20%-60% of the total adaptation.

40. Revised Guidelines for Prescribing
Cylinders
1. In children, give the full astigmatic correction.
2. In adults, try the full astigmatic correction fi rst. Give
warning and encouragement.
If problems are anticipated, try a walking-around trial with
trial frames before prescribing.
3. To minimize distortion, use minus cylinder lenses and
minimize vertex distances.
4. Spatial distortion from astigmatic spectacle corrections is
a binocular phenomenon.
Occlude one eye to verify that this is indeed the cause of
the patient's complaints.

41. • If necessary, reduce distortion still further by rotating
the cylinder axis toward 180or 90 (or toward the old
axis) and/or by reducing the cylinder power.
• Balance the resulting blur with the remaining
distortion, using careful adjustment of cylinder power
and sphere.
• Residual astigmatism at any position of the cylinder
axis may be minimized with the Jackson cross cylinder
test for cylinder power.
• Adjust the sphere using the spherical equivalent
concept as a guide, but rely on a final subjective check
to obtain best visual acuity.
6. If distortion cannot be reduced sufficiently by altering
the astigmatic spectacle correction , consider contact
lenses (which cause no appreciable distortion) or
iseikonic corrections.

42. • If the patient with moderate to high astigmatism has no
complaints about distance spectacle correction but has
difficulty reading at near, remember that changes in the
astigmatic axes of the eyes (from cyclorotations) and
changes in the effectivity of astigmatic corrections may
cause problems at near. Such patients may require separate
reading glasses.
• Finally, patients who desire spectacles only for part-time
wear may not be able to adapt to distortion during short
periods of wear.
In such cases, the astigmatic correction should be altered
to reduce distortion according to the principles outlined
above.