This document discusses best form spectacle lenses and their off-axis performance. It describes how lenses are designed to minimize common aberrations seen off-axis like transverse chromatic aberration, oblique astigmatism, and distortion. Specific lens forms are examined, including point-focal, minimum tangential error, and Percival designs. Field diagrams illustrate how different lens curves can improve off-axis power. The principles of Tscherning's ellipses, which help design lenses corrected for oblique astigmatism, are also summarized.

1. BEST FORM SPECTACLE LENSES
OPTOM FASLU MUHAMMED

2. In actual world, the eyes turn behind the lens to
view through off-axis visual points and it is
then that the form assumes importance.

3. OFF-AXIS PERFORMANCE OF LENSES
The aberrations which are significant to the
spectacle wearer are:
 Transverse chromatic aberration (TCA)
 Oblique astigmatism (OA)
 Curvature of field
 Distortion

4. Spherical aberration and coma – are both
aberrations of wide aperture systems and
although a spectacle lens may be quite large.

5. TRANSVERSE CHROMATIC
ABERRATION
Transverse chromatism can only be
eliminated by constructing an Achromatic
lens, that is, a pair of lenses bonded together,
in which the chromatism of one component
neutralizes the chromatism of the second.
Given power by selecting a material with the
highest available V-value.

7.  OFF AXIS BLUR –Under condition of low
contrast colour fringes may be noticed.
instead ,the effect of TCA is to cause a
reduction in visual acuity.

8. To reduce chromatism by bending the lens
into a very steeply curved form (the so-called
Wollaston bending –but such lenses are
expensive to produce and appear very
bulbous.

9. To a good approximation, the magnitude of
the TCA at any given point on a lens is found
by calculating the prismatic effect, P, at the
point and dividing this by the Abbe number,
TCA = P/V.
P = Prismatic effect
V = Abbe number

10. DISTORTION
It is possible to reduce distortion by supplying
steeply curved Wollaston form lenses.

11. FIELD DIAGRAMS
A most useful guide to the effects of oblique
astigmatism and curvature of field in a given
spectacle lens is obtained by studying a field
diagram for the lens form.

12. FIELD DIAGRAMS
 A field diagram is a plot of the tangential
and sagittal oblique vertex sphere powers
against the ocular rotation of the eye
viewing through the lens.

14. The plane containing the optical axis of the
surface is referred to as the ‘tangential plane’
 The plan at right angles to the tangential
plane is referred to as the sagittal plane.
The vertex sphere is an imaginary reference
surface concentric with the eye’s centre of
rotation, from which the positions of the
tangential and sagittal foci are measured.

15. The far point sphere is the imaginary surface,
also concentric with the eye’s centre of
rotation, upon which we can assume the far
point to remain as the eye rotates to view
through off-axis zones of the lens.
The distance between the vertex sphere and
the far point sphere measured through the
eye’s centre of rotation, Z, is constant and
equal to the back vertex focal length of the
lens.

16. FIELD DIAGRAM IN +4.00 PLANO CONVEX LENS
In the case of a perfect lens, such as the +4.00
design whose ideal field diagram is illustrated
in the tangential and sagittal oblique vertex
sphere powers remain +4.00 for all zones of
the lens.
Unfortunately, this performance is impossible
to obtain in a single lens with just two
surfaces, at least for this power.

18. The performance of a +4.00 design, made in
Plano-convex form. When the eye views along
the optical axis of the lens, the power of the
lens is, indeed, +4.00D.

19. OFF AXIS PERFOMANCE OF +4.00 DS LENS
When the eye rotates through 30° from the
optical axis,
In sagittal meridian is +4.25D
In tangential meridian +5.25 in the.
This is equivalent to a power +4.25D sphere
with a +1.00D cylinder.
This is so different from the paraxial power
that it cannot be ignored. Clearly, the choice of
a Plano-convex design for a lens of power
+4.00D is a poor one.

20. We shall see that, although we cannot make a
+4.00D lens for which the power remains the
same for all directions of gaze, we can
certainly improve.

22. BEST FORM SPECTACLE LENSES
A best form spectacle lens is one whose
surface powers have been specially computed
to eliminate, or at least minimize, certain
stated defects in its image forming properties.

23. BEST FORM SPECTACLE LENSES
Shows how the off-axis performance of
+4.00D lenses varies for three meniscus forms
with front curves +9.75D, +8.12D and +7.62D.

25. POINT-FOCAL’ LENS
 In Figure (BC is
+9.75 Ds)
 (OAE) = 0
 Such a form is
described as a ‘Point-
focal’ lens form.
 At 35°, the power of
the lens has dropped
to +3.75D.

26. MINIMUM TANGENTIAL (T) ERROR’
FORM
 In figure b(BC is +8.12
Ds ) :-
If the form of the
lens is flattened from the
point-focal bending, the
tangential power
increases and now the
same as the back vertex
power of the lens. Such a
form is described as a
‘minimum tangential
(T) error’ form.

27. In Figure 6c (BC is +7.62 D ), the bending of
the lens has been reduced still further to a
+7.62D base curve and it can be seen in the
field diagram.
T and S oblique vertex sphere powers have
increased to just the point where the focal
lines within the eye would lie either side of,
and equidistant from, the retina.

28. PERCIVAL LENS DESIGN’
 At 35°, the off-axis
power of the lens is
+3.85DS/+0.30DC, the
T is +0.15D too great
and the S is 0.15D too
weak compared with the
paraxial power.
 The mean oblique power
of the lens is +4.00D.
This form of lens is
known as a ‘Percival
lens design’ and is
free from mean oblique
error.

29. The same principles are involved in the
design of minus spectacle lenses.
illustrates field diagrams for -4.00D lenses
made in point-focal form (+5.00 base
curve), minimum T-error form (+3.87 base
curve) and Percival form (+3.25 base
curve).

30. FIELD DIAGRAM OF -4.00D SPH

31. TSCHERNING’S ELLIPSES
Spectacle lens design was undertaken by the
laborious means of trigonometric ray tracing
using six or seven-figure logarithm tables.
This problem was addressed during the 19th
century by Airy Coddington and von Seidel
who, amongst others, developed approximate
equations for the aberrations and for lens
forms which exhibit minimum aberration.

32. The equations for spectacle lenses, which
are corrected for oblique astigmatism, are
quadratic in form and plot in the form of
an ellipse. Such equations were
published by Dr Marius Tscherning 2 &
known as Tscherning’s ellipses.

33.  Tscherning’s ellipses show very nicely that there is a
range of powers that can be made free from a
particular aberration and that within this range, there
are two different forms of lens for each power.
 The shallower form - the ‘Ostwalt Bending’ and is
the form which is usually employed in practice.
 The steeper form is known as the ‘Wollaston
bending’ and, in view of the necessary deep
curvatures, is more difficult to produce.

34.  Lenses in the range +7.00 to -23.00 can be
made free from astigmatism .
 The Ostwalt form for a -5.00D lens requires a
back surface power in the region of -9.50D.
 The Wollaston form a -5.00 D lens needs a back
surface power in the region of -22.00D.
 Tscherning ellipses drawn for lenses made in
three different refractive indices – 1.50, 1.70,
and 1.90.

35. TSCHERNING’S ELLIPSES FOR DISTANCE AND NEAR VISION POINT
FOCAL LENSES. CONSTRUCTED FOR
N = 1.50, CRD = 27MM, L1 = -3.00 FOR THE NV ELLIPSE

36. BEST FORM LENSES OF WOLLASTON
FORM
It is for steeper form.
Near vision form is the same as the distance
vision form over a large part of the range.
In oblique gaze, bundles of light pass through
the lens more nearly in the position of
minimum deviation .

37. Wollaston form lenses. The Contour Optics lens
from Sola Technologies

38. BEST FORM LENSES FOR NEAR VISION
Similar equations can be derived for lenses
intended to be used for near vision1 and the
ellipse for near vision at -33.3cm.
Ostwalt lenses by German firm
Rupp+Hubrach.

39. BEST FORM ASTIGMATIC LENSES
Curved form for the same reason as spherical
lenses.
Three different forms of toroidal surface are
used in ophthalmic lens manufacture
1.The tyre-formation surface-mass
production
2.The barrel -formation surface –
individual surface working
3.Capstan –formation surface

40. Thank U………