Optics of Free-Form Lenses – Part 2, Single Vision
Darryl Meister is a Certified Master Optician, technical marketing manager for Carl Zeiss Vision, technical representative to the VCA and ANSI, has been a key contributor to many important industry initiatives and writes and lectures frequently on ophthalmic optics, lenses and dispensing.
completion of this program the participant should be able to:
- Learn the limitations of contemporary best form single vision lenses and their designs.
- Understand the way that prescriptions can be customized using position of wear values.
- Know how lenses can be customized for conformal optics i.e., for an 8 base wrap frame regardless of almost but extreme Rx’s.
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO).
|This course is supported by an educational grant from
Although free-form progressive lenses customized to the visual requirements of
the wearer have become increasingly common, some lens manufacturers have
recently begun introducing free-form single vision lenses that are also
customized to the visual requirements of the wearer.
Limitations of Traditional Single Vision Lenses
For over a century, lens designers have understood that the field of clear vision
through a spectacle lens is limited by various optical aberrations, particularly oblique astigmatism. These optical aberrations result in unwanted sphere and
cylinder power changes from the desired prescription away from the center of
the lens. This reduces the
quality of peripheral vision for
the wearer. For spherical
prescription powers, it is
possible to minimize these
optical aberrations either
through the proper choice of
front (base) curve or through
the use of an aspheric lens
In order to eliminate these optical aberrations completely, however, a unique
base curve or aspheric lens design would have to be used for each spherical
prescription power according to “best form” optical design principles.
Unfortunately, this represents an impractical requirement for cost and inventory
traditional single vision lenses are produced from lens blanks that are factory-molded with a limited number of
front (base) curves, upon which relatively broad prescription ranges must be
grouped (Figure 1). Moreover, for prescriptions with cylinder power, no base
curve or aspheric design can eliminate the optical aberrations produced by both
the sphere power meridian and cylinder power meridian simultaneously.
Figure 1. Each prescription power ideally requires a unique front (base) curve or
aspheric design in order to eliminate optical aberrations, yet powers are generally
grouped onto a limited number of base curves to minimize costs.
Additionally, the position of wear of the lens can also influence optics and
vision quality. The position of wear represents the position of the fitted spectacle
lens on the wearer. This includes pantoscopic tilt, face-form wrap, and vertex
distance. Tilting a lens also introduces oblique astigmatism, which results in
unwanted cylinder power and changes to the sphere power (Figure 2).
Figure 2. Although vision may be clear through the center of a lens with no tilt,
vision is often blurred when viewing
through the periphery of the lens or
through the center of the lens when tilt has been added due to oblique astigmatism.
Fortunately, with the advent of free-
form surfacing, single vision lenses can
now be designed and manufactured on
demand for each wearer. Some lens
manufacturers have begun using this
technology in conjunction with
sophisticated optical design software to
customize the lens design to the unique
visual requirements of each wearer,
immediately prior to fabrication. Using
the wearer’s prescription and fitting
parameters, a unique lens design is
calculated in “real time.” The final lens
calculations are then transmitted
directly to precision free-form surfacing
Customization for the Prescription
When the wearer looks obliquely through the peripheral regions of a spectacle
lens, unwanted sphere and cylinder power errors are introduced by oblique
astigmatism and other aberrations. This results in errors from the desired focus
of the lens. These unwanted power errors from the desired prescription produce
blur that degrades image quality and narrows the field of clear vision for the
wearer (Figure 3). Lens aberrations can also cause the field of clear vision through the lenses to become distorted in shape, particularly in the presence of
cylinder power, which impacts the binocular vision and stereopsis of the wearer.
With traditional single vision lenses, each base curve will typically deliver
optimum optical performance only for sphere powers located near the center of
the prescription range
associated with each base
curve (Figure 4). Other
prescription powers will
frequently suffer from
residual aberrations in the
periphery of the lens
because of this optical
compromise that increase
as the power deviates from
the ideal prescription
Figure 3. For many prescriptions, the field of clear vision may be significantly
reduced and distorted in shape by uncorrected lens aberrations.
Figure 4. Unlike traditional lenses, the optical performance of customized
single vision lenses is not necessarily limited by the availability of
base curves, so every wearer enjoys the best optics possible with minimal lens aberration
Recall that a unique base curve or
aspheric lens design is required in order
to eliminate—or at least minimize—these
aberrations in spectacle lenses with a
spherical prescription power. Further,
when the prescription contains cylinder
power, no conventional base curve or
aspheric design can eliminate the
aberrations produced simultaneously by
both the sphere and cylinder power of the
lens. Moreover, because prism effectively
tilts the optical axis of the lens,
prescribed prism also introduces oblique
With sufficiently advanced software
and a free-form delivery system, it
becomes possible to customize the lens
design based upon the unique prescription
requirements of each wearer. By fine-tuning
the optical design of the lens for the exact
prescription using a complex aspheric optical
design, residual lens aberrations are virtually
eliminated. As a result, fully customized lenses will deliver wider, more symmetrical fields of clear vision compared to traditional lenses. Wearers
will enjoy the widest fields of
vision possible and improved
binocular utility, regardless of
prescription (Figure 5).
Figure 5. This free-form single vision lens from Carl Zeiss Vision
is precisely customized for the wearer’s exact prescription,
which ensures wide, symmetrical fields of clear vision.
Customization for the
Position of Wear
The position of wear is the
position of the fitted lens
relative to the actual wearer,
including the pantoscopic tilt,
face-form wrap, and vertex
distance of the lens. Spectacle
prescriptions are typically
determined using refractor-head or trial-frame lenses that are positioned
perpendicular to the lines of sight. Unlike the lenses used during the ocular
refraction procedure, eyeglass frames generally place the spectacle lenses at an
angle with respect to the lines of sight once fitted to the wearer’s face.
Unfortunately, tilting a lens introduces oblique astigmatism, which results in
unwanted sphere and cylinder power changes across the lens. These are
proportional both to the
power of the lens and to
the magnitude of lens tilt.
Moreover, changes in the
fitted vertex distance will
also influence optical
performance, since the
viewing angle to a given
point over the lens design
varies as a function of the
distance between the
center of rotation of the
eye and the back of the
position of wear can have
a significant impact upon
the optical performance of spectacle lenses, particularly upon the quality of
straight-ahead vision (Figure 6).
Figure 6. Vision may be significantly degraded by the
position of the fitted lens on the eyeglass wearer.
it is possible t
customize the lens
design based upon
the unique fitting
parameters of each
wearer (Figure 7). If
face-form wrap and
vertex distance are
performance of the
lens in its position of wear may be mathematically modeled in order to apply the
necessary optical corrections across the lens surface during the optical
optimization process. Wearers can therefore enjoy the best optical performance
possible, regardless of their unique fitting requirements (Figure 8).
Figure 7. Plots of ray-traced optical astigmatism demonstrate that the optics of the lens design can also be
tailored to the exact fitting requirements of the wearer, ensuring that every lens performs exactly as intended,
with no unwanted prescription changes that could otherwise degrade vision quality through the central viewing zones.
Figure 8. This free-form single vision lens from Carl Zeiss Vision
is precisely customized for the wearer’s exact fitting parameters, which ensures clear vision straight-ahead vision.
Traditional spectacle lenses are often designed to exhibit the specified optical
performance only when measured using a conventional focimeter, such as a
lensometer. Because Zeiss Individual SV is designed to provide the wearer with
the prescribed optical performance once the lens in the actual position of wear, small differences from the original prescription are required at the verification
point of the lens. These power adjustments are supplied as a compensated
prescription, which represents the correct lens powers to verify with focimeters
in order to provide the actual wearer with the specified prescription.
Until recently, the real-time optical design benefits afforded by “free-form” or
“digital surfacing” technology were limited to progressive lenses. While
presbyopes could enjoy the clearest, most comfortable vision possible, pre-
presbyopes wearing single vision lenses were left to tolerate the inherent optical
compromises of traditional, semi-finished lenses. Consequently, single vision
lens wearers often endured reduced fields of clear vision or a reduction in visual
acuity through the center of the lens.
In fact, because of optical aberrations introduced by the tilt and peripheral
optical performance of the lens, the visual acuity of the wearer may be reduced
from 20/20 to 20/40 or even worse. In addition to losing up to three or more
lines of visual acuity on a Snellen chart, the wearer’s appreciation of contrast is
also greatly reduced. This causes colors to lose sharpness and definition.
Cosmetic and Conformal Optics
Optical customization frees lens designs from the constraints of traditional “best
form” lenses, which must adhere to specific base (front) curve guidelines in
order to maintain quality vision. Some customized single vision lenses rely on an
extremely sophisticated optical optimization process to refine points over the
entire back surface of the lens, regardless of whether a flatter or steeper front
curve is utilized. The result of this optimization process is a complex
“aspherization” of the initial lens design. A flatter profile is often chosen in order
to reduce lens thickness, weight, and magnification. Wearers can therefore
experience the clearest optics possible in a lens that is flatter, thinner, and more
attractive-looking than conventional lens designs that rely on steeper, spherical
base curves (Figure 9).
Figure 9. This free-form single vision lens from Carl Zeiss Vision
employs point-by-point optical aspherization to deliver exceptional optical performance in a flat, slim lens.
It is also possible to
manipulate the optics and form of
the lens based on the overall
“shape” of the frame and other
With the increasing popularity of
steeply curved and highly
wrapped eyewear, which often
necessitate complex atoric lens
designs on relatively steep (8.00)
base curves, this application of free-form technology is becoming increasingly relevant to achieve maximum
optimal performance. When the form of the lens has been manipulated to
“conform” to certain mechanical requirements, the lens design must be suitably
tailored to the shape of the lens in order to prevent a reduction in optical
Free-Form Lens Surfacing
Unlike traditional lenses, which rely either on a set of mold or hard lap tools in
fixed increments of surface power in either eighth-diopter (0.125 D) or tenth-
diopter (0.10 D) steps, free-form lens surfacing utilizes “soft” lap tools made,
from a compliant foam or similar material, which are not restricted to fixed
increments of surface power. Hence, a computer-controlled soft lap polisher can
polish surfaces cut by a free-form generator in hundredth-diopter (0.01 D)
steps. This results in more precise optical powers.
Manufacturing complex lens designs using free-form surfacing requires
meticulous process engineering to assure quality and ongoing quality control.
Although free-form lens surfacing can theoretically produce extremely accurate
and precise lens surfaces, in practice the kinematics of the single-point diamond
turning and soft lap polishing processes used in free-form lens surfacing can
result in unwanted variations in surface power and waviness. Dozens of different
cutting and polishing parameters must be carefully adjusted and tested for each
combination of lens design, prescription, and material in order to ensure quality
results (Figure 10).
Figure 10. Without extensive process engineering and ongoing quality control,
a free-form surfacing process can actually produce lenses with unwanted variations in power and waviness over the surface.
Failure to verify production quality of a free-form surfacing process on a
regular basis can also lead to inferior optical quality compared to traditional lens
production methods. Fortunately, lenses from the more experienced free-form lens suppliers must typically meet stringent quality guidelines and design
specifications. These free-form lens suppliers may use sophisticated optical
metrology instruments to “map” the optical characteristics of lenses periodically
in order to compare them against the intended lens design (Figure 11). This
quality control ensures that every lens delivers the precise optical powers that
the wearer requires.
Figure 11. Engineers at Carl Zeiss Vision
rely on sophisticated optical metrology tools
to ensure that
free-form lenses deliver consistently superior quality and optics.
Some free-form lens suppliers emphasize the value of free-form lens
surfacing over traditional lens surfacing because it eliminates the rounding
errors of hard lap tools. The actual optical error due to these rounding errors is
typically small, with a maximum power error on the order of either ±0.0625 or
±0.05 diopters, depending upon the tooling increments. The optical error
produced by either changes in the position of wear or the use of a limited
number of base curve designs, on the other hand, can be considerably greater.
As with free-form progressive lenses, the use
of free-form surfacing to deliver truly
customized single vision lenses is the most
meaningful visual benefit of this technology
to wearers. The full potential of free-form
technology will only be realized when utilized
in conjunction with powerful software tools
capable of “real-time” optical design using
input specific to the individual wearer.
With sufficiently advanced optical design
and free-form delivery system, lens
designers can deliver lenses that provide
wearers with the best optical performance
possible, regardless of their prescription
requirements or fitting characteristics. With
the most advanced customized free-form
lenses, patients can enjoy up to 50% wider fields of clear vision as well as
sharper, more natural straight-ahead vision.