What is Free Form (FF)?
• Free form is a manufacturing technology that allows cutting and polishing arbitrary surfaces
• A lens is free-form if
- At least one of its surfaces is made with free form technology and
- That surface is not spherical neither torical*
*Free form machines may cut standard surfaces, but as they can also be cut with standard methods, the corresponding lenses should not be named free-form lenses.
Is Free-Form more precise?
• What is claimed: 0.01 D
• Very difficult to achieve in plastic
• Well tuned process with tools, soft pads and slurries not worn out, guarantees better precision than standard milling and polishing methods.
Has a Free-Form lens better optical quality?
• Not necessarily.
• Free form is a manufacturing process, not a design method, neither an optical design software, but
• Free-form technology will allow you to get better optics, as far as you use good optical design tools.
Why there is so much expectation around FF?
• The great thing around free-form is that ophthalmic industry is no longer tied to base-curve restrictions.
• With standard base curves, a sophisticated lens must be made with a few complex surfaces (semifinished blanks) and spheres or torus.
• With free form, each sophisticated lens may have its own sophisticated surface.
Why a different surface for each lens is better?
• In Optics, asphericity is the way to correct for aberrations when you cannot use many lens elements.
• In ophthalmic optics, compensation must be made by just one lens per eye.
• Each prescription, each lens position, each object position, requires a particular asphericity to get the best possible correction.
• Standard lenses use the same asphericity for thousands of different situations: asphericity becomes useless.
• A different surface for each lens will allow a different asphericity for each lens: optimum compensation of side effects (aberrations).
What can be done by advanced ophthalmic design and Free-Form technology?
• Optimize the lens for the power that the user perceives, not the power measured in the focimeter (which is ok for quality control, but is not the power perceived by the user)
• Optimize the lens for any position of use (tilts, distance to the eye, etc…)
• Optimize the lens for any base curve (the spherical base curves used with FF lenses)
• Optimize the lens for each prescription
• Optimize the lens for particular applications
• Better control of lens thickness
• Off-centering without losing optical quality
…Important improvement to the optical quality of standard lenses
What cannot be done with advanced ophthalmic design and Free-Form Technology?
• Progressive lenses that violates Minkwitz’s theorem (PALs with wider corridors for the same corridor length.)
• Lenses without aberrations
• Lenses which compensates for third order eye aberrations at any direction of sight (those can be compensated only within a very narrow field)
• PALs without adaptation period.
• Short PALs with intermediate wide enough for computer displays.
Back-side or front-side?
• The position of the progressive surface (even if there are two progressive surfaces) is not that important.
• The important thing is the progressive surface (or both) being free-form and computed with good software, not the position.
• The performance of any front side PAL can be reproduced in a back side PAL. The opposite is also true.
• Some miss-conceptions about back-side progressives:
• “Field is wider because the surface is nearer to the eye.” Indeed the back surface is a little bit nearer, but also there is less room for progression on the back, so there is not net improvement.
• “Magnification is more stable because front refractive power is constant.” That’s true, but the effect is so small. Magnification is mainly due to power. As power increases in a PAL, so does magnification.
• “Back side PALs produces less distortion”. False. Distortion depends on the power variation. The faster the variation, the larger the distortion. This is a characteristic of the design, whether it is back of front.
•Asphericity is more effective on the more curved surface (a 0.5 D surface is so flat there is almost no range to compensate or anything):
- In positive lenses, ideal Free-Form surface should be the front surface
- In negative lenses, ideal Free-Form surface should be the back surace
- But manufacturing currently requires to have the Free-Form surface at the same side -> Back.
• FF is not magic, but is a great improvement.
• To take full advantage of FF technology, the lenses should be optimized for real user-perceived power
• Customization only makes real sense if the lens is optimized for user power
• Position of use only makes sense for user power.
• Optimization for each prescription only makes sense for user power.
If a lens is optimized for user power, focimeter readings does not fully coincide with prescription.
And correct addition in user power reads considerably smaller in a focimeter!!!
Curvature vs eye-perceived power
• A classical lens may have a nice cylinder map when we obtain it from surface optimization,
• But then ray-tracing shows that real lens performance is not so good.