Originally Posted by
Darryl Meister
The terms actually refer to pretty specific lens design features:
Is your progressive a multi design, does it change based on add alone or prescription as well? - multi-design refers to a progressive lens design that changes based upon the addition power; Design by Rx refers to a lens design that changes based upon the addition power and the base curve (distance Rx). These design changes generally include a change in viewing zone size or softness and a change in corridor length. Examples include SOLA Percepta, SOLAOne, Essilor Ovation, Essilor Accolade, etcetera.
Is your progressive asymmetrical? - asymmetrical refers to the use of a lens design with a near zone inset achieved through optical design; symmetrical, on the other hand, refers to a lens design with a near zone inset ahcieved through a mechanical rotation of the design. Asymmetric lens designs generally have significantly higher magnitudes of astigmatism in the nasal region of each lens, but much better binocular alignment between the distance, intermediate, and near zones of the right and lenses compared to symmetrical lens designs.
Does your lens incorporate horizontal symmetry? - horizontal symmetry refers to a lens design that has been optimized binocularly by minimizing differences in power, prism, and magnification between corresponding points on the right and left lenses to improve binocular fusion. This essentially achieves the viewing zone alignment benefits of asymmetry without the excess astigmatism in the nasal region of the lens, which would otherwise disrupt binocular fusion.
Does your progressive design change based on provided panto, dihedral, and vertex measurements or are these measures used just to compensate the prescritpion? - This is a very important distinction. Some laboratories may "tweak" the prescription using software in order to compensate the prescription for lens tilt. This is only effective, however, for the optics over a very small region of the distance zone. Certain free-form lenses actually calculate the correct prescription compensation at many points over the lens design in order to improve the optics of the entire lens. This requires a point-by-point adjustment to the asphericity at each point over the lens surface though, which requires pretty sophisticated optical design software used in conjunction with a free-form delivery system.
Does your design have the capability for variable inset? - variable inset refers to an adjustment to the inset of the intermediate and near zones in order to compensate for the effects of prism induced by the distance zone and the additional convergence required for higher addition powers. With traditional lenses, this is often calculated for each base curve and addition power combination. With certain free-form lenses, it is possible to compute the proper inset for every prescription combination as well as for the position of wear, which can also influence convergence. Examples include Zeiss Individual, Hoyalux iD, etcetera.
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