I have found bits and pieces here and there about the arguments against digitally surfaced molded progressives as "true" free form (Varilux, duh), but I want to explore the reasoning behind this stance. I understand that *if* you define free form as surfacing the entire lens design onto the backside of a spherical blank, then Varilux (and a few others') lenses don't qualify. However, "free form" seems in theses forums to be used interchangeably as a description of surfacing technique and a measure of quality. I have encountered numerous rebuttals of Essilor product as "not true free form", clearly implying they are inferior to "true" 100% backside free form designs.

One of the prime purported benefits of backside design is the "keyhole effect" and gain in near vision width of field. Maybe I'm a sucker for E propaganda, but they have both mathematically and clinically dispelled the myth of that benefit. What remains is the potential "freedom" of designing the entire lens freely on the backside, but that must still take into account the existing front curve. Since we're just designing with mathematical models, why should an existing progressive front curve provide excessive technical challenges over a merely spherical curve? The backside can compensate for anything on the front, and in E's theory existing frontside curves can compliment and enhance backside design for a greater combined effect than 100% backside alone.

I am as open to debate as anyone, but I have not encountered compelling arguments for 100% backside vs. hybrid in free form lenses. I know there are plenty of 100% backside proponents here; what's the definitive opticians' argument in favor of the backside vs. hybrid?