Then mold makers would take the stylist's design and have a prototype maker re-engineer it for molding considerations. Getting a prototype took a long time - and the people who could make hand prototypes are now reaching retirement age. Though these craftsmen are masters in their art, symmetry and precision can't always be guaranteed. You have to care about how the mold is made and push the limits of process. All parts have molding flaws, and you know you'll have to incorporate them for a successful design.
Optics provide another challenge in designing eyewear.
You have to pay particular attention to optical design or vision will be distorted. This allows me to verify the optical performance accurately. Ten years ago, flat front 6-base styles only were possible. The higher the number, the smaller the radius and the higher the accuracy required for good optics. Today's designer aims for the widest possible field of view, with the best coverage and ventilation, he says.