Which lens type is not defined by the strength of its material?

The correct answer focuses on soft designs, which are not primarily defined by the strength of the material but rather are characterized by their flexibility and comfort. Soft lenses, particularly contact lenses, are made from gel-like materials that conform to the shape of the eye, providing a comfortable fit. This flexibility allows them to be thinner and more breathable compared to rigid lenses.

In contrast, segment bifocals, rigid designs, and monovision designs are defined by specific optical characteristics and material properties. Segment bifocals have distinct optical zones for different vision needs, rigid designs are made from materials that provide specific vision corrections and shape retention, and monovision designs involve adjusting only one eye for near vision while the other is for distance vision, often requiring precise material properties to manage the different focal strengths. Thus, soft designs stand out as they prioritize comfort and fitting over the rigidity or strength of the material used.