Lens sag

Lens sag refers to the deformation of a lens under its own weight or external pressure, typically causing it to deviate from its intended shape. This phenomenon is particularly relevant in large lenses, such as those used in telescopes and projectors, and in flexible lenses like contact lenses. The amount of sag depends on factors including the lens material's stiffness (Young's modulus), the lens diameter and thickness, and the gravitational force acting upon it.

In optical systems, lens sag can introduce aberrations, reducing image quality and performance. It is a significant consideration in the design and manufacturing processes of precision optics. Strategies to minimize sag include using stiffer materials, employing thinner lens designs, and providing support structures to counteract the effects of gravity. Furthermore, in the case of adaptive optics, active compensation mechanisms can be implemented to correct for sag-induced distortions in real-time.

In the context of contact lenses, "sag" sometimes also refers to the sagittal depth or chord diameter, a geometric measurement indicating the overall curvature of the lens and how it fits on the eye. This is crucial for ensuring comfortable and effective vision correction.