What is a characteristic of flat lenses?

A characteristic of flat lenses is that they may rotate off axis during a blink. Flat lenses are designed with a base curve that is too flat in relation to the corneal curvature, which can lead to instability on the eye. When a person blinks, the natural movement can cause these lenses to shift or rotate, potentially affecting vision and comfort.

The other options do not accurately represent the behavior of flat lenses. For example, while flat lenses may not lock onto the cornea as well as those with a more matching curvature, this property does not solely define them. Additionally, comfort can vary based on individual anatomy and lens fit rather than curvature alone, and tear exchange is influenced by various factors including the overall design and fit of the lens rather than just the flatness of the curvature.