Focal Length

Focal Length

Focal length is the distance at which incoming parallel rays converge after passing through the microlens. It’s governed by both the radius of curvature and the refractive index—but strongly influenced by the sag height.
For a fixed diameter and material, increasing sag shortens the focal length, enhancing optical power.
Conversely, reducing sag lengthens focal length, producing gentler focusing behavior.

Trade-offs:
Short focal length: Compact systems, stronger focusing, but tighter alignment and tolerance needs.
Long focal length: Gentler optics, more depth of field, but greater system size and lower angular resolution.

Important Note: Designers must ensure sag and focal length are co-optimized, as over-aggressive sag requirements may exceed fabrication capabilities even if the desired focal length is theoretically achievable.