A singlet lens whose front and back radii of curvature are optimized for a specific conjugate ratio to yield the smallest possible focused spot. For focusing a collimated laser beam, this usually results in a distinct meniscus or asymmetric biconvex shape that balances the refraction evenly across both surfaces.
Heard on the job
Swap that symmetric biconvex out for a best form lens so we can get a tighter spot on the detector.