Where should caliper fingers be placed when measuring the minimum thickness of a lens?

When measuring the minimum thickness of a lens, the caliper fingers should be positioned on the optical center for a minus lens. This is because the optical center is the point on the lens where light passes through without deviation, serving as a key reference point for various measurements, including lens thickness. For minus lenses, which are concave and generally thinner at the center compared to the edges, measuring at the optical center provides an accurate representation of the lens's minimum thickness.

Placing the calipers on the optical center allows for consistency in measurements, ensuring accurate assessments that are crucial for fitting the lenses properly in eyeglass frames. The measurement taken at this point is essential, particularly for minus lenses, as it directly impacts the overall performance and appearance of the eyewear.