A lens can pass every visual inspection checkpoint and still fail in the field. That gap between "looks acceptable" and "performs as required" is where precision optics projects run into trouble, and it is a problem that optical metrology was specifically designed to address.
Visual checks are fast and practical, but they are bounded by what the eye and basic tooling can confirm. Surface cleanliness, obvious chips, and gross dimensional errors are all fair game. What they cannot reliably catch are sub-tolerance deviations in curvature, refractive index variation, or edge detection anomalies that only emerge under controlled measurement conditions. Perspective distortion alone can make a non-conforming surface appear within spec during a manual review.
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