to pick up, inspect, check, consider
撿 centers on the action of picking something up to examine it closely: inspecting, checking, or considering it carefully.
撿 combines 扌 (hand) with 僉, which likely contributes the sound and the idea of gathering or examining. The exact historical development is uncertain.
The hand radical 扌 beside 僉 suggests a hand picking something up to look at it closely. Imagine a hand lifting a small object and bringing it near the eyes for inspection.
For レン, picture a hand picking up a lens to inspect a tiny detail: lens -> レン, and the hand brings the lens close to check.