An opening in optics (5)

I believe the answer is:

pupil

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'an opening' is the definition.
(pupil is a kind of opening)

I cannot understand how the rest of the clue works.

(Other definitions for pupil that I've seen before include "The sort of student" , "Schoolchild; part of eye" , "feature in "Observer"?" , "School learner" , "Eye hole" .)

I've seen this clue in the Evening Standard.

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