Concentrate on what a lens does (5)

I believe the answer is:

focus

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'concentrate' is the definition.
(I know that concentrate can be written as focus)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for focus that I've seen before include "Spotlight" , "Centre of interest or activity (5)" , "Bull's eye" , "Hub" , "Adjust (camera lens)" .)

I've seen this clue in The Mirror.

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