Concentrate on what a lens can do (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 the remainder of the clue.

Can you help me to learn more?

(Other definitions for focus that I've seen before include "Main topic of attention" , "Adjust camera lenses - concentrate" , "Spotlight" , "Where light-rays meet" , "Adjust for sharp image" .)

I've seen this clue in The Daily Express.

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