Keenest photo tip must be: remains on the subject (5,2,3,5)

I believe the answer is:

keeps to the point

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'keenest photo tip must be remains on the subject' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'keenest photo tip must be remains on the subject' is the wordplay.
'keenest photo tip' can be anagrammed to 'KEEPS TO THE POINT'.
Though, I am not sure how the anagram is indicated.

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I've seen this clue in the Sydney Morning Herald.

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