How quickly you realised you'd been photographed? (2,1,5)

I believe the answer is:

in a flash

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'how quickly' is the definition.
(I've seen this before)

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

(Other definitions for in a flash that I've seen before include "pictures may be taken" , "Where American detected electricity" , "How lightning appears" , "Almost instantly" , "Instantly, straightaway" .)

I've seen this clue in the Evening Standard.

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