Like a point of interest in optics (5)

I believe the answer is:

focal

I'm a little stuck... Click here to teach me more about this clue!

'point of interest in optics' is the definition.
'focal' can be an answer for 'point' (focal point is a kind of point). I'm not certain of the 'of interest in optics' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for focal that I've seen before include "Things are concentrated at such a point" , "sort of length" , "Type of length or point" , "Relating to a centre of interest" , "Relating to a central point of interest" .)

I've seen this clue in the Evening Standard.

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