Justin Bieber -- Zooms In On Ex Chantel Jeffries

The saying "old flames die hard" clearly applies to Justin Bieber ... because he was just spotted soaking up the sun in Malibu with his ex, Chantel Jeffries.

Jeffries was the chick in the Lambo with Bieber when he was arrested in Miami for DUI, drag racing and resisting arrest.

Fast forward to Friday ... Bieber got behind the lens at the beach where Jeffries was doing a photoshoot for San Lorenzo swimwear.

So now we know who's zoomin' who.