Find lens in Mountain Temple was a mission in Jak II. Jak was looking for answers regarding the Tomb of Mar, for which in turn Onin send Jak to collect three artifacts from the Mountain Temple, one of them was the Lens.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Just the Artifacts Ma'am", completing this one and the other two related artifact missions at the Mountain Temple will reward you with a bronze trophy.
A collection of three mission spread out over a single large area, each with its own linear path. When entering the mountain area and taking the elevator down and immediately head left through a hole in the wall here. A lone (stronger than before) centurion guards this place, defeat it and climb up here to be ambushed by hoppers. They're weak and small but annoying in numbers when they jump out of the water, the Scatter Gun makes short work of them.
Continue and jump down into the open area, where a ram head attacks you. Its armor is impenetrable so don't bother attacking it that way; instead you'll want to dodge its charge attack and shoot the weak underbelly. When defeated you can press the nearby button to reveal some new platforms to get out of there. Up here a centurion guards the gap, kill it and walk towards the apparent dead end here.
Something happens and some precursor platforms appear out of the ground, use these to reach new higher ground from where you can continue forwards. After some platforming; another hopper ambush from the water and past it a bigger group of manic heads. Past this bunch is yet another centurion as well as another button which reveals more platforms to climb.
Here is the area for the Lens, surrounded by water; the last hopper ambush as well as the biggest. The Scatter Gun will make short work of these as well. Grab the Lens and take a nearby platform to return to the starting area.