When I started using Nuke, I was perplexed about how to make nice lens flares. I was confused, since every other compositing program I had ever used before had a built in 'Lens Flare' node that would 'just make it'.
Instead, Nuke comes with the 'Flare' node, a very flexible node that can create most lens flare effects that you would need.
There are several Lens Flare gizmos that you can download from Nukepedia:
In this tutorial, I go through the gizmos that you can download and install into Nuke to create Lens Flares. We'll examine the 'Flare' tool in Nuke, going into detail looking at the individual controls. Then we'll construct a lens flare from scratch.