Depth Of Field testing

I played with DOF and it works pretty nice!
I would like to see a point with mouse to focus area -function. Now the focus distance setting is
quite slow to find. But it work :)

As I mentioned before, I would now put my energy & time to RTX real-time render engine:

I found out the that Path-Traced way is the only way to have volumetric fog and it this attic scene it is the
way to go to make spectacular renderings. But rendering costs are more important to me. But with this DOF and simple fog and moving moonlight, I have good lights to start with.

What settings should I play with to solve the minor firelfy/blinking light errors in the above render?
Are they coming from some exotic light feature that I could just swap to normal light?