Thanks for reply guys <3
To answer you first vincent, i mean an effect like this:
Where light passes through some dirt and stuff in the camera lens, which then gets lighted up to create more photorealism i guess. Atleast that what i thought when people used alot of it in renders 
The thing is, that i would actually like to insert kind of an dirt lens texture in post process to be say, in the compositor. The thing however is, that im not able to let it react with light sources in the scene, so instead of showing up only within heated light, the texture itself is shown on the whole screen :S
I hope u can follow my weird explanation
Cheers !