Linking Opacity to Z coordinates in the compositor I have a particle system with tons of particles flying towards the camera. They are supposed to be emerging from a dust storm.

Is there some way to have the particles start invisible, and then become opaque as they travel towards the camera?

I am stuck, trying to take a "mist pass", using that as the factor in a mix node, and mixing the rendered image with a 2nd image. This would do exactly what I needed if the 2nd image could just be a transparent image (just emptiness).

Any help would be greatly appreciated!
The Camera Data node gives you the distance from the camera. You can then add a math node to adjust that to get the desired amount of transparency.