How to do different pass to different objects in a post process?

Hi, I have a C# script in the MainCamera Object that excecutes OnRenderImage. I have created a shader image effect and added to a material so it can be added to the render image using the function Graphics.Blit. The shader has 2 Pass. Is it possible to run the first Pass to some objects and the other Pass for the rest? How can I difference the objects? Regards.