How to get a object mask as part of the alpha channel? I am trying to get the middle sphere to render as a part of the alpha for the image. In the image below where I have the test grid image at the moment it really should be transparent. How do I do that?
If you are using cycles, all you have to do is use holdout sahder node.
 Give the object that you want to have the 0 alpha a new material like this.

Then in the render tab of the properties window, under the Film section enable Transparent. You will have to save your render to a image format with a alpha channel, such as a .png.