hello, I just spent hours figuring out how to render out a proper zPass from maya -and I nearly went insane!!! Using the Depth channel button in render globals seems to have no effect when running on a mac, on a pc I had some luck sometimes...

HERE'S AN INSTRUCTION ON HOW TO RENDER A zPASS FROM MAYA

Create a render layer, right click on it go presets>luminance depth and that's it you can even preview it in your render window before batching, don't know if that's the "prefessional" way to do it but it's easy and it works.
Maya creates a surface shader and ataches it to all objects on your layer, it doesn't seem to do that for particles though, you'll have to do that manually.

maybe this was already a well known fact, I thought I'd share it anyway

cheers