Ive come across the technique of rendering in passes
is there any benefit to this in 3dsmax using vray?
how would one do this? just wondering if anyone uses this method
I also tried to learn this technique for doing dof in photoshop with a z buffer layer but when I rendered it out it was just a white square! I have struggled to find a decent tutorial.
u need to have a target camera. and set target distance 
Turn on ‘show’ in the Camera’s Environment Ranges setting - then slide the near/far settings until it encompasses the distances in your scene that you want a z-depth for. Now enter these values into the V-ray z-depth element.
Hope that helps!