I have a very large futuristic city built with greebles, primitives, terrains and a nice atmosphere. Technically where i want it to be. It has no life however. I want to add a few flying vehicles as either still images or as an animation. Since it will only be 10 seconds or so I really don't need the clouds etc to move.
Can I put these on a seprate layer and only do a render pass on that layer? I know i can re-render the whole scene, and choose to make that second layer it's own file, but as an animation this would be a long process as the city and sky take a long time to render. Or since the sky and city is unchanged would I just do a new animation with the city hidden from render and combine passes later in photoshop? lastly is there a smarter way?

Thanks