I recently completed a couple of new interior renders for an Avro RJ using 3ds Max and Mental Ray. So I thought this would be a good time to explain the basic Photoshop Postwork I go through.
Compositing in post production is one of the most important and probably most over looked parts of CG. It was only recently I started compositing different renders to improve the quality of my final images, so I am in no way an expert but this is how I composite my images.
The most important thing is lighting, something that I am yet to master but here are some great links to get you on your way. 8 Tutorials To Make You Fully Understand 3D Lighting In CG. But this will only get you as far as the render output on the image above.
Rendering out an Ambient Occlusion Pass from your 3d application is one of the best ways to improve results. The render will look something like the one here. Put it on top of your image in Photoshop and Multiply or Overlay it in the layers, you can also adjust the opacity. I then duplicate and merge the layers and adjust the Levels and the Brightness and Contrast.
When rendering out of 3ds Max I have found the images could do with crisping up. To do this add a Unsharp Mask. (Filters > Sharpen > Unsharp Mask)
To get a Lens Blur or Depth of Field you will need to render out a Zdepth pass. The output should look something like this one. You can override all materials instead of replacing them in 3ds Max (Render Setup > Processing > Material Override > Check the Box and drag the material in.) The last thing I do is adjust the contrast and save the image out. There is so much more you can do but here are the basics of what I generally do.