Mix not to mention match:

What I tend to do on almost every project (but you cannot assume all of course - this really another thing where artistic license makes the equation and you exercise from project to project when it's relevant or not) is actually use both AO methods so you have beautiful shadows with indirect bounces AND attractive connecting shadows with well-defined corners.

If you know about rendering in passes then this will likely seem very basic, but what I actually do is setup my picture first with MR A&D components (with occlusion switched on) as well as photometric lighting then render out the best pass. I then open another version of the scene and save it as being an 'AO' version, setup my Mental Ray material while using the ambient/reflective occlusion shader and plug it inside the 'material override' section, delete each of the lights, set the experience to white, turn off FG and power down exposure control. It may seem strange to get this done in a separate file but I still find it easier to keep things organised like this, I don't like being forced to switch everything on / off all the time in the main file, keeping them separate will mean I don't forget to show things back on ever again or accidentally delete quite a few lights. There are other explanations why having your AO version as a separate file a wonderful idea but we'll go over those this section. Once this is all setup and also have your renders it's a chance to stick them together so we switch for the compositing software. I use Photoshop for stills and Results for animation (although in the event that I'm rendering Exr files I can always use After Effects the way it deals with them greater than Photoshop) but available for you whatever compositing software you're more comfortable with, the concept is the same whatever you use:

1: In Photoshop tell you lot more your 2 files, the best pass and the occlusion pass

2: Copy the occlusion forward and paste it onto a whole new layer over the primary pass

3: Change the layer setting for the occlusion pass to 'multiply' along with adjust the opacity to match your taste

4: Done! If you switch journey occlusion layer's visibility you can understand how much of an affect there are on the final look, it really adds definition and provide your shadows a great sense of depth.

The fiddly chunks:

There are a several instances where the scheme used above doesn't seriously work, these are also when possessing a separate file for this AO starts to makes lots of sense so let's briefly discuss with you them:

1: Custom AO radii

The basic approach to adding a global ambient occlusion shader for the entire scene is fun for getting quick results nonetheless, as discussed earlier, depending on their size/detail objects sometimes want different 'max distance' principles - a scene where just about every object has the exact same setting just doesn't glance quite right. So what you need is to ditch the particular 'material override' option as well as apply custom AO settings towards your objects. You don't require to set them up individually as expected, you can go with a few AO mats which happen to have different distance values - one for details and small objects, one for medium and one for large.