By default, all objects cast and receive caustics, but not all have to in order for you to achieve the look you want. You can fine-tune the look of global illumination or caustics (or reduce render times) by specifying exactly which objects should cast and, or receive photons.

Though you can use these per-object (local) settings alone in many cases, usually you use them in conjunction with scene-wide (global) overrides to give you the maximum amount of control over the rendering of global illumination and caustics. (See Set scene-wide photon tracing overrides.)

How you combine per-object global settings depends on what you want to achieve. You can, for example, flag specific objects to receive global illumination, while the rest of your objects in your scene don’t. Or, you can flag all objects to receive caustics, but only one object to generate caustics.

To flag objects for photon participation

1. In the Scene view, select the object for which you want to change global illumination settings.
2. Open the Attribute Editor ( +a).
3. Click the selected object’s shape node.
4. Expand the mental ray section, and turn off Derive From Maya.
5. Change the Global illumination flag or the Caustics flag.
6. Set scene-wide (global) global illumination overrides if necessary.

   See Set scene-wide photon tracing overrides.

7. Modify the object’s material photon attributes, if necessary.

   (Expand the selected object’s Attribute Editor ( +A), expand the mental ray section, then adjust the attributes.)