There is also the option to create a holdout matte on a per-object basis.
Enable or disable the matte effect. Disabling 'Enable Matte' returns to normal shading.
Changes the color of the matte.
This value enables you to alter the alpha contribution (0 to 1, cut out to normal).
Note that the matte is only applied to camera rays and will therefore not be visible in reflections and refractions.
Matte colors are not visible in the refractions of the glass sphere
In this example, the matte property is applied to the green sphere, which is semi-opaque and reflected by the cube. These are the starting RGB and the alpha values, with the matte disabled.
This is the matte with the default values. Note that the sphere looks the same as above when reflected.
Matte disabled. Note that the opacity and alpha are taken from the spheres shader.
Overriding opacity, set to 0. The sphere is now invisible to eye rays, but still visible in the reflections and shadows.
Further examples (rollover images).