-To increase photon or diffuse depthes for add new caustics-:
-To increase specular depthes for remove any black area-: But IMPORTANT FIRST: To decrease "ray epsilon" from default value to ideal value for remove any black area. If this doesn't eliminate any black area, try to increase specular depthes.
-- Watch with read my description - https://youtu.be/LnAUfa7tGmc
--> If you have any doubt, reply here.
Good tip, happy rendering and best regards.
