Photon Caustics

Works with Path Tracing. Render Properties → Light Paths → Caustics → Photon Caustics. It is for the sharp caustics of glass, water and polished metal, on surfaces and inside volumes such as light shafts under water. It does nothing for soft bounced light; that is what Path Guiding is for.

• Photons: photons per pixel of the render for every photon map, 2 by default, so a preview at a lower resolution traces as many per pixel as the final render. More give smoother caustics at the cost of render time and memory. Photons bounce as far as Light Paths allows, the same limits Path Tracing uses. Strength: 1.0 is the physically correct result.
• Casters subpanel: which objects throw caustics. Automatic picks refractive and sharply reflecting objects from their materials. Manual lets you pick objects and collections yourself, to put all the effort where you want it.

Every light produces caustics by default. To leave one out, untick Emit Photons in Light Properties → Settings for a lamp, World Properties → Settings → Surface for the world, or Object Properties → Shading → Caustics for a glowing object.