Q: My Final Gather scene looks noisy.
A: Take a look at the Advanced Global Illumination Tutorial, and see if it can help you tune your FG settings.

Q: I can't visualize the photons in the external renderer. (Turtle 3.1 and earlier)
A: It's likely that you are using liquid light photon mapping. In this case there are no ordinary photon map to visualize. A way to get some idea of how it looks is to render the scene ambient pass without using final gather in which case the indirect light will be a rough view of the liquid light photon map.

Q: My Final Gather scene flickers in animation.
A: Take a look at the Final Gather in Animation tutorial and see if it can help you to tune your settings.

Q: I get black dots when combining Final Gather (or Occlusion) and displacements.
A: Adjust the "Max Ray Offset" parameter on the ilrDisplace shader. A good start value is 50% of the displacement scale value.

Q: I like the look of Global Illumination but the price is too high.
A: Global Illumination is a costly technique compared to standard ray tracing, but a correctly setup Global Illumination scene does not have to be expensive, especially in animations where a lot of the Global Illumination calculations can be reused. If only the camera is animated, use the Static Final Gather render stat (or the global override) and save the Photon Maps to disk. This way the Global Illumination solution will only be calculated once for the entire animation. Use this in conjunction with the Final Gather Exploit Frame Coherence feature. Exploiting Frame Coherence will speed up the Final Gather calculation and reduce flickering.

Q: I get light leakage in scenes with lights emitting photons into a room.
A: The problem is that photons stored on the outside are included in the light calculation. When you use photon mapping together with final gather, every photon is stored (including the first bounce). This works well if the light is placed inside the room, but it can cause problems if the light is placed outside, as you will get a lot of photons on the outside walls. This in combination with a large photon search radius is causing the light leakage. The easiest way to solve this problem is to exclude the first bounce from the photon map. Set the parameter "Use Min Photon Depth" to "Specified", and the "Min Photon Depth" to 1, to remove the leakage artifacts.

Q: The amount of GI light in my scene seems to be due to its scale.
A: The photon map intensity is dependent on the size of the scene. Let me try to explain why that is; The light from the photon map is calculated as the energy of the photons found, divided by the search area, L = E / (PI*r^2). So if your scene is scaled up the search area is also scaled up, which results in a lower light intensity (if you don't scale the photon energy as well). Moreover, since the area depends on the squared radius, if you scale the scene 10 times you need to scale the energy 10^2 = 100 times, to get the same light intensity as before.

Another thing you need to adjust is the photon search radius. It's always given in cm, so if your scene is 47m (4.700 cm), the photon radius should be set to something like 300-500cm.

These two images shows two scenes with different scales, different settings, but nearly identical results.