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The "Photon Map accuracy" in Carrara change's the photon search radius (and maybe the search count?). The higher the percentage, the smaller the search radius. That does then mean more photons would be needed for good coverage.
The "Lighting quality" and "Accuracy" settings (below the photon settings), I am not completely sure, but those may control the final gather .
The Photon Map accuracy, as I understand it, is a type of global map resolution. Smaller percentages, the photons are sent out in a wider range, and with higher percentages, smaller/narrow ranges. So the higher the map accuracy, the more photons that are needed to better match the range. Is this similar?
I'm still researching the Lighting Quality setting. It seems to be linked to some kind of blending of the photon or overall GI lighting calculations. The Accuracy, I'm guessing is similar to the object and shadow accuracy settings. So 16=one sample for every 16 pixels...(8,4,2,1)etc.
The photon map accuracy changes alot when scaling down scenes. The scaled down version will show quite a bit lower photon map accuarcy/resolution than the slider value allows with the default scene render settings. This seems to confirm what PhilW said about going beyond the sliders value limits.
When scaling indirect lighting scenes,...Here are a few things I found so far. >Updated<.</p>
">Photon map accuracy changes. This is probably accounts for most of the render speed increases, but its not the only reason."
I found this is indeed the main reason. The map accuracy seems to be scene magnitude based. If you scale lower than the scene magnitude for the camera view, then it will degrade the photon map accuracy below 0% for that magnitude scale. Still is a very nice option for test renders or final renders that don't need the accuracy.
">Object distances from the main camera frame affect lighting, even if their out of frame."
This only affects the Sun, Moon and Distant lighting. It is On/Off only and it severely degrades the photon map accuracy. If you need these distances, turn off the visability of the object(s) until they are closer to the camera.
">Rendering artifacts(some extreme) occur when rendering far from the origin, especially for rigged objects."
This is a reproducable bug that needs to be fixed. I don't think it has any relation to other issues atm. This bug was reported before the bug tracker went offline.
">Lighting is generaly darker. Possibly due to photons counts being reduced and/or distance from origin."
This only affects the Sun, Moon, and Distant lighting. Scaling down reduces the scenes brightness related to objects distances and/or distance to origin. It is On/Off only and degrades the photon map accuracy. Increase the lighting brightness values if you can't adjust distances etc.
">Lights that are scaled down with the scene might stop rendering at certain scales..."
This only affects the ranges of the Bulb, Spot, Tube, and Shape lights. Increase the range of the light if it stops rendering at certain scales.
With every software there are limitations. These are usually mentioned in the manual, but some like the above are not(or I missed them). I'm still not sure of why or the technical reasons these occur. They could be a bug, or simply the way Carrara's render engine is setup. At the time, I only knew that rendering was much faster for comparable results to more accurate longer render times. Even blurry reflections, one of the slowest render time features, can be viewed and tested to some degree with this method. I'll keep adding new info here as I find it. Thanks for replies, and feel free to continue discussion, test, or list any new information etc.
tbwoq - A very useful summary and thank you for putting in the time and effort to understand all this.