On the first pass, the estimated time is wildly inaccurate, however it does improve with subsequent passes. However, as jfike pointed out, the elapsed time and percentage are lost whenever the render is interruped. The other problem is that it does only update at the end of a pass, in some cases the later passes can take a few hours, which means your estimated time left and elapsed time don't update for a few hours. Mahray

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Ignore the shooty dog thing.