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The linear mode never works, the images are always black.

What are your settings? Try 100 for sensitivity, 1/125 for exposure and 1.4 for f-stop. If it's still black, the lights in your scene are not bright enough.

As for the fireflies: you've probably heard this already, but make sure no color value in your scene is higher than 0.8.

-- I'm not mad at you, just Westphalian.