I get asked this approximately once every 6 months and I always forget which means I have to spend a bunch of time looking it up again. This kind of conversion usually comes up going back and forth between tweaking lights in the render and color correcting the individual outputs in the comp. I’ll jot the math down here so I know where to look for it next time. Oh, and since it’s about relentless play and hacking in general, maybe I’ll build some quick little javascript/HTML calculators, cuz I just like to party like that!
Converting from stops to a multiplier
This is useful when you want to use f-stops with a color correct node that multiplies your color values.
newGain = oldGain * pow(2,exposureChange)
| New Gain: | 666 |
Converting from a multiplier to stops
This is useful when you have color corrects in the comp and want to bake them back into your render’s lights.
newExposure = oldExposure + log(gainChange,2)
| New Exposure: | 666 |
Adjusting for distance
While we’re at it, you often need to move a light backwards or forwards while keeping the same light intensity on a subject. Assuming that “oldDistance” is your light’s current distance to subject and “newDistance” is the new distance to subject, you can use the following formula to figure out the new exposure required for the light to have the same intensity on your subject from the new position.
newExposure = oldExposure + log(pow(newDistance,2)/pow(oldDistance,2),2)
| New Exposure | 666 |
Adjusting for light area
In some renderers the light intensity is not normalized to the area of the light itself, which means that your light becomes brighter as you scale it up. If you want the amount of light to remain similar as you scale it up or down, this is your formula:
areaRatio = (oldWidth * oldHeight)/(newWidth * newHeight)
newExposure = oldExposure + log(areaRatio, 2)
| New Exposure | 666 |