regarding calibrating hdris - is it neccesary to do even if I use separate light source (Corona Sun) for direct light? - for example noemotion hdris have no shadows from the sun
Hey!
I would still adjust RGB Level to match Corona Sky values at least. Corona Sun will eat all the sky light from those low exposure HDRis.
And just to be sure, I would remove the low level sun. It will only get stronger if you adjust RGB Level.
If we combine Corona Sun and this NoEmotion HDRi without increasing the RGB Level, we won't even notice the HDRi.

If we do a quick sky calibration like this.


We can finally see the HDRi again.


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I had a quick look at the free sample 3DCollective HDRi.
Looks like someone painted the sun with a large soft brush.
The sun value is 1079 times dimmer then Corona Sun.

Here is Corona Sun for size comparison

