Translating VFB exposure to a % of light intensity

V-Ray V-Ray Common Render Theory
1

Couldn’t find this via search, probably because I cant think of a good title for it either.

I have a dusk heli render which has lighting from different eras - the hdri/envrionment was balanced to the background footage and looks good with a VFB exposure of 0.
The hotel lights are all balanced relative to each other, but need a vfb exposure of 2.5 to match the footage.
The landscaping lights are similar and match footage with an exposure of .89
The tower lights are also different in their own set… you get the picture.

Is there a way to translate the vfb exposure increase into a % multiplier so i can work through each set of lights via a spreadsheet instead of doing it by eye?

2

Never mind! I fucked up my first test - i just tried again in a fresh scene and the difference between an exposure of 0 and 1 is double the light power.
I assumed that would be the case but got a different result when i tried originally.

3

The multiplier is 2 raised to the power of the exposure value (i.e. if exposure is 1, then the multiplier is 2^1=2), as you have found out. Glad that you got it working.

Best regards,
Vlado

4

This is why an exposure slider on the lights, even a hidden one, would be useful

5

Hijacking the thread a little… how does the VFB Exposure slider correlate to Physical Camera EV numbers? Does it correlate at all? If I have a render with the VFB Exposure set to “2.5” and I drop this down to “0” but then drop the corresponding camera EV down from 7 to 4.5 I get a much darker image. I expect minor differences in noise, etc due to sampling threshold differences but I’m getting differences greater than expected. Anyone have any advice?

EDIT: I think I’ve sorted this out. The VFB Exposure value does indeed seem to be an inverse adjustment of the camera EV value. The problem in this scene though was the Max Ray Intensity Value was set to 50. As this scene is an interior and has some large, sunlit windows in it, it seems like the ‘50’ value was just too low to allow vray to compensate for the change in exposure. I raised the max ray value to 283 (50x2^2.5) and it now seems to be rendering essentially identical to the original image. BTW-my initial interest in lowering the camera AV was to clean up the noise in the some of the darker areas, which it is now doing nicely.

6

Hi Vlado,

How does this work with negative numbers? I would like to write a python script that normalizes the exposure control from the VRFB.
Using Maya, rect lights with default units and Maya camera.
Is it:
-1 EV lightMult/2
-2 EV (lightMult/2)/2
-3 EV ((lightMult/2)/2)/2

7

It’s 2 to the power of MINUS exposure value.
So
2^0 = 1
2^-1 = 0.5
2^-2 = 0.25
and so on.