Vray Cam and Max Cam exposure equivalency

[Clifton Santiago]

Through trial and error, I have discovered that the VrayPhysicalCamera with Vray Sun/Sky with intensity of 1.0 at at these settings: f-stop 8, shutter speed 200, ISO 100, vignetting disabled, Color mapping Linear defaults (dark 1, bright 1) is almost identical to the Max camera with Color mapping Linear of dark 0.031, bright 0.031 (using 1.5 SP 1 w/ Max 2008 ).

Is there a (simple) formula or chart somewhere that can take the guesswork out of this? I need to have diffent exposures (interior vs. exterior) with Max cameras that match Vray physical cameras. I do not want to adjust the intensity of the sun, because it just feels wrong.

If I wasn't mathematically challenged, maybe I could write a formula...

[DanielBrew]

If the function is linear then try multiplying 1/'shutter speed' by 6.2 and use that value for your light and dark multipliers.

[Clifton Santiago]

Yeah, if I keep the aperture and ISO constant, its pretty simple. I think I'll do more tests, and make a chart for reference. And then do one for Reinhard, which I often use for exteriors. I guess that will be trickier though because it would plot a curve...

[vlado]

The formula that V-Ray uses for the camera exposure is this (the attenuation value in the code below):

  // Take f-number and focal length into account;
  // based on ISO 12232:2006(E) standard for measuring ISO
  // of electronic sensors (defines the ISO number in respect
  // to the amount of light necessary for producing a unit
  // response in the sensor)
  
  aperture_dist=focal_length*focus_distance/(-focal_length+focus_distance);
  R=focus_distance/aperture_dist;
  A_eff=((R+1.0f)/R)*f_number;
  
  attenuation=(65.0f/(78.0f*100.0f))/(A_eff*A_eff);

  // Take into account the tone operator exposure
  attenuation=attenuation/pi();
  ToneOperatorInterface* toneOpInt=
    static_cast<ToneOperatorInterface*>(GetCOREInterface(TONE_OPERATOR_INTERFACE));
  if (!toneOpInt) attenuation=attenuation*1500.0f;
  else {
    ToneOperator *toneOp=toneOpInt->GetToneOperator();
    if (!toneOp) attenuation=attenuation*1500.0f;
    else attenuation=attenuation*toneOp->GetPhysicalUnit(t);
  }

  // Take the shutter speed into account
  attenuation=attenuation/shutter_speed;

  // Adjust the exposure according to the user specified ISO
  attenuation=attenuation*f_ISO;

Best regards,
Vlado

[Clifton Santiago]

Thanks Vlado. I will try and figure out the code...