thirdpersonDistance is the distance of your camera when in third person, it has a default value of 4.0F, it is private but with reflections we can utilize it.
In older versions we used
double d3 = (double)(this.thirdPersonDistancePrev + (thirdPersonDistance - this.thirdPersonDistancePrev) * partialTicks);
Now we use
double d3 = (double)(this.thirdPersonDistancePrev + (4.0F - this.thirdPersonDistancePrev) * partialTicks);