Exposure conversions
A simple flash app to convert between intensity and exposure.
I included a sample scale of the Inverse Square Law as well: for each doubling of the distance, the intensity falls by a factor of 4.
How can we define intensity 1 or exposure 0 in CG?
Because of the non linear nature of perception, the human eye perceive something as being half as bright when it has 0.18 times the original intensity.
Photographic light meters, including those found in digital cameras, are designed to read an 18 percent gray card as a neutral (middle tone) of gray and calculate the exposure needed to create an image of the card as a neutral tone (half the brightness of the available light) midway between solid black and pure white.
Using the 1981 edition of "The Negative", Chapter 3 "Exposure," Section "Metering Exposure," by Ansel Adams as a reference, we could say:
Making a reading from a professional mid-grey card surface (which is referred to as 18-percent reflectance neutral gray and which matches the forth gray from bottom left on a macbeth card) and using that reading to determine an exposure 0, will cause that surface to be reproduced as middle gray in the final print. If we place the gray card within a scene and take a meter reading from it, we are assured that the meter is measuring a middle reflectance value at that time of the day, and we can avoid the pitfalls of a single averaged reading of the entire subject.
If pressed, the manufacturers of some exposure meters will acknowledge that they depart from standard calibration of their meters by incorporating a "K factor." This factor is supposed to give a higher percentage of acceptable images under average conditions than a meter calibrated exactly to an 18 percent reflectance. The practical effect of the K factor is that if we make a careful reading from a middle gray surface and expose as indicated, the result will not be exactly a middle gray!
So this method has really sense only within the custom color calibration cycles of a live action CG production.
But independently, 0.18 mid gray is a good starting point with wich consider an exposure 0 response and with wich render shaders should be setup to match the mid gray scanned live plate.Here is a sample of a GretagMacbeth card:
