If your main studio light is set at F11 what would you set the filling flash at? F8?
You are asking about exposure for a specific lighting ratio. The main should be placed to illuminate most of the face, thus it is set high and centered or slightly off center. The idea is to have this light play on the subject in a flattering way.
Because the main lamp casts shadows that will likely go dark and thus be void of detail, we add a second lamp called the fill. Since the job of the fill is to illuminate shadows cast by the main, the best location is at lens height as close to an imaginary line drawn lens-to- subject. This is because we are filling shadows from the camera’s viewpoint.
Most portrait photographers soon discover that images sell best if an illusion of one light, from above is preserved. To this end, we set the fill subordinate to the main.
For this discussion, assume the main casts 1000 units of light on the frontal portion of the subjects face. The shadows receive zero light. Now we place the main subordinate by 50%. That’s one f-stop since each f-stop is a 2X change in light brightness. Now we have 1000 units on the front of face from the main and 500 units in the shadow areas. We also have 1000 + 500 = 1500 units where both play on the subject. Thus the ratio is 1500:500. We handle this ratio as if it were a fraction and reduce it by dividing both sides by 500. This reduces the ratio to 3:1.
The 3:1 ratio is considered the “bread & butter” ratio because it sells best because it is flattering.
How about setting the exposure, main reads f/11, fill reads f/8?. We turn off the main and take the reading only with the fill is operating. Thus the exposure will be f/11.
Seems weird? An old axiom in photography is “expose for the shadows. Try it; you will like it.
If you move the fill further back away from the subject by multiplying its subject to lamp distance by 1.4 you will cut the light in half. Now the exposure for the fill only is f/5.6. In this setup, the main delivers 1000 units, the fill delivers 250 units. On the frontal area of the face they deliver 1250. The ratio is 1250:250. This reduces to 5:1 a more contrasty lighting. Try it; you will like it. The exposure is f/5.6.
Move the fill further back by again multiplying the lamp to subject distance by 1.4. Now the main delivers 1000 units, the fill delivers 125 units. The combined units are 1125 the ratio 1125:125 = 9:1. This is high contrast theatrical lighting; the exposure is f/4.
Don’t think I don’t know what I am taking about! Try this all for yourself. In years past I was a teacher at the Professional Photographers of America school for continuing education. My subject was color print and process, but from time to time, I helped the great masters place their lights during a class. A tiny bit of knowledge from those masters rubbed off. Now you might call it pixel dust.