Why accurately measure incident light in studio photography?

Question

I wanted to understand the value of incident light meters and tried web search for "Why measure incident light in studio photography?". I found into that such meters measure light more accurately. I understand the logic, I know accurate color representation is important to show objects on photo as they are in real life. But in studio total light exposure is what is variable (not "natural") and depends on powers of light sources and amount of light is just one number (lux), no need to measure complex relationships like for RGB correctness.

What is the point of correctly measuring amount of incident artificial light? IMHO it is the amount of reflected light that defines how photo would look like.

Answer 1

Reflected light meters by necessity assume a normal reflectance level. At one time this was referred to as 18 percent gray but the exact value is commonly disputed.

The average reflectance level is not important. Consider two very different portraits, one of a white wedding dress, the other a black cocktail dress. An incident meter will tell you the light hitting the subjects. A properly exposed shot will produce white whites and black blacks.

Metering reflectance will give different exposures for the two cases, dropping the white dress exposure more toward gray and increasing the black dress exposure more toward gray.

Incident metering removes the guesswork as to reflectance effects.

Answer 2

Incident metering is generally more accurate than reflected, since the actual incident light is independent of the various reflection factors from the subjects several colors. Theoretically, black will reflect near 0% and white will reflect near 100%, however MOST IMPORTANTLY, both will come out near correct if incident metering of the actual light. Metering the actual incident light centers the exposure range so that both black and white extremes will come out correct. If a color reflects near 0%, it shows as black. If it reflects near 100%, it shows as white. The eye also sees this at the scene too. This is NOT just about black and white, for example blue and green meter quite differently too. This considerably disturbs reflected light metering. But incident metering (of the actual light on the subject, and if flash or indoor lighting, at the correct subject distance) is about fail-safe. The photographer can still choose to favor one end or the other.

However, for studio use, there is another very important practical matter. Outdoors, there is generally only the light from the one Sun. In studio use, multiple lights are often used for planned purposes. And then actually metering each light at the subject is the only way to know precisely what each light is actually doing. Metering each light is how each lights actual intensity is adjusted and set to be the planned value for planned effect. For example, the Fill light may be set say 1 1/3 stop less than the Main light, both metered at the subject. Metering each light is how the lighting ratio is carefully created. This is how studio work differs from outdoors snapshots (the indoor light is planned and created, and controlling the chosen lighting ratio is all important to studio work).

In use outdoors, reflected metering is more conveniently done (depending on some unknown variable "average reflectance factor" of the scene, but at least it can be metered at the camera location). And incident metering is more awkward outdoors (at least for distant subjects), since incident must be metered at the subjects actual location (the light actually incident on the subject). This "inconvenience" is not important in the studio, where the subject is quite accessible, and the lighting is more carefully planned, and controlling each light requires more precision.