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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.

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  • @Mark M Exactly the opposite interests me - what value from incident light meter when actual reflected light might differ? What value in shooing black and white with same exposure setting? It might be for "realism" (as we see nature), but my question pertains to studio. Sep 5, 2021 at 4:05

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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.

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  • I see, it is useful if photographer assumes small part of scene as "standard" exposure, human face in your example, right? And there are no practical means to measure reflective light from this small part of scene only? Sep 5, 2021 at 4:27
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    @Martian2020: cameras allow spot metering, which will integrate the light from a small area and properly expose that area. If you have a standard reflectivity area, this may suffice. I do a lot of birds and find the spot is still too large, so I have to guess what exposure compensation to use. I am sure reflected light meters can be purchased or set to look at a small area, but sometimes you don't have any area that is standard reflectivity. Sep 5, 2021 at 4:33
  • As a rough standby, you can always take a (reflected) meter reading off your own (non-camera) hand. Just be mindful of any difference in position between the lights, subject, and your hand, as that will affect the metering. Personal preference though… use an ambient meter in a studio.
    – John
    Sep 5, 2021 at 8:59
  • @RossMillikan Spot metering still doesn't know if the spot being metered is a dark colored thing or a light colored thing. Any kind of simple reflectance metering will attempt to expose whatever is metered as a medium brightness object, whether it's a black object, a white object, or any other color/reflectance in between. In the case of your birds, incident metering done properly is not possible unless you have a way of placing the light meter directly at the bird's location. Thus, reflectance metering, with all of its foibles, is the best we have available.
    – Michael C
    Sep 5, 2021 at 18:32
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    Is 18% supposed to be a "normal" or "average" reflectance level, or is it intended to yield an exposure which will most often stay within the latitude of the film? I would think light metering should be based upon the latter goal, and the presumed "average" light level of the scene should be chosen based upon the latitude of the exposure medium.
    – supercat
    Sep 6, 2021 at 15:22
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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.

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  • Thank you. However, from your answer it is not clear what is the value of "correctly" measuring each light source. Could you add for your example of two lights setup: what is it supposed to achieve and what for? TIA Sep 13, 2021 at 15:40
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    The concept of lighting in Studio portraits: Two lights are used for the Main and Fill, with a frontal Fill metered to be a planned measured amount less than Main, to create desired lighting ratio, to generally produce soft gradient shadows on the face, for a natural look showing shape and curves (not a "deer in the headlights" look). Often another light is used for a hair light and another for the light on the background. A formal portrait is carefully controlled. The only way to design what each light is actually doing is to individually and "correctly" meter each light at the subject.
    – WayneF
    Sep 14, 2021 at 3:14

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