The photographer's view of brightness is based on how a camera will expose and is usually expressed as an f/stop (aperture) at a given shutter speed and film/sensor ISO (100, 200, etc). A light meter is used to measure brightness and is essentially measuring lumens and displaying the measurement as an f/stop. When using a basic light meter, one sets the shutter speed and ISO, then makes the measurement and reads the f/stop.
Some meters can display measurements as an exposure value (EV). An EV is a relative measure that expresses brightness without specifying a particular shutter speed, ISO, or aperture. Like an f/stop, a change of one EV is either a doubling or halving of light intensity. The EV is referenced to zero, where 0 EV is equal to f/1 for 1-second at ISO 100. Since an EV is a relative measure, it is mostly used to express an increase or decrease in exposure. For example, if I want to open up the shadow areas (while risking blowing out highlight areas) I might increase my exposure by 2 EV.
Knowing the brightness of a light source is essential to any photographer. When using ambient light, the light meter built into most cameras is more than adequate, but in the studio, where we create our lighting more often than not with strobes, we want to set the power according to our vision, be it brightly lit, dim and dramatic, or anything in between. This is why I am an advocate of using a light meter in the studio. In a future post I would like to explain why a meter is superior to using a digital camera's histogram, but in a nutshell, it is because the histogram is based on a jpeg file generated in the camera, to which a contrast curve has been applied. It is the application of the contrast that makes the histogram inadequate. More than a few times I have bracketed in the studio because the histogram indicated that my light meter had given me an incorrect reading. To date, the light meter has always proven to be right, so I will hang my hat on my light meter and use the histogram as an approximation.
When using multiple lights, one must consider areas of overlap and calculate the brightness accordingly. For example, I set my key (main) light for f/5.6 and my fill light for f/5.6 and the fill light overlaps the key light. In this case, for a standard exposure my aperture must be set to f/8 because in the overlap area I am getting twice the light (+1EV) compared to the key light, so the proper aperture would be f/8 and the lighting ratio is 2:1.
Here is another example. If I set my key light for f/8 and my fill light for f/5.6, then for a standard exposure, my aperture must be set to f/9.5 (1/2-stop smaller than f/8). The key light has twice the brightness of the fill. Thinking in terms of units of light, I get two units of light from the key and one unit from the fill for a total of 3 units in the overlap area. The lighting ration is 3:1 and the correct exposure is f/9.5, 1/2-stop smaller than what would be needed for the f/8 key light by itself.
One last note on lighting ratios. The ratio refers to the brightest area of lighting (key) compared to the shadow lighting (fill). When someone refers to the "key light" they usually mean the physical light unit on the key side. However, when the "key light" and "fill light" overlap as they typically do, the brightness for the key lighting is the sum of both light sources, so the ratio expressed as the sum of the two lights -Vs- the fill.
Distinguishing between a key light and key lighting can be confusing because of the vagueness and similarity of those terms. Unfortunately, this is not the only place where semantics fail us in the studio. If I tell you that I used a 22" reflector am I indicating that I put a beauty dish or some such large reflector on my flash, or am I telling you that I used a 22" reflective surface to bounce light from the key light to use for fill? ...sigh.
Hopefully this illustration makes things more clear.
Continue on to "Quality of Light - part 1"