based indeed on the reflected light the camera records when the sources overlap. Very early analog ratio meters consisted of a card with two holes, one blank and the other covered with a strip of neutral density. The blank hole would be held over the shadow side of the face and the hole with the filters over the highlight side and adjusted until the appearance matched visually. Each .30 difference in neutral density was equal to a 2x difference in reflected light. In the case of two equal sources overlapping in a key over centered fill strategy the incident strength is 1:1 between them but the overlap creates a 2:1 reflected ratio. So when expressing ratios it is important to make the distinction between incident and reflected to avoid confusion. Negative fill / subtractive lighting In cases where the fill light is desired to be darker than what is available without artificial means, a flag or frame may be used to block ambient light and thereby provide what is called negative fill. In omnidirectional open shade or an overcast day where the light creates few highlight or shadow clues regarding 3D blocking the light on one side will have the net effect of making the light from the other direction the more dominant "key" vector in the lighting pattern using a process also referred to as "subtractive lighting". Reflectors as fill sources An alternative to using a direct light source as a fill is to re-direct or "bounce" the key light towards the subject by using a reflector. When used with artificial key light it can be difficult to place a reflector where it can both catch and reflect the light and have it bounce back onto the subject at the ideal "neutral" near-axis angle, often resulting in dark, underfilled areas in the lighting pattern. This is less of a problem outdoors where light usually comes from many directions. Spill fill The term "spill fill" refers to fill light which results from the footprint of light sources bouncing off surfaces in the shooting environment. It can, if not observed and understood, lead to erroneous assumptions about lighting strategies and modifier choices. For example, a difference between a softbox and shoot-through umbrella of identical size used in a small reflective space is that the softbox is designed to limit spill and the shoot through umbrella to maximize it. The umbrella will