It is the role of the key light to create the highlights and shadows with model the features of the face. Fill is a "silent" partner used only to modulate the amount of detail visible in the shadows and the match the range of the scene photographed to the dynamic range of the camera.
The ideal fill is as invisible as the atmosphere, the fill source of the sun. In other words you ideally shouldn't be able to see any highlights or shadows created by the fill. That doesn't mean there can't be a secondary source of highlights beside the key light (e.g., a hair light) but I classify that type of light as an accent light rather than fill.
Positioning the Fill Source: Ideally the fill light should come from the camera axis so it will illuminate everything the camera sees evenly and falls off front to back relative to the camera. This is called "neutral fill" because it does not influence the modeling of the subject. The second best position for fill, applicable to oblique facial views, is to align the fill with the subject's nose so it falls off front-to-back relative to the front of the face.
Pitfalls of side fill: When fill is positioned to the side opposite the key light to catch and reflect it the resulting fill becomes uneven, overfilling the shadow side, particularly the ear. Side fill can also create unflattering cross-shadows in a direction opposite the shadow created by the key light. If you can see cross-shadows from the fill it also means the fill is canceling the modeling effects of the key light. When side fill is used it is possible for body parts such as cheekbones to shade it from other areas such as the "smile line" crease next to the mouth, or the area of the cheek where the nose shadow created by the key light falls. When fill is shaded like this there will be two-tone key light shadows; lighter in areas getting fill and dark voids were the fill is getting shaded by some other body part. The problems of side fill can combine to create a very unflattering mottled pattern of highlight and shadow which looks unnatural. If you must use it, do so carefully watching for uneven dark "dead" spots.
Reflector Fill: The dilemma with reflectors is positioning where they are in front of the subject but still able to catch and reflect the key light source or other ambient light such as the sky outdoors. It's difficult to move a small reflector in front of the subject and keep it out of view of the camera. One solution to that problem is to use a very large reflector panel behind the camera, or a "shoot-through" panel with a hole large enough for the camera lens. Neither is very practical unless you have a phalanx of assistants to hold the reflector. Regardless of the size reflector used try to keep it as far forward, relative to the front of the subject's face, as possible.
- White: A white reflector is best for portraiture because its light is soft and less likely to create distracting specular reflections within the shadows it is filling.
- Silver: A silver reflector will produce more light than white, but the character of the light will be more direct and more likely to produce specular highlights on skin and other shiny surfaces. This is not desirable or general portraiture, but may be effect desired for fashion and glamor work.
- Gold: A gold foil reflector will have the same general characteristics as silver, but make the filled areas warmer in tone. This is not desirable or general portraiture, but may be effect desired for fashion and glamor work.
The use of a flash for fill eliminates all of the logistical problems of transporting and positioning a reflector for neutral fill.A fill flash which links to the camera's metering system to provide TTL or "Through the lens" control of exposure will allow independent adjustment of flash output relative to the ambient exposure via Flash Exposure Compensation (FEC). This allows the photographer to "dial-in" the desired amount of fill.
Fill from a direct hot shoe flash will produce unflattering harsh shadows which are cast sideways if the camera is turned sideway into "portrait" mode. Mounting the fill-flash source on a bracket which keeps it 12-18 inches above the lens at all times gives the light a downward angle which hides shadows behind the subject and in more flattering places on the face in both landscape and portrait modes. Equipping the fill flash with a reflection diffuser will soften the edges of the shadows and reduce specular (mirror-like) hot spot reflections on shiny surfaces. See the separate Fill Flash, DIY reflector-diffuser, and other hot shoe tutorials.
High-speed (HS) flash: Standard flashes limit shutter speeds to the x-sync limit of the camera, the shortest time the sensor is fully exposed to light. X-sync speed varies depending on shutter design and sensor size by is generally 1/250th or lower. A typical sunlight exposure at ISO 100 is 1/250th @ f/11 which limits the photographer's creative control of depth of field via aperture. Wide apertures can't be used and a more powerful flash is needed to match the ambient light.
Flashes units with HS capability pulse the flash as the shutter curtains move across the sensor allowing flash exposures of 1/1000 or greater. These faster shutter speeds allow wider apertures and more create control over composition and DOF when fill flash is used in bright ambient light.
Lighting Ratios: The X:Y lighting ratio notation used for still photography is based on the REFLECTED relative brightness of facial highlights, created by the overlapping fill and key lights, vs. the amount of fill light present. X represents the strength of the sky light and varies from 2 to 5 or more. Y represents the strength of the fill and is always 1, e.g. 2:1, 3:1, 4:1, 5:1 See my lighting ratio tutorial for a more detail on the difference between INCIDENT and REFLECTED ratios.
Holistic Concepts for Lighting
and Digital Photography
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