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Angle of incidence equals angle of reflectance.
Auto is useful if you know the reflected value needed from the source, subject and angle combination; just set it an go.
Angle of incidence equals angle of reflectance.
Using auto becomes difficult when the reflected value seen by the camera and the reflected value seen by the flash receptor are different.
For instance, if the light coming from the flash (incidence) is "skipping" off the subject in such a way that mimics the intensity of the source causing a specular reflection. Sunlight reflecting off concrete can change in apparent intensity as the viewer changes angle. If the sunlight or flash is at the viewing position then the light reflecting off concrete will appear to be reflected somewhat evenly, however if the viewer positions them-self opposite the source (sunlight or flash or filament or neon whatever) they will see the
source more or less reflecting off the surface as they change their viewing (camera) angle. The
source as opposed to the incidental light emitted from the source.
Angle of incidence equals angle of reflectance.
This is also seen when shooting (looking) into a mirror or a window or at another highly reflective subject which can bounce (reflectance) the light directly back to the camera/viewer; possibly showing a hotspot (specularity).
In both cases the reflected value can be greater than what is wanted, resulting in drastically blown out highlights. This can be mitigated by reducing the intensity of the light, such as turning it down; similar ways to do this include increasing the distance between the light and the subject, adding a black netting material between the source and subject (this can effect the
quality of light as well as intensity), adding diffusion or bouncing onto the subject from a wall or ceiling or sheet ect (this will effect the
quality of light) or altering the angle of incidence to gain a more manageable reflectance.
Shooting a fujiroid or a digiroid and considering the limited dynamic range of the jpeg, negative or print can offer insight into the reflected values one may get on film; but consistency in output media (film) and methodology and tools along with practice increases ones success when comparing various mediums.
Angle of incidence equals angle of reflectance
A flash set to auto will generally provide a reasonably consistent output (incidence) as read when reflected back to it's position (reflectance) when set to that value; but adjusting for the angle of incidence (flash output angle relative to the subject) and the angle of reflectance as viewed by the camera or reducing the flash output may be necessary to achieve the reflectance value needed to show detail in the form of texture, tone and/or color.
edit to add:
Light is generally refered to as "soft" or "hard" based on the edge transition from light to shadow; a soft light will have a more gradual transition than the abrupt or "hard" transition of a hard light.
A hairlight may be a more clear example than the concrete. Hair is generally roundish, so when viewing the effects of a hairlight from its position few specular reflections may be visible, depending on the reflectance caused by natural sheen or hairspray etc and whether this light is inherently soft or hard. But when viewed from the camera position the reflected light may show increased specularity due to the angle of reflectance. If the hair is dark the specularity may be useful, however if the hair is white, gray or blonde the specularity may overpower the texture and color requiring the hairlight to be modified in intensity, position or quality or a combination of some or all these.
This is also clearly evident when looking at light coming in through a door or window and illuminating a living room floor such as tile or wood. The apparent intensity will change as one moves around the room, thus changing the viewing angle. By moving around the room one is witnessing differences in angle of incidence equals angle of reflectance and greater or lesser specularity. The same effect as moving a light for a greater or lesser reflection effect or ƒ value.
edit to add:
This does not really consider æstetics such as skin color and reflectance value; white people generally do not look good when recorded at medium grey or 18%. For white skin I usually prefer something like 36% or greater. So for a white person I will generally seek a reflected reading 1 - 1.5 (ish, 1.3 or 1.6 maybe more depending) stops greater than my shooting ƒ-stop.
If someone sees I have made an error somewhere please correct me as soon as possible, I don't want to pass on bad info.