Part of the problem with terminology, \"terms of art\", and conventions in any craft is they get into common usage without any understanding of the underlying cause and effect.
For example any flash on camera is referred to as most as \"fill\" flash, especially outdoors. But in terms of actual cause and effect it actually the \"key\" light on the foreground because it, not the ambient, create the highlights. The distinction between \"key\" and \"fill\" isn\'t the position of the two lights but which creates the foundation of shadows and which creates the highlights on top of those shadow values.
A \"diffuser\", precisely, is a physical object placed in front of a light source to scatter it\'s rays. Sun / overcast sky is the original model. God said \"let there be light\" and created the Sun. Adam looked pretty good in the Sun because he was a stud dude. But when God took the rib and created Eve she looked down and said, \"Gee that hard light isn\'t flattering Eve at all\" and then created overcast to diffuse the light.
The sun isn\'t a point source, but in terms of relative size from anywhere on earth it looks and acts like one from the POV of the observer . When the sky gets overcast the size of the apparent source gets bigger. The actual cause and effect underlying the more diffuse light overcast produces it that instead of having one strong vector from a single direction it comes more or less equally from all directions. On an overcast day when an incident meter is used dome down in single source measurement mode there\'s only about 1/2 stop difference seen between pointing at where the sun is behind the clouds and the opposite direction. On a clear day that comparison produces a 3 stop difference between sun and skylight opposite.
Even on a clear day the single source sun creates more than one light vector. The direct rays of the sun that reach the earth are parallel (from the POV of the observer) and created distinct shadows. But we are able to see detail in those shadow because that same single source bounces light off water molecules and other stuff in the atmosphere creating many different secondary \"fill\" vectors.
But wait.... as they say on the Slicer commercial... If you put a face in open shade on the side of the building you\'ll usually see the eyes are shaded. Shadows indicate that one vector of light is more dominant than others. The dominant vector in photographic lighting parlance is the \"key\" source creating the modeling over the \"fill\" which is revealing the shadow detail.
In open shade the modeling result from the fact skylight is stronger from directly above than the sides. Like flash used on camera is thought of as only \"fill\" but in terms of cause and effect it is acting like two sources: key vector from above and \"wrap-around\" fill vectors from all directions.
Bounced flash indoors is similar in cause and effect to open shade. There is a dominant downward vector off the ceiling and many \"wrap-around\" fill vectors from the sided and often below off the floor if it is light in tone. Like open shade if the angle of the light is too steep relative to the faces the brows shade the eyes.
Many seeing that cause and effect think, \"Hmm eyes are dark, I need to add fill flash.\" But in terms of actual cause and effect any flash added to lighten the eyes will also hit the cheeks below. The eyes will be lighter, but compared to the highlights on the cheeks they will remain darker.
The ONLY solution to get THE SAME light in eyes and cheek is recognizing the source of the problem is not a lack of fill in the eyes it\'s a lack of key lighting and raising the face into the key light so it will get past the brow into the eyes.
But that cause and effect does not occur to most photographers judging from the number of portraits in shade I see with dull dead looking eye sockets darker than cheeks.
I see the same thing in many indoor flash shots. The flash creates catchlight reflections that give them sparkle but the brow shades both the ambient and the key vector of the flash creating the highlights and the orbits surrounding the catchlight remains dark because they are only illuminated by whatever \"spill fill\" was bouncing around the room.
The cause and effect of shaded eyes and adding \"fill\" flash is more easily seen outdoors on a face is cross lit by direct sun. The incident ratio on cheek and shaded eyes is about 8:1 because direct sun is 3 stops brighter than the skylight creating the fill. The reflected ratio is usually less because the cheeks act like a reflector bouncing fill into the eyes (why athletes put those black strips under their eyes to avoid glare).
H:S
8:1 Incident ratio cheek:eye sockets
What happens when flash 1-stop under the intensity of the sun (4 units) is added?
H:S
8:1
4:4
==
12:5 = 2:1 rounded to the nearest integer
The flash changes the contrast of the overall highlight:shadow pattern to make the shadows lighter and \"softer\" / \"flatter\" (dimensionally) but the eyes will still be darker than the cheeks. Why? that variable is controlled by the orientation of the face and eyes to the key light, not the contrast of the lighting.
The only way to get highlights in the eyes is to get them into the KEY vector of the lighting. That boys and girls is why it\'s important to develop situational awareness of both the ambient and flash lighting in terms of the vectors of key and fill it creates.
First thing I do entering a room is look at the eyes of the people in it. Is the ambient source in the room shading them? If so I know I\'ll need to cancel it out nearly completely with shutter then light the space and face with flash to avoid the same shaded eyes in my photo.
Outdoors shooting a portrait in open shade I do the same thing, look at the eyes, but I can\'t cancel out the ambient with the shutter. So outdoors I need to find a way to get the faces looking up into the sky to get the light in the eyes. That\'s why I bring along a ladder on an outdoor portrait session. If shooting candids I look for a rock, bench or some other higher POV knowing the subjects will look up at me to see what I\'m doing up there at some point.
If shooting in ambient lighting indoors without flash I do the same thing for the same reason; stand on a chair and catch the faces looking up to get light past the brow into the eyes. Not any light the KEY VECTOR of the light.
What will determine how \"diffuse\" the lighting is holistically (what affects what it hits)? The ratio of KEY to FILL intensity (the relative amplitude of the vectors) and the number of different directions the KEY and FILL vectors come from.
Nearly all discussions of diffusion / diffuser I read here focus only on whether the KEY vectors are collimated or diffuse (distributed outward from source)
Jan 28, 2013 at 08:36 AM
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