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If you look at a globe or a world map you'll see three lines: Equator, Topic of Cancer, and Tropic of Capricorn. Due to the tilt of the earth relative to the sun the sun tracks between those three lines during the year. That's what causes the change of seasons and the difference in the amount of daylight in summer vs. winter. That movement of the earth relative to the sun is what changes the angle of the sun as indicated on Brian's diagram.
I've posted these before, but they illustrate how the sun changes lighting patterns of faces at different times of day. A few years ago just before the winter solstice, when the sun is the furthest south in the sky, we had a big snow storm. So I created this head, facing it due south and tilting it back slightly so at noon on the solstice it would be illuminated with a centered, symmetrical lighting pattern when the sun was due south:
But here's what the lighting looked like around 10AM when the sun was in the SE quadrant of the sky, viewed from the shadow side...
and here's what it looked like later in the afternoon at 2PM when the sun had tracked into the SW part of the sky...
At 10 and 2 the angle of the light winds up being about a 45° downward angle, which due to the shape of human faces is the ideal angle for getting the light past the brow and into the eyes. The reason I created the snow head tilted back is because at noon the angle of the sun is greater than 45° and the brow shades the eyes.
Here's a similar exercise the year earlier done for amusement and edification after shoveling the driveway. Here I faced the nose to the west because I wanted the light to hit it from 45° downward and 45° to the side relative to the nose in the afternoon sun at 2PM. Here's what that angle looked like from various directions walking around the face:
Sun over the shoulder (flat lighting):
Sun at 45° relative to nose, camera 0° relative to nose (i.e. 45/45 lighting viewed full face)
Light at this angle creates natural looking 3D modeling on a face, but because one side is highlighted and the other shaded the net effect of facial angle and light pattern isn't as symmetrical looking as with the centered pattern at noon on the other head above.
Moving around more to the shadow side the light is still hitting the face at 45/45 relative to the nose, but now the camera is 45° relative to the nose and 90° relative to the sun..
What is interesting about this combination, and why it works well for portraits, is because the combination of camera angle and light angle wind up highlighting only the front part of the face, similar to a mask over the face. That and the fact the side of the face is shaded works like a magnet to pull the viewer in to see the eyes and mouth which are well lit. The only problem with this in terms of body language is that we don't usually interact with people with faces cocked at 45°. It's more typical of the angle we might observe someone from who we aren't having a conversation with. So while this is a more symmetrical combination than full face with 45/45 lighting, the full face pose will come across as more sincere if used for something like as business head shot. The oblique 45/45 lit view works well for things like glamour, fashion and character studies where its not critical that the subject be looking at the camera, which can look "shifty-eyed" in a the photo.
The trick when using this combination and needing the subject appearing to be looking at the camera is to first pose the face to the light, move the camera to find the most balanced oblique angle, then coach the subject to keep their head still and just move their eyes to a spot about half way between the direction their nose is pointing and at the camera until looking through the viewfinder at the face you see white on either side. It's a perceptual thing. Because we usually interact with people face-to-face we normally see centered eyes, so when then aren't centered they seem odd. Sometimes you need to fake it to make it look more real.
The final combination is the profile view. The face and sun haven't moved: the sun is still 45/45 from the spot between the eyes. But the camera is now looking at the pattern from a POV about 135° (90+45) from the sun...
Note the shadow on the side of the nose. A mistake I see many make when lighting a profile is to turn the face too much towards the "key" modeling light to the point the side of the nose facing the camera is highlighted. It's the shadow on the side if the nose that set's up the contrast that draws attention to single highlighted eye and the lips in the profile view. If the nose is lit up like a 60W light bulb it will become the center of attention instead.
As in the oblique view you need to cheat the eyes for a profile. With the person looking straight as the normally would you don't see the iris or pupil as you would expect to. So here you have the subject look at the camera out of the corner of their eye and it winds up looking more normal in the photo.
The point here is that lighting a face isn't rocket science, its more like basic geometry combined with basic situational awareness of the direction natural light comes from at various times of day and year. The snow head examples where done with direct sunlight, but the indirect light of open shade or a window will have the same direction. With a north facing window you'll get a similar pattern by angling the nose 45° to the window. Ideally you'd want to use a window higher than the face, or mask off the bottom, so the light also comes from more of a downward 45° angle like the sun in the snow head shots, vs. sideways.
Here's an illustration from my tutorial on window lighting: http://photo.nova.org/Window/ showing how the camera would move around the face turned 45° to the window similar to the second set of snow head shots: