Both of these images are shot in exactly the same window light, but the mood is totally different. It's all about the use of shadows and the camera position.
In the first image, the model is sitting on the floor with a "normal" size window (about 2' x 4’) about 4 feet away. I'm shooting with a Nikon D700 and the 85mm f 1.8 lens, and I'm shooting wide open for the smallest depth of field. Only her eyes are sharp, and it's all that really matters. I'm standing with my back against the wall right next to the window, maybe even blocking it a bit and the model's face is turned fully into the light. That direction of light makes the skin tones really creamy, not much texture, and the quality of light is very soft and pretty. The soft light, and eyes looking up creates a sweet, dreamy image, and the shallow depth of field enhances the effect.
In the second image, the model stood up and the camera angle is now slightly below the face. She's turned her nose away from the light, towards her right, so that makes more shadow on her cheek. When we see a face from an angle, not straight on, and we see the side that has more shadow, it's called a "short" lighting. The short lighting has several effects: it both creates more drama and somber mood, and it slims out the face a bit, too, because we see less light. There is no fill in the shadows, so they are darker, also adding to the drama. Since she's now looking down, the eyelids are more closed so it's very introspective in feeling.
Always be aware of the direction and quality of light, and how the shadows really control the mood. Make sure that you guide the subject's expression to match the mood. And remember that one light can look very different from different angles, so walk around the subject!
September 22 - September 25, 2010