How does ambient light affect the quality of a picture?

Answer 1

Natural light is comprised of sunlight or skylight. The former will produce distinct shadows, sharp delineation and enhances subject textures if it comes from one side (across the surface). This can emphasize the perception of depth in the scene. Direct sunlight often forces people to squint while the photographer places the sun behind his/her back. Without the use of corrective filtration, subject colors can shift towards warm/yellow hues quite noticeably when under direct sun in the early morning or late evening.

Skylight on the other hand, produces softer shadows and less delineation or texture enhancement. A slight color shift towards blue (cooler) may be noticed (compared to sunlight) since the sky color is closer to the balance of color film or digital sensors than the color temperature of direct sun. However, skylight can have one drawback when used for outdoor portraiture:

When a person stands outdoors in the shade of a tree, the area around their eyes often turns out very dark (raccoon like) in the photograph. This is because the effect of skylight is pleasing and easy to observe, but the amateur photographer doesn't notice the dark eye sockets produced by the large overhead source (the sky). This is why you will see any wedding photographer worth his salt balancing the ambient natural light with frontal electronic flash. If no flash is available, positioning the subject under the tree canopy will help. To produce this effect for observation, look at the subject's eye area while he/she dons and removes a Baseball cap. The visor on the cap will restrict the lighting to become much more horizontal, thereby reducing the raccoon effect.