Sometimes you can’t wait for the perfect light, and the only option is to modify the existing light. There are two excellent tools which every photographer may use to modify light: diffusers and reflectors.
Diffusers
Diffusers are light modifiers which go between the light and the subject to soften the light that passes through. Diffusers might be a bit complicated to set up as they have to be placed between the light source and the subject of the photo. When used outside in natural light, this usually means that they need to be positioned above the subject in order to diffuse sunlight. Think of a diffuser as an artificial cloud which you may have complete control over. In general, the material of the diffuser should let roughly 50 percent of the light through, yet this is a matter of taste. Since direct sunlight, or any direct light, creates harsh lights for portraits, if you can’t avoid taking photos in the midday sun, you should try to use a diffuser of some kind. In a pinch, you can use any light-colored, thin material such as a white bed sheet. When shooting indoors, the bright sunlight coming through a window may be softened by hanging a white sheet in front of the window. The output from studio lights might be modified using diffusers such as a softbox or umbrella in front of the flash head.
A softbox is a large light diffuser that softens the light of a studio flash, and although these cost more than umbrellas, they do a better job keeping the light exactly where you want it. A softbox connects to a flash head using a speed ring, usually sold separately from both the softbox and the strobe. Softboxes come in a variety of sizes and shapes, from a small square to a large octagon.
An umbrella is a light modifier which connects in front of a studio light and looks just like an umbrella used to protect you from the rain. The umbrella is positioned so that the light is aimed through the umbrella. The light is then aimed 180 degrees from the subject letting the light reflect back from the inside of the umbrella toward the subject. Although this method is cheaper and easier than having a softbox, it doesn’t have the same level of control—but it is still better than nothing at all.
Reflectors
A reflector does exactly what its name implies: It reflects the light onto your subject and fills out the shadows to create a more even light on the subject. The bigger the reflector, the more light is reflected onto the subject. Reflectors work best when the original light source is very strong.
Different reflector surfaces create different light, and using the right reflector for the right job is important. Although very shiny material is highly reflective, it is very hard to control and does not reflect evenly. A plain white reflector has a much softer effect, and the spread of light is more even. When gold is added to the reflector’s color, the reflected light is warmer and has the properties more associated with the natural light available during the golden hour. When silver is added to the reflector, the light has a harsher quality.
Reflectors come in a variety of sizes, but what really makes them useful for location work is that most types are collapsible. The reflector material is sewn around a sprung steel hoop, which keeps the reflective material taut, but still allows for the reflector to be folded and stored in a much smaller space. Many reflectors come with multiple covers as well, so that the color can be changed when needed.
To add more light to the shadow portions of the subject, the reflector needs to be positioned directly opposite the main light source and aimed toward the subject. The biggest challenge in using reflectors is positioning them in the right place and still being able to take the photograph. Some reflectors now come with a handgrip to help keep the reflector steady. If it isn’t going to interfere with the composition, you can even have the subject hold the reflector for you.
One great use for a reflector is to minimize shadows under a subject’s eyes. When a person is lit from above by natural or artificial light, shadows can form under the eyes, which is not a very pleasing look. Positioning a white or silver/white reflector in front of the subject, aiming up at the face, reflects some of the light back up at the subject and fills in those shadows.
Have a look at other articles describing the influence of lighting conditions on photography:
The Color of Light, The Diffusion of Light, The Intensity of Light, Light Direction, Light types, Photography and light.