Light Ratio - What is the best to use?The determination of what is the best light ratio to use in portrait photography is subjective. Portrait photographers like to use different lighting ratios for different effects.
Traditional portraits, like this portrait on the left, usually have a lighting ratio between 2:1 and 4:1. A 4:1 ratio indicates that there is four times (or two f stops as much light striking the highlight areas of the face than is striking the shadow areas. Having such a range will still produce good details in the shadow areas and provide enough contrast to reveal your subject's form. This diagram shows a typical lighting set up that is popular today. Umbrella lighting is a very affordable way to create attractive portraits. Instead of using another light, a large reflector can be placed close to your subject. Placing it closer to your subject will give you a lower lighting ratio. Placing your reflector at a farther distance from your subject will mean that less light will bounce back into the shadow areas. This will produce a higher lighting ratio and a more dramatic portrait. With the advent of digital photography, it becomes much easier to "cheat" and simple look at your digital camera's LCD screen to preview the lighting effect you are achieving. Because digital camera sensor's are getting so sensitive and capable of shooting in low light situations (high ISO number), it is becoming more popular to shoot without flash.
The curtains diffused the window light to create this beautiful, softly lit portrait. I used a large reflector on the right to bounce the window light back on my subject. This is one way to get this relatively low light ratio. Another advantage to using this photography technique is that you don't have to wait for a flash to recycle. The light is continuous. You can make many exposures in a short time frame and you won't miss any great portrait expressions. Happy Shooting! Back from Light Ratio to Photography Definitions |
A lighting ratio is the ratio between the main light (also known as the key light) and the fill light. 
I use natural light from windows when I am shooting portraits inside whenever possible.