Studio - Lighting ratios.

In portrait photography, lighting ratios are comparisons of the main light in a photo to the lesser light which fills in the shadows.

Considering lighting ratios is important for portrait photography. The four most common lighting ratios for portrait photography are 1:1, 2:1, 3:1, and 4:1.
The lower ratios, 1:1 and 2:1 can be used for most photography needs. These ratios make the entire photograph easier to see, so they are often used for family photographs or headshots of small children and women. This is called flat lighting. 
For portraits of women, 2:1 is the ratio that is commonly used. 
As with the ratio 3:1, 4:1 works well for pictures of men. This ratio gives a more “dramatic” impression to a portrait.