Balancing light with ratios
Balancing light with ratios | ||
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When lighting a subject, the most common way to balance light is to use ratios, which are easy to calculate by measuring the exposure of each light source alone (either with your camera's exposure meter or using an external flash meter). Once you have the light calculated for each source alone, you can figure the lighting ratio. For example, suppose that the main light for a portrait provides enough illumination that you would use an f-stop of f11. The supplementary light you'll be using is less intense, is bounced into a more diffuse reflector, or is farther away (or any combination of these) and produces an exposure, all by itself, of f5.6. That translates into two f-stops' difference or, putting it another way, one light source is 4 times as intense as the other. You can express this absolute relationship as the ratio 4:1. Because the main light is used to illuminate the highlight portion of your image, while the secondary light is used to fill in the dark, shadow areas left by the main light, this ratio tells us a lot about the lighting contrast for the scene. (I'll explain more about main and fill lights shortly.) In practice, a 4:1 lighting ratio (or higher) is quite dramatic and can leave you with fairly dark shadows to contrast with your highlights. For portraiture, you probably will want to use 3:1 or 2:1 lighting ratios for a softer look that lets the shadows define the shape of your subject without cloaking parts in inky blackness. If you use incandescent lighting or electronic flash equipped with modeling lights, you will rarely calculate lighting ratios while you shoot. Instead, you'll base your lighting setups on how the subject looks, making your shadows lighter or darker depending on the effect you want. If you use electronic flash without a modeling light, or flash with modeling lights that aren't proportional to the light emitted by the flash, you can calculate lighting ratios. If you do need to know the lighting ratio, it's easy to figure by measuring the exposure separately for each light and multiplying the number of f-stops difference by two. A two-stop difference means a 4:1 lighting ratio; two-and-a-half stops difference adds up to a 5:1 lighting ratio; three stops is 6:1 and so forth. | ||