Lighting Calculators

Light is the raw material of photography. Understanding how to measure, modify, and control it is essential for consistent results. The guide number (GN) of a flash unit quantifies its power: GN = distance × f-number (at ISO 100). A flash with GN 58 can illuminate a subject at 7.25 metres at f/8, or at 14.5 metres at f/4.

The inverse square law governs how light intensity decreases with distance. When you double the distance from a light source to the subject, the light intensity drops to one quarter. This principle is fundamental to understanding light falloff across group shots, background exposure, and the effective range of flash units and continuous lights.

Lighting ratios describe the contrast between the key (main) light and the fill light on a subject. A 2:1 ratio gives subtle shadows suitable for beauty work, while 4:1 or 8:1 ratios create dramatic, high-contrast results. The ratio is measured in stops of difference between the lit and shadow sides of the face.

Softbox size relative to the subject and distance determines the quality of light. A larger source relative to the subject produces softer shadows with gradual transitions. As a general guide, the softbox should be at least as wide as the subject for flattering portrait lighting. Our calculators help you plan your lighting setup before you start shooting.