Fill-in flash is a technique used in photography to reduce harsh contrast by introducing additional light into the darker or shadowed areas of a subject. It is particularly useful when shooting in bright, directional lighting conditions - such as harsh sunlight or backlit scenes - where the difference between the highlights and shadows would otherwise be too great for the camera sensor to handle effectively.
Fill-in flash can be achieved using either an on-camera or off-camera flash unit, or by using a reflective material such as a reflector board to bounce available light back onto the subject. When using flash, the output is typically set lower than the ambient exposure so that the flash supplements rather than dominates the natural light, resulting in a balanced and natural-looking image.
This technique is widely used in outdoor portrait photography, wedding photography, and product shoots where shadow detail and even skin tones are important. Getting the balance right between the ambient light and the flash output is key to achieving results that look natural rather than artificially lit.