. When the subject is underexposed due to the deficit of sunlight and other lights or sometimes due to the backlight, fill flash is supplemented to correctly expose the subject. For many photographers, using fill flash is a daunting task as they apprehend overexposure of a photo. But with the appropriate use of a fill flash, you can nail your photography.ill flash can produce a stellar outcome if leveraged deftly and accurately. No matter how the subject looks like, the utilization of the fill flash can ramp up the overall aesthetic of a portrait and provide a high-end vibe.