It is almost common knowledge these days that flash underexposure is caused by the unit having insufficient power to cover adequately the flash-to-subject distance. For example, an important youth sports photography tip for you to know will be that no small flash unit can light an object that is more than about 10 m (33 ft) away, and even quite powerful flash units cannot adequately light an object that is more than about 30 m (100 ft) distant.
Uneven lighting is caused when the flash is unable to cover the angle of view of the lens - a problem most often experienced with wide-angles, another problem that is usually encountered in youth sports photography when an attached lens or lens accessory blocks the light from a camera-mounted flash.
Solution
For close-up work in youth sports photography, reduce the power of the flash if possible. When taking youth sports photos for distant subjects in the dark children playing in the field, for example, or at different sports games if you run a youth sports photography business, using flash is usually a waste of time and is best turned off. A better option is to use a long exposure and support the camera on a tripod or rest it on something stable, such as a wall or fence. With accessory flash units - not built-in types -you can place a diffuser over the flash window to help spread the light and so prevent darkened corners when using a wide-angle lens, which can be very useful for youth sports photography.
