Fumbling flash settings
If you’re using fill-flash outdoors and find that your pictures are coming out too bright, then it’s likely that either you’re too close to the subject or the shutter speed required to expose the shot correctly exceeds the flash sync speed (the fastest shutter speed that flash can be used with – typically 1/200 sec). Switching to a smaller aperture is often the easiest way to fix this problem. The aperture controls the flash exposure, with smaller apertures reducing the flash exposure.
On very bright days, this may mean that you end up using a smaller aperture than you’d like, bringing more of a scene into focus. In this situation, fit a standard ND filter to your lens; this will let you use large apertures without over-exposing.