Creating a natural looking portrait outside with flash limits your options regarding your camera settings. A photographer might be stuck at their sync speed, usually 1/180 to 1/250, and at a low f-stop of around f/22. This creates a balance of flash to ambient that will not over expose your subject, but who wants to shot portraits at f/22 all the time?
Shooting portraits with your lens wide open, especially with fast glass, creates the subject/background separation that is so desired by photographers and clients alike. Photographer Joel Grimes shows us how to use an ND filter to overpower the sun while using a wide open aperture.
To achieve the desired result Grimes used a variety of gear:
Paul C. Buff Einstein
Some cameras have ND filters built in like the Fuji X100T, which has a three stop ND filter that moves in between the sensor and the lens. Being able to shoot portraits inside then switching the ND filter on for outdoor shooting with out adjusting the settings makes it an indispensable tool during a fast paced event like a wedding. If your camera does not have a built in filter, Shivani wrote up a great list of 5 Tiffen ND Filters to fit every budget that is worth a read. See link below:
Source: Joel Grimes via Youtube