Cut down light levels
Combine flash and large apertures with an ND filter
While high-speed sync has the potential to transform your outdoor flash photography, there’s one big drawback: power. The light from the flashgun has to be pulsed at a lower power during the exposure, which means that it doesn’t reach as far as it does during a normal, single burst of flash. The faster the shutter speed, the more the problem is compounded. A flashgun that allows you to zoom can give you more reach, as will using a larger aperture or a higher ISO. But you may already be shooting at the lens’s maximum aperture, and increasing the ISO will also increase the shutter speed, as well as the amount of noise.
An alternative approach is to dial in smaller aperture settings until the shutter speed falls below the camera’s x-sync, then you can deactivate high-speed sync and use the full power range of your flashgun. The trouble is that smaller apertures increase the depth of field, or the amount of front-to-back sharpness in a scene. This may be undesirable when you’re shooting a close-up or a portrait, where you’re more likely to want details in the background and foreground to melt away.
One technique to get around this is to fit a neutral density (ND) filter to the front of the lens. These filters block the amount of light coming through the lens, giving you slower shutter speeds even when using large apertures to blur distracting backgrounds. A two- or three-stop ND is a good place to start – or try a polarizing filter, which, when used at full strength, reduces the light to a similar degree.