“Guide Numbers” specify a flash unit's ability to illuminate subjects at a distance. The guide number is equal to the maximum flash-to-subject distance multiplied by the appropriate aperture F-number: Guide Number (GN) = distance (m) x aperture (F-number)

 A guide number of 60 (in meters at ISO 100), for example, means that a subject 15 meters away will be properly illuminated with an aperture setting of F4 (60 = 15 x 4). If you decrease the aperture size to F8, the distance will be halved so that subjects at a maximum distance of 7.5 meters (60 = 7.5 x 8) will be properly illuminated. Boosting the camera’s ISO setting can produce results similar to using a flash unit with a higher GN.

Although an off-camera flash setup can enhance visual depth, it is usually easier and sometimes just as effective to bounce the light from an on-camera flash unit off a nearby surface such as a wall or ceiling for more natural lighting with enhanced dimensionality. The difference can be dramatic, and since the flash unit stays on the camera you retain maximum mobility and speed. Bouncing the flash also provides broader coverage, and is ideal when you need to light a group of people, for example.

To create background bokeh in bright light you need a fast shutter speed with a large aperture. If you also need fill flash the maximum shutter speed will be limited, and that can make it impossible to use a large enough aperture. High-speed sync (HSS) makes it possible to use flash at all shutter speeds, so you can increase the aperture as needed.

Slow sync is used in situations where a slow shutter speed is required to properly expose background scenery indoors or at night. The flash fires to expose the subject, and then rather than closing immediately the shutter remains open for long enough to expose the background as well. This gives you well-balanced images in which both the subject and background are properly exposed.    

Multiple off-camera flashes can dramatically expand your lighting options. Ratio control, via infrared or radio communication, can make it easy to achieve ideal light output from each light.

Although infrared remote light control is fine for close-range setups where ambient lighting is appropriate and there are no obstacles to block the control signals, radio remote will reliably handle a much wider range of environments. Sony’s flash with a radio-based system (HVL-F45RM) and a wireless radio commander/receiver (FA-WRC1M/FA-WRR1) make setting up advanced multiple flash setups easy, and can deliver detailed, precise control data to up to 15 flash units within up to 5 groups (3 groups in TTL mode) over distances of up to approximately 98.4 ft. (30 meters). The control signals will reach the receivers regardless of prevailing lighting conditions, receiver location, or obstacles. Radio wireless ensures the most consistent results in the widest range of shooting situations.