What to look for…
Here are the features to consider when buying a dedicated flashgun
The maximum output power, expressed as a Gn (Guide number), increases at longer zoom settings, when the light is focused at a narrower angle.
Diffusers & Reflectors
Apart from the Nikon SB-500, all the flashguns in this group have zoom heads with built-in wide-angle diffusers and pull-out reflector cards.
A zoom range of 24-105mm is pretty standard, but some flashguns extend the range to as much as 24-200mm, offering greater telephoto reach.
Bounce and swivel
Most flashguns offer a bounce range of 0-90 degrees, but some add a slight downward angle for close-up shooting. Swivel is usually 180 degrees to both the left and right.