If you’ve spent any time trying to take decent photos with the built in flash on your camera you’ll know that it is virtually impossible. Which is why most high-end DSLRs don’t even have a built in flash. Instead, you should be using a separate flash unit.
There are many makers of these – including Branded ones by Canon, Nikon, Sony, etc. but also third party flashes from people like Nissin, Phottix and my favourite Yongnuo. Branded flashes are typically more robustly made although the Yongnuo that I have are very sturdy. The cost of the flash increases with flash power – referred to as Guide Number (GN) and top end flashes have built in radio triggers that allow you to place the flash off-camera and still fire it.
For the smaller and cheaper units without built-in triggers, you can buy separate flash triggers. Manufacturers like Pocket Wizard lead the way along with Phottix and are used by most pros. For me, I’ve had a cheap set of Yongnuo RF-603 triggers for several years and found them to be excellent. I’m now looking to upgrade to the YN-E3 triggers and transmitter that allows more creative control of the flashes directly from the camera – handy if the flash is buried inside an umbrella box!
Just make sure you buy the set that are compatible with the camera you have, as some are manufacturer specific.
The two other things you’ll need for off-camera flash are a Light Stand (unless you have a willing volunteer with you all the time) and a Cold shoe mount to connect the flash (and trigger) to the stand. Most come with the ability to change flash angle and usually the ability to attach an umbrella too – we’ll be covering light modifiers at #3.