Tip: if you use a wide angle lens – and I hope you are, because you will get pictures like this, that look very three-dimensional, with the scene “wrapping around you”:
…then you need to know about your flash’s wide angle diffuser.
This is the piece of plastic that you can pull out, that looks like a diffuser:
It looks like a diffuser, but it is not. All it does is make the light go to a wider angle, when you are using a lens wider than the flash’s internal zoom mechanism can handle. Else, you would get vignetting.
When you shoot with a lens wider than around 24mm (on full frame), you need it to ensure the entire picture is lit. Like in this 16mm (on full frame) image, the sign would not be lit if I had not used the adapter.
But here comes tip 2: you can also choose not not use it, when shooting with a wide lens. This might be a good choice if you want vignetting, or if you are short of power, like on a sunny day, and your subject is in the middle. Then why waste power lighting up the side?