How to overpower bright sunlight with on-camera flash
The photograph on the left shows one of the toughest lighting conditions you get to deal with. Your subjects are half in the sun / half in the shade ... and there is no way you could interrupt and change things in your favor. There is no way to have the flower-girls move. No way to bring in additional, off-camera Read more inside...
Flash photography: applying the Sunny 16 Rule & Flash Guide Number
In essence, if you know the GN of your flash, then you could use (bare) off-camera flash to match the sunlight, without even metering!
There is a super-useful shortcut built into those two simple values: Sunny 16, and the Guide Number.
Now, I am pretty sure that when you hear mention of the Read more inside...
Tutorial: How to use the guide number of your flash
GN = distance * f-stop
Your flash's Guide Number (GN) is determined at 100 ISO, when it gives correct exposure at a certain distance, multiplied by the f-stop
The idea that we can figure out the manual flash exposure by the combination of distance and aperture (for a given ISO setting), was covered in these Read more inside...