I am covering this photography topic today because I find rather interesting that most people I know do not use flash in their photos. It can be a very powerful ally when used correctly.
Basically, you will want your flash on whenever you are shooting subjects outdoors. For people and animals, this is a must, as it fills the dark spots of the image with light (fill flash). When there’s movement, even better, as it freezes the the moment and guarantees a sharp image. I use it whenever I need sharpness, like the photo above.
One important note: when you use flash, that is responsible for freezing the image, not shutter release. In this case, the shutter release will effect background lighting and the aperture, foreground lighting.
For indoors, you should either turn your flash off or use it in slow-sync mode. I personally prefer to use it in slow-sync mode because when it’s off, the tones tend to be biased towards amber. I have a color preset in my camera to correct that, but I prefer the results I get with slow-sync.
In slow-sync mode, the flash fires at the beginning of the exposure and then allows the background light to fill in, resulting in a more natural result than simply blowing white light away on people’s faces.
The problem with slow-sync is that the average human being is a moron when it comes to photography, and they will think that the photo is done when the flash is off and go away. To avoid this in casual shooting, you can also try rear-curtain mode, which fires the flash at the end of the exposure. Both modes cause a bit of (background) motion blur, since the exposure is a bit longer to allow the natural light to fill in.
Red-eye reduction is useless in external flashes and it’s particularly annoying, so don’t use it if you don’t need it.
If you find the results too harsh, you may try using a diffuser on your flash to soften the white light. Or use your hand for that as I do. The diffuser is one piece of junk I avoid carrying around. If your flash has a tilt-head, use it. Pointing it at 90 degrees towards the ceiling gives you excellent results indoors.
And that covers the basics of using flash. I hope this helps!