Get so good at flash that nobody knows you're using it.
I had an interesting chat with another photographer the other day, who wondered why I used on-camera flash indoors. They felt you could always tell a shot was 'flashed,' which made it look artificial.
I countered that you could often tell indoor shots without extra lighting, as crudely applied higher ISOs created obvious noise. So we agreed to disagree, but their point was telling.
If you are going to use flash, it shouldn't be painfully obvious, unless you going for some arty strobist effects. To paraphrase Bruce Lee, you should aim to master “the art of flashing without flashing...”
Simple fill flash on portraits
One of the easiest and quickest ways to use built-in flash is to add a 'pop' of flash to fill in unsightly shadows on a sunny day. You will need to manually activate your flash, and then shoot in either aperture priority, program or manual mode, so you can control the depth of field.
Use the flash exposure compensation adjustment to control the power of the flash output. Use pop-up flash conservatively and it will be hard to tell you've used it at all.
Bounce flash indoors
By bouncing it, I mean angling the head of the flashgun so you bounce the light off a white or pale coloured wall or ceiling, or even direct it behind you. So you will need a dedicated hotshoe-mounted flashgun rather than trying to use your camera's pop-up flash.
TTL or manual exposure?
You can either trigger your flashgun in TTL (through the lens) or Manual mode. TLL is like aperture priority or shutter priority mode on your camera, it's semi automated and will try to work out the correct overall exposure for you. With Manual, you get no help from the flashgun and have to make all the adjustments yourself.
Both TTL and Manual enable you to adjust the flash output, and I advise you to be conservative until you have found the right settings. Don't be afraid to keep adjusting the angle and direction of the flashgun head and try to bounce it off white or place surfaces, rather than coloured ones – these will give your images a colour cast. Another good way to minimise the risk of this is to pop up the white bounce card that is usually built into flashguns.
The benefits of skilled bounce flash
While it's not a panacea for every challenging indoor lighting scenario, bounce flash can help to boost colours and sharpness and add a nice catchlight in the eyes of portraits. You don't have the chore of having to remove muddy digital noise from shooting at high ISOs, either.
That said, you will need to adjust the 'holy trinity' of aperture, shutter speed and ISO to get the best results with your flashgun, and be realistic about the area it can illuminate – don't expect your flashgun to be able to light up a wedding service if you are stuck right back in the aisle.