I used to think of myself as someone who didn't use the flash on the camera, but that was because I was completely unaware of the difference an off camera flash can make. I almost always carry one with me whenever I'm out now, even in the day time, as there's a ton of different uses for it. We're gonna start by looking at possible uses of the flash and then look at when you wouldn't want to use it.
When you Should use Flash
This is the most obvious time to use a flash, but I thoroughly recommend that you use an external flash unit and bounce it off the ceiling of wall to get a much more natural look. Firing the flash at the same angle as the lens results in a very dull and flattened image, so if you can get the flash off the camera and shoot from the side, then that's even better. I'll refer back to the first tutorial I ever wrote for back when my blog was on my personal site which explains exactly how to take better photos indoors: Here it is.
Slightly less obvious then indoors, but still very important. We've all been there when you're shooting into the sun and your subject is just a silhouette and you don't know how to fix it. I've written a very good article with lots of photos that you can view by clicking on that link if you want to know more. The flash acts as a second light source and fills in where the camera has been underexposed because the camera's metering mode has prioritised a different part of the frame such as the sun in the photo below.
Cool Night Photo Effects
I love night photography as it really allows you work with a blank canvas and make up your own colours and light, and using a flash can be part of that. Long exposures allow you to move around the scene without leaving a trail and an external flash fired manually will freeze certain bits of light around the frame. Have a look at the photo below to see what I mean. If you want to know more about this, check out my tutorial on light painting.
The cool thing about flash is that it allows you to freeze the motion of a photo with a short burst of light. This works especially well if you're shooting in low light like in the photo below. I couldn't set the shutter speed too high or it would have been too dark, so instead I used a flash and it caught the droplet of water at it's peak with ease.
Flash has the ability to freeze the motion of a photo and let you play around with the light trails. This is especially handy if you're working in a low light situation and all you have is a nasty on camera flash as it allows you to produce something cool and creative from very little. Have a look at my example below taken at f/11, for 0.8 seconds, at ISO 250.
When you Should NOT use Flash
I see this way too often and i'm sure you have too – people 100 feet away from a stage trying to use their on camera flash to take a photo in low light. This is utterly pointless and the flash will probably have only reached about 10-15 feet before maxing out. You're much better off putting your camera into manual or a priority mode and doing it properly. Have a look at my low light photography tutorial to learn more.
Nothing says 'look at me' like a big flash attached to and even bigger camera going off in the corner of your eye. If you wish to go unnoticed, widen your aperture and raise your ISO. This will allow you to take well exposed photos in low light conditions, such as indoors. I recommend an ISO of about 400, and you can widen the aperture as much as you want; it’ll give your photos a nice shallow depth of field which means the focus will be on the subject rather then the surroundings. If you want to learn more about candid photography, check out this tutorial.
Unless you're planning on doing some cool effects like the ones mentioned above, I recommend turning your flash off. You'll have to take your camera out of full auto mode for this, otherwise it'll fire automatically, but the differences is clear. Instead of getting a bright overexposed foreground, you'll end up will a well exposed photo like the one below.
I'd say that about 95% of gigs don't allow you to use a flash on your camera as it annoys the band, distracts the fans and ruins the lighting designers hard work. Instead, you should widen your aperture and lower your shutter speed so that the camera picks up more light. Flash casts ugly shadows when shooting at gigs as you're on the ground and the artist is on the stage, making it a very unnatural angle for the light. If you want to learn more about gig photography, click here.
I know I mentioned using a flash in the daytime above, but that's only to point out the situations where you may want to use it. The majority of the time shooting outdoors doesn't require the flash to be fired, even in the shade as the sun does most of the hard work for you. If you have a subject that you can move, try and get them to change their positioning so that the sun is hitting them from the side, rather then from behind and if you're having trouble getting the lighting right, try using a polarising filter.