Let me preface this post by saying I am by no means an expert at using flash. I only dig the speed light out when the days get shorter, and natural light indoors becomes less. Despite my lack of authority on the subject, I thought that some of you would still find it useful to have a basic understanding of using flash, for those times when you might want to use it indoors to supplement the existing light. Of course there are lots of other ways you can use flash (as fill flash, main light etc) but this is what I use it for, and I'm willing to bet that is why most lifestyle photographers reach for their speed light! So, this is just an introduction to it to get you started.
(If you are unsure about why you would even want an external flash unit, i wrote about this recently with some examples here so you can read that first to get some more ideas)
The other thing to note is that for this to make any sense whatsoever, you really must understand your exposure triangle and how everything goes together - but for the purposes of this article I'm just going to assume that you are confident about what each of the elements does!
With all that caveats neatly out of the way, let's start on how to use flash!
There are three main elements I look at when taking an image using flash: the background / ambient light, the amount of flash needed, and where I can bounce the flash off.
Here's my steps:
1) Set Camera Exposure for Ambient Light
The strange thing about shooting with flash is that is some ways it is like you are setting two separate exposures - one for the background, and one for your subject. Use your camera settings to expose for how you want the ambient light / background to look - the more ambient light you let in, the less "flashy" your image will look (where you have a bright subject against a dark background)
First, I set my ISO as high as I can without introducing noise - this will be different for each camera model and the amount of light available and so on.
Next, I set my aperture as low as I can for what I am photographing - the lower the better. I want to let in as much light as I can but still with workable settings.
Last is my shutter speed. You can afford to have a lower shutter speed than you would normally since flash freezes the motion somewhat (but also bear in mind that you will need to keep your shutter speed slower than you camera's max sync speed - this is usually around 1/200 but varies from camera model - if you don't know, start at 1/125!) If I am indoors with a lot less light, in order to keep my ambient light levels good, I will choose a relatively low shutter speed - less and I might get motion blur from camera shake (I'm naturally shaky!). You can take a test shot at this point if you want, just to make sure you have your background looking the way you want it.
The image below has settings of F2.0 / ISO3200 / SS 1/200. You can see that my subject is still too dark, but I have the ambient light the way I want it, or just a little bit darker (to account for the fact that the flash will brighten it a little)
2) Set the Flash Exposure for Subject
Now, turn your flash unit on. We are now going to expose for our subject using flash. If you are new to flash, I think you will be best starting with TTL. Basically, it's a bit like AUTO for flash: you are letting the camera / flash decide on how much light to add to the scene. However, you have something called Flash Exposure Compensation which allows you to tell your flash that you need more light or less light than it decided on AUTO. This is an easy way of controlling the amount of flash used - you just add power to get more light, and less to reduce the amount of light produced, in the same way as using exposure compensation on your camera.
Here's the image with the flash power at 0:
Which I then dialled down to -1
This is the steps for adjusting the power on my flash unit (The Canon 430EXII) but if you have a different model you will need to dig out the manual that came with it - they are all pretty similar though.
3) Bounce Flash to diffuse light
In order to get a more natural look to your images, it's useful to bounce the flash off something else, rather than pointing it straight at your subject. So have a look around you and see what would be best to bounce your flash from. You can either have it pointing up and slightly behind you, or up and slightly in front of you, or to the side. Bear in mind that if you bounce off a coloured wall, you will get that colour flashed back onto your subject - white or neutral is best to bounce off so keep that in mind when choosing!
In the first two shots above the flash is pointed above and slightly forward, and this shot below I moved the flash to bounce from the side.
4) Test and Adjust!
Once you have your camera settings and flash settings, and bounced your light, take a test shot and see what you need to do. If the flash is looking strong on your subject, move your flash exposure down. If you are getting shadow behind your subject, try bringing up the ambient light by reducing your shutter speed. In both cases you can also try moving the angle of your flash, bouncing it elsewhere to see if that helps. If everything is looking too dark, then adjust your flash power to increase exposure and / or increase your ambient light. Just keep in mind that in some ways you are setting two exposures - one for your subject which is controlled via your flash, and one for your background / ambient light which is controlled via the camera settings, usually by changing your shutter speed.
This is just a quick introduction to using flash to get you going - there is more to it so if you are interested in getting a more solid understanding, this book is a great option: On Camera Flash for Digital Wedding and Portrait Photography - the steps still pertain to other types of photography, including lifestyle, so don't let the title put you off!
If you have questions, I can try to help, just leave a comment below :)