So you probably have heard people talking about bouncing your flash and you may not have fully understand what they quire meant. Basically in an ideal situation you won't be aiming your flash directly at your subjects because what tends to happen is that you cause harsh shadows and you lose depth. It also gives just an all round rather unflattering look for most people. Do note that if you're using off camera flash, then aiming it at your subject isn't such a big deal and is often what you would do depending on the look you're going for. Also sometimes you'll just have to do it, such as when you're outside with no walls or ceiling to bounce off.
So we've established under regular circumstances you'll want to avoid aiming your flash at your subjects, which begs the question, well where do you point it then. Well effectively at something else like the ceiling or a nearby wall. What will happen is that the flash will hit the wall and bounce back onto your subject.
Okay, so why bother with this at all? You might argue, "In the studio professionals have lights that face us not the wall". Well yes, you are correct they do do that. However, it comes back down to the difference between hard and soft light, which I explained in my post about lighting modifiers. You get hard light if your light source is small relative to your subject and soft light when you have a big light source relative to your subject. When you bounce off the wall, you effectively increase the size of your light source to become the size of the wall which will then give you soft light. That's why studios have umbrellas and softboxes because they're already increasing the size of the light so you don't need to bounce it. You'd only bounce it if you don't have such an item and you're working with just your bare flash.
If you can't avoid pointing your flash at your subject, and there will be a lot of occasions where this is the case, then you can use a diffuser to soften the light. Be mindful that your flash probably has a wide panel that is used to help spread out the lights when you're shooting ultra-wide. This panel is not an effective replacement for a diffuser and I wouldn't recommend using this for such a purpose.
A Flash Wide Panel
When determining how to bounce your flash, the difficultly is sometimes determining what you should and shouldn't bounce off. Usually low, white ceilings are the perfect. This is because they will be close to the subject so the flash can reach the wall, and cos it's white, it won't leave a colour cast that can occur when you shoot off something that's not white. However, it's really up to you to determine what you should use and in some instances you will need to adjust the white balance in post to compensate for when you shoot by bouncing off a wall/ceiling that's not white.
I've found aiming the flash up at the ceiling and at a slight angle behind you gives the best look when shooting with bounce flash. What you can do if your ceilings are too high is to use the fill card which will allow you to bring some light forward to infill and it also gives good catchlights.Catchlights are the little white dots in the eye you see from the bright flash source and can make your subjects eyes pop. I often drag my subjects near a wall and keep the wall behind me that I can then use to bounce light off.
What a flash fill card looks like
To give you an idea of what different options there are, I took the following four shots with exactly the same camera settings. I just changed the direction of the flash to bounce off difference objects. Do note that I leave my flash of ETTL when I shoot on the run which means the flash will automatically determine the correct output for exposure based on the settings you've chosen.
35mm 1/80s F2.8 ISO 200 Flash Fired (Aimed up and backwards)
35mm 1/80s F2.8 ISO 200 Flash Fired (Aimed at ceiling)
35mm 1/80s F2.8 ISO 200 Flash Fired (Aimed at wall to the right)
35mm 1/80s F2.8 ISO 200 Flash Fired (Direct flash)
The following is what my flash looks like when I said upward and slightly backward.
This is my preferred setup but admittedly it doesn't always work so you'll need to use your judgement to determine what works best in what situations. This setup works best if there's a wall behind you but often works okay if the ceiling is low. Bouncing directly up at the ceiling is popular but can cause shadows under the eyes and chin so I'm not a fan of shooting like this unless I can a fill card to compensate.
Usually when bouncing the flash you'll need to increase the flash power to compensate for the loss of light associated with bouncing. In these cases you can manually bump up the flash power or just use ETTL. As I said earlier, ETTL should automatically compensate but if doesn't you can use the flash exposure compensation option to deal with it. It will usually be a + and - sign on the back of the flash.
Another thing to note is that your flash is only so powerful and your aperture will affect how much light the flash to fire. If you shoot at really small apertures, e.g. F8, then you might find your flash is struggling because it has to fire so much light to fit through the small aperture. SO you can open up your aperture or adjust your ISO to assist your flash in working better. For ISO I would put it somewhere between 100 and 400 depending on the scene. Opening up your shutter won't affect your flash power but will affect the ambient light so I like to leave it around 1/80s so I can get some ambient light of the room in the photo. Do be careful not to use too slow a shutter speed as this can induce camera shake.
Here's another example of a shot taken with the flash straight up. As you can see this caused some shadows to appear under the eyes and cheeks which I thought were not very flattering. Unfortunately I didn't have a fill car at the time which would've helped so I moved my subject so that I had a wall behind me and then I bounced off the wall. As you can see the different is quite dramatic.
35mm 1/80s F2.8 ISO 200 Flash Fired (Aimed at ceiling)
I hope that has helped clarify what bouncing your flash actually means and the purpose behind it. Do try to experiment to get a better feel for it as there are no hard and fast rules about these things because the second you go outside, your situation is completely different. However, this should definitely help you to get better photos at your next indoor event.