Using Canon flash system (camera and flash) can be a nightmare for someone who don't have at least some basic knowledge.
When using Canon flash system you should know two important things:
1. Camera meters just ambient light. Ambient light is whatever light comes thru the lens WITHOUT the flash. Whatever you set on camera are for ambient light. So use camera settings to make ambient (background) looks as you want. If you wish dramatic sunset, than make sky a stop darker (for example). And again - this setting is just for background (ambient). If sky is not appropriate exposured, change settings a little and try. Why not? It is learning time and we live in digital world.
Be careful - in darkness you can expect slow shutter speed. This is a warn to you, that hand holding can lead to shaken photos. Use tripod if shutter speed are too slow.
2. When flash is on - flash will add lights for foregrounds (subject in focus). Background will be exposed with ambient light (look above), but foreground will be exposed with flash. And - you guess - every change on your flash gun will have affection on foreground. In first step we set camera for desired ambient light, now we have to catch desired foreground exposure (with setting on flash!).
Simple isn't it? Well yes and no. There are many different combinations of settings and lights. And photography is playing with light, remember?
With experiences you will understand why some settings are different than others. And what causes overexposed background and dark person in front of you.
But this is just a beginning. Before you start, you should know (at least :-)) how Canon camera acts in different modes. Here we go - in a short way. We assume that you have switch on your flash and it works in automatic E-TTL mode (what is default behavior).
The camera and the flash work together to get the best illumination for the subject in focus (foreground), whether or not it means that the background becomes totally dark. In most cases this will results in properly illuminated subject, but dark background. Use this setting if you are in hurry or don't want to think about exposure. Be here if you don't have enough practice and have an important once time event. You can stay here and forget everything (you can have an eye on composition :-)).
In this mode - the camera exposes for the ambient light. Shutter times can be slow, especially if there is dark background (evening, dark church ...)!! Use tripod if you can. And be careful - if camera meters dramatic sky, it can "brighten" it. In that case, stop down a stop or more, to get more dramatic sky.
In Av/Tv modes flash goes to fill-in mode, and adjusts itself to available light and illumination of subject in focus (foreground).
The meter reading of the camera should be the same with or without the flash!
This is the MODE. Again - setting on camera are for background, setting on flash are for foreground. The flash adjusts itself to available light and illumination of subject in focus (foreground). When you came into room, use settings (both aperture and shutter) on camera to take the best possible shot of ambient. Raise the ISO settings if you must, and usually you can even handhold the camera. Switch the flash on and try. If background is properly exposured, then you should just adjust your flash to get properly exposured object. The ambient won't change and you on flash you change just a foreground exposure (Well this is not true in all case. If room is small, flash will also affects on background illumination. Practice, practice ...). When flash is set, you win.
Now, you are equip with some knowledge and if necessary, you know what to change to get what you want. With a little practice everybody can take a good flash picture (to some level - than the equipment plays the rule (softboxes, more flashes, extra lights, diffusers ...)).
Look at two examples:
Background (ambient light) set on camera and foreground set on flash.
In this example flash was used to lighten up face. In this case, we can talk about "fill flash". But basic behavior of flash system is the same (separate metering of background and separate of foreground). If I want to darken sky, I dial down (+/- exposure compensation on camera) to darken (for example -2/3) ambient and flash will automatically add enough power to lit face the same. And the opposite - if I want more bright face, I use some clicks on flash (for example +1/3). The face will be brighter, the sky won't change. And there are many combinations between ...
We will talk about fill flash in one of next chapters ...
Comments are welcome.