Hey, I have a 430EXII and I know you said the handbook explains, but what would I be looking for in the handbook for this exactly?
I often want to try and shoot things similar to this at night and I've not tried this method before. Is it setting the correct exposure? Why point it at someones face? Sorry for all the questions, just really want to get to grips with my flash gun!
The handbook explains how to use the flash exposure lock button - FEL, which is marked * on Canon.
First off, you need to be aware that with all auto-TTL flash systems there are basically two flashes. They go off very rapidly and so close together you cannot distingush the two with the naked eye. The first pre-flash is for exposure calculation. It can actually be a multiple stream of morse-code like pulses, depending on the auto-TTL mode, but it's primary purpose is to send out a measured flash that the metering system can read and compare to ambient light data. In less time than it takes to blink, the camera assesses this and sets the power output for the main flash, which then goes off immediately afterwards when the shutter opens.
Flash exposure control is much more difficult than ambient light, because of the inverse square law fall-off (as described above) and other environmental factors. With daylight for example, it doesn't really matter much how far away the subject is as the light from the sun remains constant. With flash it varies with distance, often a great deal - as explained above - and the environment makes a big difference too (small light room, big dark room, outdoors etc).
Manufacturers use various methods to try and get this right, much of it quite sophisticated, comparing dynamic range as well as light levels and factoring in the focusing distance with lenses that report it. Details of the algorithms they use are never published but the fact remains that, at the end of the day, it's often not much more than a best guess and in tricky situations, such as the OP describes, they need all the help they can get.
Flash exposure is often heavily biased around the centre, quite rightly, but if you have an off-centre subject that can mean that the flash assumes the background is more important and that results in over-exposure of closer subjects.
The FEL* button gets around that by firing the pre-flash, which calculates the exposure, independently on command. Point the camera at the subject, press the FEL button which fires the pre-flash and locks the exposure, then recompose the shot the way you want and press the shutter release.
It generally works very well but if you need to use it a lot, you might get an even more reliable result by setting the camera's metering pattern to centre-weighted, which is a better 'fit' to the the way flash metering works. And as a last resort, if all else fails you have over-ride options of +/- exposure compensation both on the gun and the camera, and full manual control of both if you prefer.
I find the big advantage of auto-TTL is that it generally sets the exposure pretty well, and it does it first time without any faffing. If you have to use manual, you'll need to do a few test shots and make adjustments, and the settings will vary (a lot!) if the subject distance subsequently changes.
