erm OK, but I'm sure someone else could do a better job. I'm not claiming any great expertese here - just passing on what I've figured out.
Originally Posted by sparx
Regular flash (as above) assumes that the ambient light is negligle - we ignore it, and use the flash for the full lighting. Fill in more or less works the other way round: The ambient light is fine, but we're going to add a bit of flash to fill in the shadows (for example someones eyes may be in shadow on a sunny day).
In this case we need to set the exposure for the ambient light as usuall, then add flash, making sure that the flash is deliberatly not bright enough to affect the lighter parts of the scene.
The easiest way to do this is with a smart flash. A good flash will allow you to dial in the film speed and aperture, and it will meter the flash as it's going off. In this case I'd set the camera exposure as usual (making sure that I'm slower than the sync speed of the shutter). I'd then LIE to the flash.
Assume it's a bright sunny day: iso 100, 1/100th, f/16.
If I told the flash I was on f/11 or f/8 it would think it's work was having more of an effect than it actually is, and so would put out less light than it would normally to fully light the scene (the ambient light has little effect on the flash's built in meter, as the flash is VERY intense for a very short time). This would have little effect on the high lights (two stops down is 1/4 of the light. We'd need to double the light to get to the next stop, so the flash adds 1/8th(ish) of a stop to the mid tones), but has signifigant effect on the shadows.
So if you've got a programable flash, just set it one stop GREATER exposure than the camera, and it will put out one stop LESS light. You can go one, or two stops depending on how much fill-in you want. It's not too critical, as you're (hopefully) correctly exposing using natural light.
With a non-programable flash, to get the same effect you'd need to start with the flash calculation I showed earlier :
eg GN20 at 2 metres=f/11.
then take the aperture down one or two stops so the flash isn't overpowering -> f/16.
Now calculate your exposure based on that f/stop, so on a bight sunny day with 100film, 100th f/16 would be fine. If a very light cloud cover arrived, I need to increase teh exposure a stop - but I can't change the aperture without affecting the flash, so I'd got to 1/50th, and everything should be the same.
It's a bit more fidley, but the basic rule is expose as normal, and set the flash to put out less light than you need.
As I said earlier this is a bit more subjective, and I'm sure someone else can give more professional advice, but this should get you started.