meter for flash ?????????

[Cutter]

Me again
When you are taking a photo of a person using flash its say meter for the model or subject for the flash, how do you do that? and how do you know you are and have properley exposing your model or subject ?

realy sorry about these questions just cant get my head around a few things.

I understand if you point you camera to the sky with a fstop of say F5.6 and you need a shutter speed of 1/100ss to get the right exposure for the sky, then point you camera using flash on your subject you will get a properly exposed sky. The question is how do I know the flash has not over or under exposing my subject ? Is there a setting method to achieve the correct flash exposer or is it just trial and error?

I do hope I am making sense

[PaulMontgomery]

It depends on the flash whether its got an auto setting or not. If you have to do everything manually, set the camera to ISO 100 (because that's usually what the flash guide number is quoted at) then use the guide number:
GN=aperture* distance.

or jiggling it round as an example, a guide number of 50m used for a subject 10m away = f/5.