Flash power and flash exposure compensation

[Goldcoin79]

Can anyone answer a question for me on the differences between flash power and flash exposure compensation on a built in flash.

If I wanted to use a fill flash and I lower the power of the flash to low for example would it have the same effect if I changed the flash exposure compensation to -2 instead of adjusting the flash power?

My main reason for asking is I am trying to establish the difference between flash power and flash exposure compensation and when you would use one over the other.

 

[KmH]

The only difference is the shooting modes flash compensation are available in.

Manually changing the flash power is available in any shooting mode.

 

[Goldcoin79]

Thank you.

 

[Big Mike]

There really is no difference...the flash fires at a certain power level...it's just a matter of how it gets there. If you are setting the flash power, you'd probably be in a manual flash mode. You tell it to fire at 1:1 (full) or 1/4 etc. It will fire at that power, whether it's correct for the photo or not.

Flash exposure compensation only works with the flash in an auto (TTL) mode. In this mode, the flash fires a pre-flash, the camera reads the reflected light 'Though The Lens', and then decides on a flash power based on the amount of reflected light. (if you shoot a bright white subject, more light will be reflected and thus the flash will fire with less power).
So when you use FEC, you are telling the flash to use 'that much' more or less power, from it's metered value. For example, if you set it to +1, it will pre-flash, read the reflected light and then give you one more stop than it would have at zero FEC. So if you are shooting a bright subject, the zero vaule would be underexposed, so you would want to dial in positive FEC. But if you are shooting a dark subject, less light would be reflected from the pre-flash, thus the camera/flash will want to put out more flash power that is actually needed...therefore you might want to use negative FEC.

It pretty much works the same way that auto exposure modes and Exposure Compensation work. The camera is calibrated to give you good exposures if your subject is the same tone/reflectivity as middle grey...but if your scene/subject is brighter or darker, you will need to compensate via EC and/or FEC.