Flash exposure lock

[Skynet-T800]

Not sure what i'm doing here i've set my D700 up to use this feature so am I right in thinking when I press this button and the Sb-910 emits a flash is this the exposure/meter reading fo my shot? then it's a case of composing and fire the shutter?

 

[Beepcake]

I don't shoot Nikon, but nobody else has answered yet and I assuming they work pretty much the same...

FEL locks the flash exposure, but the metering it uses to work the flash power out is based on a much smaller area (either the spot metering zone, or the active focus point), rather than an average/evaluative approach for the entire scene.

I use it when I'm using ETTL and the contrast or distance between my main subject and the rest of the scene is quite large, or the background lighting is changing a lot, I put the centre point over the face, hit FEL, recompose and take the shot.

The downside is you need to flash once to get the FEL, then flash again to take the shot, which can be surprisingly confusing for the people you're shooting - even if you explain it to them