This picture is made on a stormy afternoon on
the beautifull island of Ameland out of the North of holland.
To get the stormclouds on the background in good light,
you measure the
light from the sky and set the apeture and shutterspeed
according what your lightmeter tells you to set.
and then underexpose a little to bring out the gray sky more.
Turn on the flash and let it light the scene as is,
set the flash exposure (under or over) on the flash depending on the distance
to the subject.
Some more examples :
See more pictures on Ivar Vicenzoni on Flickr