Flash Tip: Creating the Illusion of Distance by Adjusting Brightness
From Adrien-Luc Sanders, former About.com Guide
To make it look more natural for a sunny day, make the brightness adjustments in reverse: with the brightest range farthest, and the darkest range closest. It may seem to oppose the general assumption of how one thinks it would work, but give it a shotit looks a bit more right to the average eye.
Objects also tend to blur with distance, so if you want to do a little softening, go right ahead. Now you've got three clearly distinguished areas in your scene, a foreground and background--and a sense of depth that it lacked before.