  The recent advent of wave-shaping methods has demonstrated the focusing of
light through and inside even the most strongly scattering materials. Typically
in wavefront shaping, light is focused in an area with the size of one speckle
spot. It has been shown that the intensity is not only increased in the target
speckle spot, but also in an area outside the optimized speckle spot.
Consequently, the total transmission is enhanced, even though only the
intensity in a single speckle spot is controlled. Here, we experimentally study
how the intensity enhancement on both interfaces of a scattering medium depends
on the optimization area on the transmission side. We observe that as the
optimization radius increases, the enhancement of the total transmitted
intensity increases. We find a concomitant decrease of the total reflected
intensity, which implies an energy redistribution between transmission and
reflection channels. In addition, we find a qualitative evidence of a
long-range reflection-transmission correlation. Our result is useful for
efficient light harvesting in solar cells, multi-channel quantum secure
communications, imaging, and complex beam delivery through a scattering medium.
