  We describe a new method for measuring galaxy magnification due to weak
gravitational lensing. Our method makes use of a tight scaling relation between
galaxy properties that are modified by gravitational lensing, such as apparent
size, and other properties that are not, such as surface brightness. In
particular, we use a version of the well-known fundamental plane relation for
early type galaxies. This modified "photometric fundamental plane" replaces
velocity dispersions with photometric galaxy properties, thus obviating the
need for spectroscopic data. We present the first detection of magnification
using this method by applying it to photometric catalogs from the Sloan Digital
Sky Survey. This analysis shows that the derived magnification signal is
comparable to that available from conventional methods using gravitational
shear. We suppress the dominant sources of systematic error and discuss modest
improvements that may allow this method to equal or even surpass the
signal-to-noise achievable with shear. Moreover, some of the dominant sources
of systematic error are substantially different from those of shear-based
techniques. Thus, combining the two techniques addresses the major weaknesses
of each and provides a substantial improvement over either method used in
isolation. With this new technique, magnification becomes a necessary
measurement tool for the coming era of large ground-based surveys intending to
measure gravitational lensing.
