This master thesis combines theoretical studies and observational results of the gravitational lensed quasar SDSS J1004+4112. The first chapter provides an overview of the principle of gravitational lensing along with different lensing phenomena and their applications in modern astrophysics. Also some insights on active galactic nuclei with a specific focus on quasars is given as well as a short summary of the historical background. In Chapter 2 the fields of gravitational lensing as there are weak-, strong- and microlensing are depicted and in Chapter 3 a general theoretical description of gravitational lensing can be found. Chapter 4 is devoted to the description of the data and the observatory where they were taken at. In Chapter 5 a summary of the methods used in this thesis are described, containing the preparation of data, producing magnification maps and calculation of the accretion disk-size of gravitational lensed quasars. The main part of this masters thesis focuses on the applications related to the wide-separation lensed quasar SDSS J1004+4112. Eight monitoring seasons of the four brightest images of SDSS J1004+4112 observed between December 2003 to October 2010 are presented. Using the measured time delays of Fohlmeister et al. for the images A, B and C and the model predicted time delay of Oguri for image D, the intrinsic quasar variability was removed, finding microlensing events of about 0.5 mag in the images C and D. From the statistics of microlensing amplitudes in images A, C, and D, the half light radius for the accretion disk has been inferred that is in agreement with the size predicted from the black hole mass in SDSS J1004+4112 using the thin disk theory.