Weak lensing is sensitive to both the distance and the growth factor as a function of redshift. The lensing effect can be measured using either the shear or the magnification. Despite not being specifically designed for this task, the J-PAS camera is expected to achieve high quality measurements of galaxy ellipticities, which will be combined with our ultra-precise photometric redshifts to form an extremely powerful weak lensing survey, uniquely well suited to accurately separate the lensing shear signal from intrinsic galaxy alignment. Gravitational lensing modulates the observed spatial distribution of galaxies. Dim galaxies that otherwise would not have been detected are brought into the sample by the lensing magnification. This increases the observed number density of galaxies. On the other hand, magnification also increases the apparent area, which leads to a drop in the observed number density of galaxies. The net lensing effect, known as magnification bias, is controlled by the slope of the number counts. The J-PAS Survey will be able to measure this effect by cross-correlating galaxy samples defined by separated redshift slices. Given the excellent seeing measured in the Pico del Buitre, J-PAS will use the best times (seeing<0.5-0.6") to perform a full high-quality image map of the Northern Hemisphere.