Based on a 3-D prestack depth migration we built depth error gathers to analyze the focusing condition for a velocity field. In this paper we used two different data sets, a 2-D synthetic and a real 3-D marine data set. For both data sets we analyzed the depth error gathers. These depth error gathers probe to be useful tool in evaluating the focusing condition of the migration velocity to obtain a good 3-D image. In this paper we show one iteration of the depth focusing analysis methodology, future works will show more than one iteration in the migration velocity estimation for 3-D data sets.