1. A method of generating a three-dimensional (3-D) image of a sample using an optical microscope, the method comprising: projecting a pattern onto a focal plane of an objective lens of the optical microscope; varying a relative distance between the sample and the objective lens at pre-determined steps; capturing a first set of images of the sample with the pattern at each pre-determined step; recording the relative distance at each step; removing the pattern from the focal plane of the objective lens; repeating varying the relative distance between the sample and the objective lens at the predetermined steps; capturing a second set of images of the sample without the pattern at each step; and analyzing the first set of images and the second set of images to create a 3-D image of the sample.