Differential Interference Contrast (DIC) microscopy is a powerful non-invasive interferometer for visualizing live, transparent biological cells. The need for reconstruction and quantification of visualized objects out of DIC images requires that an image formation model is developed and that the inter- action of light waves with biological matter is solved. A common solution is expressed via the Born approximation either explicitly, or implicitly, within a point spread function of the microscope. The Born approximation validity region is limited and is currently set by a theoretical bound to very small objects. This bound is based on restricting the wave field to be within the object, which is not realistic to DIC microscopic apparatus or general light microcopies. It is also derived for object shapes of a sphere or an infinite cylinder. In this work a numerical framework is used to solve the Born approximation via the Helmholtz equation. A numerically-based analytic criterion for the validity region is presented for round and cubical objects. Different than the theoretical bound, the suggested criterion considers the field external to the object, corresponding to microscopic imaging.