Image characterization in microscopy, in particular, the estimation of its resolution, requires detailed knowledge of its relation to the object. For objects with sizes comparable to or smaller than the operating wavelength, such a relation can be obtained only by considering electromagnetic scattering described by the Maxwell equations. Here we follow precisely the steps involved in the image formation in microscopy with broad angle illumination-starting from the Maxwell equations to find the scattered far fields for each plane wave, projecting them into a sensor array, and finally assembling the incoherent image by adding all coherent contributions. We consider a classical object-a narrow slit in an absorbing screen, which is taken as a very thin chromium film deposited on a glass substrate. The inapplicability of the Kirchhoff approximation for such a slit is addressed, and the calculated image is subsequently analyzed to evaluate its intrinsic resolution using a point spread function. The difference in image intensities defined using the Poynting vector and the electric field intensity is also discussed.