We present a theoretical analysis of the image formation in conventional brightfield reflection microscopes with optical sectioning property via structured illumination. We show that the optically sectioned images obtained with this approach possess the optical sectioning strengths comparable to those obtained with the confocal microscope. We further show that the transfer function behaviour is directly comparable to that of the true confocal instrument. Some effects of spherical aberrations are included in the analysis. The theoretical considerations are compared with and confirmed by experimental results.