If you bring a positive charge near a thin conducting sheet, it will induce a negative charge on the side nearest to the charge. And in order for the conducting sheet to remain electrically neutral, a positive charge of equal magnitude will be induced on the other side. How could the conductor have any significant effect at all on the total electric field? It seems to me like the field from the induced positive and negative charges should "cancel out" everywhere except for points very close to the conducting sheet (i.e. points not much greater than the thickness of the sheet).