Thanks everyone for the reactions.
Rudolf is right about how my ShenHao is constructed: the back has a provision for putting in a fresnell before the ground glass, without changing the register of the ground glass. The fresnel I bought seem to fit perfectly before the ground glas of the camera when checked with a caliper. What i wanted to ask is how I should position the fresnel: with the plain side towards the ground glass or towards the lens?
My second question is what the disadvantage are of using a fresnel lens: some people seem to find it harder to focus the lens. Are there any other disadvantage sin using the fresnel i should aware of?