I have, among my Mamiya TLRs, a c3. I wanted to add a clear glassine grid to it as I have on my 4x5 to help me with composition. I removed the housing on the focusing screen and found that it has both a fresnel lens and a ground glass screen. BTW, my c220s only have the ground glass. What is interesting is that the fresnel lens is in the light path between the viewing lens and the ground glass, whereas the fresnel is outside the ground glass on my 4x5. On my 4x5, the glassine grid is between the ground glass and the fresnel, but the fresnel is abousolutely outside the ground glass. The screen arrangement on the c3 is held in place with some dried glue in each of the corners, so my assumption would be that this is the arrangement that came from the factory, but it seems contrary to everything I understood about the way fresnel lenses worked and the position of the image forming surface. In addition, even though the c3 has the fresnel lense, my experience with is is that it has been harder to focus than the c220s, which I would now say was counter-intuitive. Can anyone offer some clarity on this? Should I just put the clear grid between the 2 lenses and put it back the way it was? I would be afraid that switching the two screens (putting the ground glass inside the fresnel, as I would expect it to be) would now place the ground glass too close to the viewing lens and alter the focus point, so I do not want to do that. On the other hand, if this fresnel lens was an "after market" modification, I would have no qualms about rigging some type of clip and putting it outside the ground glass on top of the focusing screen housing so that the focusing screen is still in the right place. Anyway, I'd like any insight that anyone could offer. Thanks.