Some time ago, I was given a fresnel screen for my Zone VI 4x5 camera. The scre en (original Zone VI equipment) is the type which permanently replaces the exist ing groundglass. It consists of two parts. The first part is a plastic sheet w hich has fresnel rings on one side and is frosted (apparently) on the other (smo oth) side. When installed as directed, the smooth side faces the lens. The sec ond part is a clear glass sheet which is installed behind the screen (toward the photographer); I assume its function is simply protective.
Since the smooth side of the screen, which appears to be the frosted side, will occupy the same position as that formerly occupied by the ground side of the reg ular GG, it stands to reason that using this screen will not change the focussin g accuracy of the camera. However, I would like to confirm this before installi ng it. Can anybody confirm this, either from experience or from optical theory?
Alternatively, is there a fresnel available which can be used on the Zone VI cam era and which is of the type that fits behind the existing groundglass, to be in stalled or removed as needed? This seems to me to be a more sensible approach t han permanently replacing the existing screen.
Edmund scientific sells fresnel lenses in various sizes and focal lengths. Coul d such a lens be used for this purpose? If so, what would be an appropriate foc al length?