View Full Version : Fresnel Screen

Rob "John Henry" Rothman
27-Jan-1998, 09:58
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?

Birck Cox
17-Jun-1998, 10:32
It seems a bit counter-intuitive, I know, but this works for me: years ago, when I first got an old Crown Graphic for copy work, I blithely added a fresnel to p erk up the image(I didn't know what I was doing). It's been there ever since, an d the 4X5 transparencies I get off the camera are crystalline in sharpness.The c onfiguration: fresnel is trapped between ground glass and graflok frame, i.e., t he closest surface to the back of the lens is the concentric-ring side of the fr esnel; the glossy side of the fresnel is in contact with the grainy side of the conventional ground glass; the surface I examine with a loup is the glossy side of the g.g. So either the image is correctly resolving on the ring side of the f resnel, rather than on the g.g., or the 2-3mm thickness of the fresnel doesn't make enough difference to affect f ocus. I lean toward the former conclusion. Birck Cox