Some time ago, I was given a fresnel screen for my Zone VI 4x5 camera. The screen (original Zone VI equipment) is the type which permanently replaces the existing groundglass. It consists of two parts. The first part is a plastic sheet which has fresnel rings on one side and is frosted (apparently) on the other (smooth) side. When installed as directed, the smooth side faces the lens. The second 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 regular GG, it stands to reason that using this screen will not change the focussing accuracy of the camera. However, I would like to confirm this before installing 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 camera and which is of the type that fits behind the existing groundglass, to be installed or removed as needed? This seems to me to be a more sensible approach than permanently replacing the existing screen.

Edmund scientific sells fresnel lenses in various sizes and focal lengths. Could such a lens be used for this purpose? If so, what would be an appropriate focal length?

-- Rob Rothman (rrothman@riag.com), January 27, 1998

Answers

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 perk up the image(I didn't know what I was doing). It's been there ever since, and the 4X5 transparencies I get off the camera are crystalline in sharpness.The configuration: fresnel is trapped between ground glass and graflok frame, i.e., the closest surface to the back of the lens is the concentric-ring side of the fresnel; 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 fresnel, rather than on the g.g., or the 2-3mm thickness of the fresnel doesn't make enough difference to affect focus. I lean toward the former conclusion. Birck Cox

-- Birck Cox (birckcmi@philly.infi.net), June 17, 1998.