Fresnel Lens Problem?

[SchwinnParamount]

In the processing of designing and building a back for my new camera, I've run across an interesting problem. I don't know if it is bad design execution or a miss-understanding of the nature of the fresnel lens.

As you all know, the distance from the inner surface of the ground glass to the edge of the ground glass frame must be the same as the distance of the film in the holder to the front edge of the holder itself. So, I made a holder that holds true to this rule. The problem ( I think ) is that I put a fresnel lens between the glass and the rabbited edge of the glass holder. Now the glass is a couple of mm further back than the film would be. But, the fresnel lens occupies the place where the glass would have been. By the way, the fresnel lens is within tolerance for focus.

I seem to be having a focus problem. Is the image from the lens transmitted through the fresnel lens as though it wasn't there? Or is the image being projected on the front of the fresnel lens with the ground glass just acting as a second layer of transparent material?

Does this make sense? I'm not sure I know how to explain my problem any more clearly.

 

[Ole]

Both, I think. Wasn't there a discussion about this just a few days ago?

The basic problem is that the Fresnel has both a physical thickness and an optical thickness, and they are different. So you are now focusing on the ground glass which is moved back a few millimeters pysically, and about 1.3 times farther back optically.

If the ground surface of the ground glass is not the front surface of the whole GG setup, you have a problem. A Fresnel can be fitted to the front, but only by changing the focal distance by a very carefully calibrated amount.