Fresnel

[Absinthe]

Is the fresnel anything more than one of those full page magnifiers they sell in the book stores?

[Jim Jones]

They are similar in theory, but different in construction. Fresnel lenses as applied to ground glasses have been frequently discussed. Try a search (at the top of this page) for much more information.

[Eric Woodbury]

A fresnel is a flat lens. It has the same curvature, but the thickness is removed. There are lots of little rings with a curved surface on one side. Those page magnifiers are one example, so are the backup lenses for campers, the spinning fresnels in lighthouses, and the several different kinds for view camera. At a fixed focal length and on axis, they work fine. Off axis and at other focal lengths, they are a compromise.

[Mark Woods]

The fresnel lenses are used extensively on motion picture lights. Lenticular fresnels are used for 3D stills and also those stills where the image changes. The circular ones you're describing are the magnifiers and also used on van windows for a better view.

[Emmanuel BIGLER]

Is the fresnel anything more than one of those full page magnifiers they sell in the book stores?

You can use such a cheap loupe as a fresnel lens for your ground glass. The main parameters are the focal length and of course the size.
The rule of thumb used for the compromise is that you take a focal length equal to the diagonal of the format or equal to the standard focal length of the format e.g. 100 m for 6x9cm, 150mm for 4x5", 210mm for 5x5" and 300 mm for 8x10"
Large A4-size (or US-letter-size) fresnel loupes have a focal lenghs close to 300 mm and fit well a 8x10" camera.
Their f-number is huge, about f/1 but nobody asks such a Fresnel lens used as a taking lens to make decent pictures of photographic subjects