Fresnel lens question

[Greg]

The simple answer to this has eluded me in figuring it out.

OK, on my 8x10 I have a fresnel lens mounted behind the ground glass. If I view the image projected on the ground glass by my 12 inch lens, the fresnel lens allows me to view pretty much all of the image. Now if I view the image projected on the ground glass by my 120mm lens, only the central part of the projected image is viewable.

In order to view more of the image that my 120mm lens projects on the GG, would I have to substitute a fresnel lens with a shorter or longer "focal length"?

thanks in advance

[photog_ed]

Shorter focal length. I think the focal length of the Fresnel should be about the same as that of your taking lens.

Ed

[mdarnton]

Visualize what is happening: Light from the lens is normally directed outwards at the edges, in an increasingly-larger cone, away from your eye, right? The fresnel works by directing all light hitting the ground glass back converging to the eye rather than letting it continue to diverge. Most longer lenses can get by with an average fresnel, but a wide angle lens fans the light outward at a very wide angle and needs a stronger fresnel to pull it back inwards to the point of the eye. The normal fresnel isn't strong enough to accomlish that thus at the edges the light is diverging too much for the weaker normal fresnel to bend it back to your eye. Thus, dark edges. Stronger, shorter FL fresnel required.