If you line up the suns rays parallel to a Fresnel lens, the light is concentrated, and the focus directly underneath. However, what happens if the sun is off to the side, making the light hit at an angle (ei. 45 degrees)? Will the only difference be the focal point, of will the light be less concentrated? If so, by how much? What about a convex lens? Thanks
A Fresnell lens is essentially a "collapsed" plano-convex lens. If you were to raise each ring of the Fresnell lens so the outer diameter of the ring were at the height of the inner diameter of the next larger ring, you'd have reconstructed the original plano-convex lens.
Obviously, there are some diffraction effects and various other higher-order aberrations, but at the simple level I think you're asking, the Fresnell lens produces a full image of the object. That's another way of saying Yes, it would produce an image of the sun even if the sun were not on the optic axis.