Meant to be Seen

There has been quite a bit of interesting discussions here on the use of fresnel lenses in HMDs as of late, as well as foisi's excellent example (http://www.mtbs3d.com/phpBB/viewtopic.php?f=120&t=16434) that I thought an open discussion about what may be possible would be worthwhile - specifically in terms of multi-element "LEEP" style lens arrays.

Since fresnel lenses in some cases can be used in place of more standard type lenses, it seems to me that they might also be well suited to LEEP lens arrays where they might be able to replace some or all of the standard elements, and in turn reduce the overall weight.

In the LEEP On The Cheap article (http://www.vrtifacts.com/hmds/leep-on-the-cheap/), Tone uses a 2 lens array consisting of a double convex objective lens, and a plano-convex eyepiece. I'm certainly not a lens expert (or even much of an amateur for that matter ), but I do know that theatre spotlights use either plano-convex or fresnel lenses. If those could be substituted for one another, then shouldn't a fresnel be able to at least replace the eyepiece in Tone's example? Even if the objective lens remains quite large, the reduced screen and HMD housing size should be able to make up any weight penalty over a single element + large screen design.

I'd like to hear some thoughts on this from those that have a better understanding of optics to let me know if there's any substance to this or if I'm way off base. The potential for a small HMD with a wide FOV is really exciting.

Leep on the cheap lenses have a focal length in the range of 70-80mm, maybe they could be replaced (both lenses) by this reference : http://www.optolife.com/diy_projector_lens/FRL002.html (specs : Ø80mm, focal length 70mm; but it seems they are out of stock for now)

the idea is to set the screen at the exact distance of the resulting focal length of the optical system to be able to focus through it as if it was at a infinite distance (I tried to explain why here)

the resulting focal length of setup like this can be estimated with the formulas of compound thin lenses

but the best way is to experiment in real life (the focal length of the InfinitEye's lenses is 66mm (measured) instead of 60mm (calculated)

Thanks for the explanations. Your post in the "Why the Rift works" thread was very informative (though I admit, I'm still trying to wrap my head around it).

In the case of compound lenses, and specifically in the LEEP on the cheap example, does the first lens converge the image, while the second lens is placed somewhere behind the first's focal point (where the undesirable beams from the outside edges of the display are refracted to an area outside the surface of it) converging the image again before it reaches the eye?