hutchphd said:
Do you now understand what I am talking about? I am happy to discuss.
I'm happy too; it is sorting out my mind usefully.
Yes. We both see the same thing but I think your interpretation is 'not right' or perhaps 'limited'. Looking closer, you can see that there is dispersion which forms, say four separate orders (just looking at a single colour).
Imagine that the grating (representing what the OP has) behind a screen so that all you can examine is those beams. They all emanate from, apparently, a single location - the grating. However, a close up of one beam will show a beam with parallel sides (same angle all the way along - ignore the diffraction due to the limited aperture). If you had to use that beam to estimate where the source is, you would say that it's at 'infinity'. So you would have conflicting observations; beams spreading out from a nearby 'point' but single beams with apparent sources at infinity. You are seeing one thing and I am seeing another.
Now replace the grating with a box with a halogen bulb plus simple filter and an array of holes in it and look at one of those mimic beams. The beam will be diverging as if from a source that's inside the box; that's conventional non-coherent optics. You could definitely say that the source is a point. No conflict here and we will both interpret it in the same (conventional) way.
If you use a convergent lens on both of those bundles of light, you will get different results. The individual split laser beams will form sharp spots at F and there will be a scaled image, consisting of defocussed spots, formed in the plane at v according to the f and the u.
For the halogen bulb arrangement, you will get a sharp image of the array of holes at v. Point sources to point images.
There are a couple of caveats, concerning the information we have been supplied with. Was the "Image Plane" described in the diagram, actually identified? Probably the least worst result of focussing using the fresnel. My reasoning is based on a good lens and it fits with what has been reported. It could be that all that's being seen is due to the aberrations of the fresnel lens so a good conventional lens should be used as a reference.
However, the whole situation as described is, itself, very defocussed and the description (including that strange diagram of the odd shaped figure) is in very unfamiliar terms.
Cheers