Kevin has I think touched on what I have seen before and am concerned about here - it is mostly "veiling flare". From general recollection, it seems to often accompany the "substandard contrast" (to which Alexis refers), at least in non-ideal light situations. If memory serves me well, the Conquests are guilty of both, as well as the symmetrical circle phenom you and Kevin seem to be describing. It does seem that bins which have a glare/flare/stray-light problem often have more than one of the manifestations. I've typically attributed it to coatings; however commentary here suggests that there are other major contributors.
1. I now have FrankD's 8x30 Conquests (and I really like them). I saw no examples of this "ring" when testing in the same environment that I saw the ring with the Discovery.
2. This ring effect is different from veiling glare. I tried to make this clear in the post above. This "ring" effect is precisely that a ring in the FOV centered on the center of the FOV. Inside and outside the (relatively thin) ring the view is unaffected (as far as I can see). This is different from veiling glare where the section of the view affected a "circular chord" across the FOV i.e. a circle intersecting the FOV circle but not concentric with it though in the workst case it might consume the whole fo the FOV. That is it is not a ring around the center of the FOV but a portion of the FOV.
We really need a standard definition of these effects (I have ranted about this before )
For me glare, flare are three different effects.
0. stray light: the generic name for all problems associated with light going where it shouldn't. Like stray color (e.g. not all stray or false color is chromatic aberration in the lense some of it, especially in roof prism bins, is dispersion in the prism).
1. Glare aka veiling glare. The loss of contrast across part or whole of the FOV due to stray light usually off-axis light. This is also used for on-axis light scattered in the lenses. I'm happy to agree to any usage for glare that involves loss of contrast from non-discrete images
2. Flare aka ghosts: discrete false images of bright sources generating on-axis rays. You know the stuff they put into computer animation to make it more like a real camer (those circles the other side of the image to the sun.
And there is even a third effect that I call "glints" i.e. a discrete reflections off a metallic part either at the objective or the ocular. Really the Promaster suffers from these too. Not seen them in other bins. Who would put a non-matte-black part on a bin in those locations? It looks a bit like a flare with a starburst or diffraction effects (out of focus).
Once you get to this point (with the nomenclature) you can see that these effects have two different origins: one is on-axis (i.e. lens and coating related) and the other is off-axis (and more related to good baffling and keeping off-axis light off the objectives.
The former (ghost image/flare) problem is essentially solved with good current multilayer AR coatings. I noticed yesterday that the Diamondback can show ghosts (I'd not seen them before) but my other (better) bins don't show ghost images.
The latter problem (off axis light reducing the contrast) is a baffling problem and I think comes down to careful design and testing.
Stray light is one of the last issues to get fixed in a bin because it doesn't appear on the spec sheet. So without hands on experience (or feedback from a good reviewer like Frank) you don't know about the problem until you are in a marsh on a fall day at 5:30pm looking into the sun and seeing a whiteout not shorebirds. When the designers of bins like the Promaster ED or the Hawke ED take this seriously (and from comments here I think they're already doing that though they may compromise on the wrong side of the stray light/FOV line for some of us) then the alpha bin guys will have to get worried because all they'll have is their brand.