There was more discussion of this phenomenon in a thread I started a week ago about the Swaro SLC 8x 42 HD. You might note the similarity of our diagrams: http://www.birdforum.net/showpost.ph...10&postcount=8
, although I presented a binocular rather than cycloptic view so as to include monocular perspective cues and stereopsis.
In general, it seems to me that the longer the eye relief the further the observer's eyes must be placed from the ocular, which facilitates rays from unmagnified peripheral objects being imaged from either side (assuming they are not blocked intentionally by the eyecups). However, design factors generally dictate that long eye relief comes at the expense of either a narrow (true) field or a lower magnification.
The Swift 8.5x 44 HHS Audubon (narrow field) and the Zeiss 7x42 BGAT (lower magnification) are examples. Because of their long eye relief each is comfortable to use with eyeglasses, and IMO each presents a much reduced tunnel view using glasses.
Henry might wish to comment here, but it seems to be technically difficult, and hence expensive, to design binoculars with both
a wide field and long eye relief. This is now a distinguishing characteristic of the new Swaro 8x 42 HD, and even more so the 8.5x 42 SV, — all of which justifies the cost for me.