Let's say you are focusing in on an object about 30' away with binoculars. Should the background that is another 30' behind it be in focus as well or does it depend on the binoculars? If the background is slightly out of focus does that mean the binoculars are faulty or could it be just the design?
[QUOTE=lvn600] just the design?[/QUOTE]
Most likely you have the answer there. Hm, 30' is about 10 meters, right? In a good pair i expect that, if focused at 10 m, the background should still be, while not perfectly sharp, well visible.
Depth of field is one of the things to look at in a binocular. I recently looked at a Steiner Peregrine 10x42. It wasn't all that bad, better that a Zeiss Classic actually, but a very shallow depth of field dismissed them immediately. Best in this respect was a pair of Swift Kestrel 10x50.
Hopefully someone will come along and give you the complete answer but the bins are not faulty nor is there something amiss with the design, it's why you have a focus control, even your eyes can't manage that depth of focus(field) you are just not aware of the change if that makes sense.
Mick