Often in binocular reviews you will see the words flat field used in the description. So what does flat field mean ?. It simply means that focus is constant across the field of view. With some binoculars you will see that you have a perfect focus in the centre, yet out of focus on the edge. You get this on some high power compacts, and on lesser quality binoculars.
I find that larger binoculars with low magnification give the best flat field images. 7x50s are a prime example. With 7x50s you get constant focus across the field, and require very little focus or right eye adjustment. This is why you can buy 7×50 binoculars that have no focus wheel attached.