We should probably all aspire to be more clear about matters of edge vs central area sharpness. There are two issues being discussed, albeit they are related. One is actual edge sharpness. For simplicity's sake, we might say it is the outermost 40% or so of the viewfield. No wide-angle binocular I have ever seen has a sharp image here. Some, though, are considerably better, others worse. Nikon 8x42 HG and the SE series is much better than average in this respect, so was the 10x40 Victory II, and Leica 10x32 is very good also. Swarovski EL 8.5x42 is perhaps a bit better than average, and most Leica models, both the older and the Ultravids (which have the same optical construction than the older models but improved optical coatings) are below average for premium binoculars. Poor edge sharpness is not particularly significant when viewing birds, and most birders do not bother to twist around their eyes enough to notice it much, let alone be troubled by it. Since edge sharpness is virtually always poor, it is hard to remember exactly how poor it was in a particular binocular, and reliable assesments can only be made by side-by-side comparisions or by taking systematic notes.
The other issue, the one Henry has been talking about and I have made some comments upon, concerns how far from the center does the image retain all or most of the sharpness it achieves in the dead center. Although it is true that we tend to move the binoculars to center any bird or other object we are trying to view, anyone willing to excercise some introspection while viewing will note that their eyes in fact move around the image a lot, and much more than their hands do in trying to maintain central orientation of the object. Therefore, it does make a difference how wide is the "sweet spot", the area where your eyes feel as if no extra work is needed and the image remains 100% or so (in practice, perhaps the area where image sharpness is at or above 90% of the maximum?). In my mesaurements of the Zeiss vs. Nikon SE, there was a rather clear difference in the SE's favor in the width of the area where the eye-measured resolution of a tripod-mounted binocular did not drop noticeably. However, I must add that my visual acuity (visus) is about 1.5-1.6, or about one-and-a-half times better than "average normal vision". For someone with a more average visus, the perceived "sweet spot" would be wider, and for someone with a better visus, narrower. I would like to emphasize, however, that overall I think the FL's exceed the performance of the SE's (by a very small margin), but, like Henry, I'm not sure which I would be more comfortable using in the long run, since the wider sweet spot does make the SE somewhat more effortless for me to use. I also believe that the chief reason why the 8.5x42 EL has such a fine repuation among discerning birders is that due to a wide sweet spot, it provides an immediately accessible and relaxing view, and since the image in other respects is also on a par with other premium binoculars, that ease allows birders to forget the binocular and simply enjoy viewing.
Kimmo
