Whenever I acquire new binoculars, which is frequent nowadays, I take them out back and view familiar things. This is not only convenient, but also provides a ready means of comparison. Several views are of well-manicured bushes, like oleander or pittosporum, and can be viewed purpendicular (i.e., normal) to their surfaces at distances of 20-60 ft. Well, here's the thing. When I stare at such a dense, full-field, leafy surface for a few moments, it invariably becomes dish shaped, rather than flat, and resembles the inside of "Chinaman's Hat," the outcropping of lava off the coast of O'ahu, Hawaii. It's an amazing and strong effect. http://www.hawaiiweb.com/oahu/sites...inamans_hat.htm
Years ago I thought something might be wrong with my binoculars, or maybe me (still possible), but every binocular I have produces the effect — although not to the same extent. Some hats slope up from the center almost to the edge. Others are bowl shaped for about 1/3 the field diameter and then flatten out to a disk, which forms the hat's brim. The illusion takes somewhat longer to develop with one eye, and seems to be facilitated by two.
Although I'm anxious to hear any explanation that makes sense, it seems to me that the perception may be the result of field curvature, or possibly astigmatism, where the brain tries to make sense of differential focus radiating from the center of the view. So, the Chinaman's Hat percept is created.
Other than the desire for an explanation, one thing that interests me is the possibility that the illusion might be used to measure some meaningful optical property of the binoculars. It wouldn't be difficult to standardize scores for the hat's crown depth, crown diameter, rim, and so forth.
Well, anyway, that's my New Year's contribution for those with discretionary time — and a manicured pittosporum bush.
Ed
Link doesn,t work.