Before everyone jumps down my throat, in my defence I'm in my 50s, have advancing presbyopia, and need all the help I can get on depth of field.
I'm not knocking flat fields of view. I know it's technically challenging and expensive to achieve, and appreciate the quality when it's done well. (I use top quality microscopes and macro photograpy as part of my day job). I also understand that for some applications, like astronomy, there is a good reason for some binocululars to have a flat field of view as well, but now I am rather puzzled that it appears to be one of the main criteria by which the different models are judged on a birding forum. (I don't use binoculars to look at flat objects.)
As I'm begining to appreciate, it seems to me that field curvature has a number of practical advantages for the (particularly aging) terestrial user. I'd seen that experts like Edz acknowedge this, but to be honest I didn't really understand the benefits untill I saw it for myself.
When I looked to replace my cheap/rubbish bins and scope, I read quite a lot of reports here and elsewhere before checking out likely candidates.The Bushnell 7x26 Elite (Custom) were an immediate Wow! It's just taken a bit of time to figure out why I liked them so much more than the competition including more expensive and technically superior bins. It seems any optical design has endured for 30 or 40 years must have got something right. At least part of that answer appears to be a pronounced field curvature.
At 10' the field is (nearly) flat. The curvature increases with focal distance. At about 200 yards, the center 30% is sharp at that distance, but the edges are sharp at about 40 yards. This is teriffic! It means that while I can see all the flocks of birds/ herds of deer etc. that I want to, and, providing the the target is on the ground, bush or tree, pretty much all the foreground is in focus too. If not true 3D (the objectives are very close) then a stunning perspective. Focussing on the bushes at the back of my plot (40 yards) it works out that about 75% of the total view is in sharp focus. With some well regarded roofs I've looked at, it might be a 30% strip across the middle... at least with my eyes. (The image also looks very vivid and three dimensional, for which you probably need more than field curvature to explain.).
There is another practical plus. Get the angle of peripheral view right and you have a hughly extended depth of field. Saves a whole lot of focal hunting when you are waiting for something to show.
It could be my eyes are just a bit wierd, but I hope I'm not alone in appreciating the benefits. I guess it would be usefull for the younger observers, but a real plus for us oldies. It someone could point me in the direction of a full size 8 or 10x with similar properties I'd really appreciate it.
Regards,
David