[typo]

Ok, I'm bracing myself for a thundering NO from most of the posters here.

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

[Kevin Conville]

Good post David,

I'm one who thinks this current obsession with flat fields is a bit of a fad. I'm not disputing that some prefer it and deem it worthy of pursuit, just that whatever "benefit" from it is exaggerated. You have pointed out several of the potential drawbacks, and you didn't even mention swirlvision.

[Kevin Purcell]

you didn't even mention swirlvision

And why would he? Pincushion distortion eliminates the rolling globe effect (which is I presume what you mean by "swirlvision").

"Flat field" and pincushion distortion are two separate optical properties. You can have the latter without the former.

The problem with some bins is you have both and that gives it's own odd edge effects that look like too much pincushion or two separate rings of pincushion. You can even get this in the opposite direction i.e. with field curvature but with low pincushion distortion so you get some rolling globe. Swift Eaglet 7x36 and Nikon Extreme ATB 7x35 have this effect.

Why have the 7x26 been around for so long ... perhaps Bushnell's inertia and decent profit from them as they continue to sell, perhaps? Why redesign the optics when you don't have to (even though today you could optimize the whole optical system unlike the past). That said I wouldn't mind seeing 7x26 from other makers (with a wide flat field )

I can't agree with the OP. I prefer to use the focuser to focus than swing the bin to pickout just the right amount of field curvature. As for bigger bins that have this effect look at the lower end of the market as they tend to have more field curvature than the upper end.

[elkcub]

Kevin,

He may be referring to Swarovision (if that's the term) and the much touted flat field with zero distortion. There is a lot worth considering in typo's post, and I agree with him on several points related to terrestrial viewing. I'm not so sure that aging eyes are a requirement, altho I'll put mine on the table at 74.

Ed

[ronh]

David,
The Fujinon FMT-SX 7x50 has considerable field curvature (3 diopters), but almost no other blurring off-axis aberrations. It behaves much as you have described. This works wonderfully as long as the light is not too harsh, but if it happens to be, there's a strong color fringing effect out near the edge. Of course 3 pounds is killer, but the views are killer too, so you will be doubly dead.
Ron

[typo]

Thanks Ron,

The Fujinon sounds blinding! Does it come with gimbal mounts?

More seriously, is that the right model? Holger Merlitz praises it for astronomy and it is aclaimed elsewhere for it's flat field of view.

http://www.holgermerlitz.de/miyauchi7x50.html

It sounds like the F in the model number refers to an additional field flattening lens. Did you mean the MTR or some other model? (Or are you trying to convert me?)

Regards,

David

[ronh]

Well, baloney. It might supposed to be flat, but has 3D from center to edge. Of course this might strike a tolerably young person who still has good accomodation as flat, since their eyes could focus it to a sharp image all over.

I think there is a fair amount of misunderstanding of what a flat field is. In fact, if this is flat, then I am certain that I don't either! But like you said, focus the center on infinity, and with my zero D of accomodation, the edges are in focus at about 25 yds.

Yeah it's big. I have mine mounted on my tank. Going for a ride around the neighborhood now, see if I can spy the first Say's Phoebe to pass through this spring. Driver, where'd I put my helmet?
Ron