Here is the latest review of the EDG 8X42 updated by Tobias, for those interested.
I always thought the term flat field meant the focal plane across the visual field was in focus from edge to edge, whereas a curved field meant that some % of the periphery of the focal plane was not in focus when the center was, but could be brought to focus, by re-adjusting the focus wheel, so the plane of focus was curved, by design.
.... It is not yet clear to me whether curvature of the focal plane and barrel or pincushion distortion of a grid are inextricably linked when it comes to binoculars. In other words, can one have a relatively flat focal plane in a visual field, combined with barrel or pincushion, especially with multiple glass elements?
.... So, what does flat field refer to then when it comes to binoculars? A curved or flat focal plane, or a visual distortion that is seen when looking at a grid? (or both)
To me the impression of 'flatness' in an image has more to do with depth of field, and value structure, rather than relying on an engineered spatial distortion across the entire field, introduced by optics.
The term "flat field" is thrown around a lot; some use it to mean "sharp to the edge", other times people use it to mean "straight lines (no pincushion)." But my understanding is that a "flat field" most accurately refers to field curvature, as you noted.
I've read enough of Henry's posts to know that correction of field curvature / astigmatism (which impact sharpness as you move to the edge) is NOT related to rectilinear distortion (barrel vs pincushion).
I have a feeling these concepts have become linked on binocular forums in no small part because of the "Swarovision" prominence and dominance. When Swaro introduced the EL SV series, all the talk was "flat field! flat field!" and these binoculars happen to have excellent correction of astigmatism + field curvature (ergo very sharp to the edge) and ALSO they chose to aggressively correct pincushion distortion so they have high AMD (rolling ball effect).
So I don't think it's "inextricably linked", other than the colloquial linkage that has arisen as a consequence of the Swarovision defining the "flat field" paradigm for the modern roof. So in many people's minds, thanks to the EL SV, the term "flat field" makes them think "sharp edge to edge, straight lines, but also rolling ball".
As above, I think it's important to distinguish between "technical" terms (low field curvature / pincushion vs barrel distortion / etc) that are precisely defined, vs. colloquial subjective descriptors. If someone compares two binoculars and comments "wow the image on Binocular X feels so much deeper, whereas Y just feels flat in comparison" ... well, there's a lot of things they could mean by that!
I currently have in my possession a 10x32 EDG, an 8x42 Monarch HG, a 10x42 Trinovid BR, and an 8x32 Ultravid HD, listed in order from "flattest" to "most curvalicious". I absolutely notice what Tobias described as "images render space in a flat, compressed way" with both Nikons comparing to the two Leicas. Something about the view in the Nikons (which both have "field flatteners") makes the depth of the view feel a bit "compressed"; I am very confident it's not my imagination... as you noted, with photography you can create a sense of artificial deepening or compression by manipulating the visual plane / focus depth, so it's a real phenomenon in theory.
If I had to describe it, my best analogy is a "diorama effect", where layers of depth feel flattened as though real objects appear to almost be flat cut-outs vs 3D. For example, looking over the tops of trees, where there's a lot of depth from the closest to the farthest tree, the full depth of the trees feels compressed/flattened towards the viewer, and the trees themselves don't present as "deep". The EDG (which most aggressively corrects pincushion + field curvature, although neither as much as a Swaro SV) shows it the most, the MHG to a lesser extent. The UVHD 32, which has plenty of pincushion AND field curvature with zero AMD, feels incredibly "deep" and "3D" in comparison when I swap. Now, I realize the 8x mag of the UV gives it a "depth" advantage vs the 10x of the EDG, but since I have two other models (one 8x and one 10x) with which to compare, I do think there's something to it.
As an interesting side anecdote here -- the 8x32 UVHD are my wife's binoculars. She's a one binocular gal, and she LOVES those Leicas, and only uses them. When I get a new binocular in house to play with, I like to hand it to her to try out just to get her off-the-cuff impressions (she doesn't know anything about technical optical stuff, just raw subjective reaction).
I was really hoping she would like the EDG, since she had used Nikons for decades prior to getting the Leica this year, and more importantly the EDG have a very narrow minimum IPD, and my wife is on the extreme narrow end (52-53mm range). When she tried the EDG, however, she immediately said, "oh, these seem weird! they are very clear but it feels like I'm looking at a microscope slide!" (she is a lab scientist so this wasn't a random analogy). She switched back to her Leicas and said "yes, this feels so much more natural!" When I pointed out the 10x vs 8x focal plane difference, she grabbed the 10x42 Trinovids and compared and she definitely felt it was something unique to the EDG, not 10x magnification (and the Trinovid has a smaller FOV than the EDG, so it's also not the 8x UV's advantage in FOV).
After probing a bit to understand what she meant (I tried my best not to "lead" her by revealing anything about flat fields vs curvature etc) it was clear she was experiencing the phenomenon noted above, and her reaction of "like looking at a microscope slide" was her trying to convey that it felt like the image was "compressed" and felt "too close" to her, like a picture being shoved in front of her face, whereas with the Leica it felt more "natural" as though she saw the extra detail, but the subject still felt "properly placed" in space.
So while she could recognize the quality of the EDG optics, she just found them subjectively distasteful and they were her least favorite of the 4 options noted above.
EDIT: I also wanted to note that, while I do NOTICE the effect, it doesn't really bother me too much. The EDG view is otherwise just phenomenal, and thankfully they implemented a very "tasteful" approach to flattening, retaining just enough pincushion and field curvature to not make it feel excessively flat in real world use (e.g. panning doesn't feel unnatural, in fact the 8x42 MHG to my eyes exhibits more "rolling ball" than the 10x32 EDG).
Eitan, thank you for this as it helps clear up things that I know I have sometimes carelessly and, to be honest, without realizing, lumped together in my mind.
I was very interested in your wife's reactions to the Nikon EDG vs. the Leica. I have a 7x42 EDG as well as a Leica UVHD Plus 8x32 and UVHD Plus 7x42. So I am familiar with the 32 your wife likes to use, and have also been able to compare a like mag. x objective size (7x42) EDG with UVHD plus.
I'd be interested if you and/or your wife feel the same as me, which is not just to do with the microscope slide feeling (I get that too) but with the two dimensionality and lack of depth making me feel a bit uncomfortable, a bit queasy as I focus in on the subject. It's not rolling ball as I am not panning in these circumstances. It's is as though the 'unreality' of the view affects me physically. That's not to say I dislike it once I get there; in fact I like the EDG very much indeed for its colour and other qualities but it is as said a bit uncomfortable to start with when altering the view and adjusting the binoculars to it.
Yes Eitan, this is exactly my impression of "flat field" and how (largely thanks to Swarovision) it got confused with "less rectilinear distortion". The only thing I would add regarding field curvature itself is that adding field flatteners also introduces complex distortion transitions near the field edge which some can find distracting ("mustache distortion", "Absam ring"). I don't find it as disturbing as the obvious astigmatism of the pre-SF Zeisses for example, but I dislike it and will gladly accept a modest loss of sharpness at the edge instead.
Since you have both the EDG and UVHD in 7x42, do you agree with Tobias' assessment of how the Leica feels "wider" and "deeper" even though they have the same FOV? I have been musing over a high end 7x42 for a long time, but after reading Tobias' assessment and also experiencing the EDG view for myself, it made me decide to avoid the EDG version (there was one on eBay recently for around $1100 which I would likely have bought otherwise to try) ...
... more pincushion seems to make the view feel wider than it is (the AFOV gets close to the simple mag x fov calculation vs high AMD binoculars which are closer to the "tangent" condition, yes I read Holger's article and tried to refresh my ossified trigonometry skills!).
My feeling is that the reason to get a 7x42 is to have maximum "depth" and "walk in" FOV, so why would I get a design that seems to "fight against" the benefits of a 7x binocular? I like the EDG view in a 10x where I'm just trying to key in on detail at longer range, so I don't care about the "diorama effect", but if I'm going to splurge on a 7x42 I want it to feel natural and 3D.
So I decided to hone in on the Leica, since the Zeiss FL 7x42 is like a rare unicorn on the used market, the UV will have a distortion pattern that will maximize the "3D depth" and AFOV of the image, and I already know I love the color and contrast and build quality of Leicas. And, it just so happens, I just won an auction on eBay for a 7x42 UVHD for a great price, so let's see how this goes! I'm very excited for this one, and if I love it as much as I hope I will, it will likely replace the MHG 8x42 as my "I don't need 10x mag and/or need a larger exit pupil" birding optic.
Here is the latest review of the EDG 8X42 updated by Tobias, for those interested.
I can't remember if you just recently got yourself an EDG 8x42 - or if you have had one all along. How do you like it yourself? I like my 7x but Tobias originally much preferred the 8x so hopefully it's everything you wanted from it.
I'd agree that the Leica feels wider and deeper than the Nikon 7x42. I haven't had a chance to check this since reading your post, but I have always had the sensation that the EDG had a narrower view. However... the more I use different bins the more I feel that the biggest mistake for my enjoyment of any of them is to go to and fro to compare them; that way I end up dwelling on negative points as well as not using them for what they are intended!
That said, if I decide to thin down a bit, this point you raised would be a good basis for deselection, though in line with what I just said in the previous paragraph it might come down simply to a feeling of how much I like or don't like the overall effect - colour, contrast, lack of glare; and the Nikon is good at lack of glare....
As far as "wider", both the 7x42 UVHD+ and EDG II have a 420' FoV if I recall correctly but according to the listed specs I found the Nikon has an AFoV of 52.2 vs 56.0 for the UVHD+. I agree this difference is readily apparent - although not bothersome to me - when comparing them.
As far as "deeper", I don't notice any difference between the two and am not sure what "deeper" means here since the DoF of both 7x's is the same, unless it is referring to a different impression of depth of the image some may experience resulting from the "flat field" design of the Nikon vs the pin cushion approach of the Leica (?). If "deeper" is referring to a 3D like effect, the Zeiss FL 7x42 does seem to have a bit more 3D effect than the other two.
As far as "deeper", I am not sure what "deeper" means here since the DoF of both 7x's is the same, unless it is referring to a different impression of depth of the image some may experience resulting from the "flat field" design of the Nikon vs the pin cushion approach of the Leica (?).