A wide angle camera lens could assess edge sharpness if aligned with the optical axis of a binocular.
The human eye could not. It would not only have to be diverted towards the field edge, but also moved laterally to avoid occultation.
John
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Wide flat field and edge sharpness an illusion? (1 Viewer)
- Thread starter Tringa45
- Start date
The human eye could not. It would not only have to be diverted towards the field edge, but also moved laterally to avoid occultation.
John
This could be a Swaro issue though, not one common to flat field binoculars in general. I haven't spent a great deal of time with the EDG but it's supposed to handle glare very well. Isn't the issue with Swaros and glare thought to be baffling (or lack thereof) rather than eyepiece design?
There almost certainly is. Fortunately I can look from edge to edge, left to right, top to bottom, of all my binoculars without taking my eyes away from the center and without getting blackouts so long as my eye distance is right. When searching for distant targets I look around the field of view in that way, to make sure everything within it is seen, before moving the binoculars.
Okay.
I really wonder if back then (or even now), optical engineers took this into account when designing wide field eyepieces. Whenever you take a picture through the eyepiece (I tried with my Nikon EII and Aculon) all the field looks sharp and in focus when it is aligned with the optical axis. However, when you actually see the edges with your eyes, the eye has to be decentred so it's pupil coincides with the exit pupil, and the image isn't necessarily sharp. It makes me wonder if the correction of aberrations was done assuming the eye would be aligned with the optical axis. We can't know this because we can't tell if the image projected onto the retina is sharp unless it is the fovea.
Ignatius
Texpat
For thousands of years humans, like pretty much all other animals, have moved their head to focus in on things of interest. Their eyes focus nicely straight ahead and through peripheral vision they can notice things at the edge. The eyes can then move in the head to a certain extent to focus on such things on the periphery. If that is not enough, the whole head moves while the eyes lock onto the thing of interest until it is more or less straight ahead again.
Why would we stop doing so just because we use binoculars?
For me at least the process is still the same. I look through my binoculars. Straight ahead everything is in focus. Around that focal area is the sweet spot through which I can move my eyes and see without blackouts. Beyond that is a, sometimes thinner and sometimes thicker, ring where the image defocussses progressively - until I get blackouts. In that ring I may still notice some movement or colourful splash, but to focus on that I will have to move my head with the binoculars. A wider FOV and/or larger sweet spot allows me to have more latitude before I have to move my head, but eventually I will have to move it. Additionally I find it quite uncomfortable to try and look a things by just turning my eyes to their limits when I am looking with the naked eye.
I have never looked though a flat field optic, but I daresay going by descriptions here and elsewhere I would not find it particularly useful. It is alien to my normal way of looking at the world.
YMMV of course.
Why would we stop doing so just because we use binoculars?
For me at least the process is still the same. I look through my binoculars. Straight ahead everything is in focus. Around that focal area is the sweet spot through which I can move my eyes and see without blackouts. Beyond that is a, sometimes thinner and sometimes thicker, ring where the image defocussses progressively - until I get blackouts. In that ring I may still notice some movement or colourful splash, but to focus on that I will have to move my head with the binoculars. A wider FOV and/or larger sweet spot allows me to have more latitude before I have to move my head, but eventually I will have to move it. Additionally I find it quite uncomfortable to try and look a things by just turning my eyes to their limits when I am looking with the naked eye.
I have never looked though a flat field optic, but I daresay going by descriptions here and elsewhere I would not find it particularly useful. It is alien to my normal way of looking at the world.
YMMV of course.
@Ignatius
Your mention of the so-called "sweet spot" leads me to question whether this is merely a function of the binocular's exit puil and instantaneous size of the user's own pupils. It would usually be generous on an 8x56 and less so on a 10x30, and could indeed be largely independent of the eyepiece design.
John
Your mention of the so-called "sweet spot" leads me to question whether this is merely a function of the binocular's exit puil and instantaneous size of the user's own pupils. It would usually be generous on an 8x56 and less so on a 10x30, and could indeed be largely independent of the eyepiece design.
John
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A couple of days ago I tried to stop down the objectives of my binoculars down to ~1cm to see if my IPD settings were fine (if you can't see with both eyes at the same time, it means the IPD is not properly set).
I am aware that for my Nikon EII 8x30, the exit pupil of would be around 1.25mm and that is much smaller than my pupils during the day. However, I was surprised to see that the whole 8.8º field became sharp, and the chromatic aberrations were completely gone. If this is the case with other binos too, it would suggest that the size of the sweet spot would indeed change depending on the eye pupil size.
Ignatius
Texpat
Looks like I am getting myself into uncharted waters here because I do not have enough knowledge about optics/physics to speak to HOW the thing most people call sweet spot comes about. Whether it is indeed a quality designed into the optical formula of eyepieces, or whether it comes about, as you say, as an ad hoc interaction between the viewer's eye and his optical tool.
My assumption, based on how people very often agree about a binocular's sweet spot, is that that is something that is basically a matter of design, and that the viewer's interaction with, and perception of it plays only a very minor part - if at all.
As I said, I would not know how tto investigate this or test for it, but I can certainly try it out with some of my binoculars to see what my own experiences tell me. Probably not much use to the scientists here.
Perhaps we have different interpretations of the term "sweet spot".
Stopping down the objectives to something like f/10 would reduce several aberrations, in particular SA and CA but the attendant 1,25 mm exit pupil would require very precise eye placement and impair viewing comfort and brightness.
John
By the sweet spot I mean the percentage of the apparent field of view that is sharp. Normally the EII is like 60% of its field sharp, but stopped down, it was like 100%. It was like a dim NL Pure. Of course, because of this tiny exit pupil, proper eye placement becomes very important. If the ipd setting is just a bit wrong, you don't have image in one eye. Furthermore if you just roll the eyes to see the field edges, without decentering them, you get immediate blackouts. Perhaps you meant sweet spot of the eye movement without blackouts?
William Lewis
Wishing birdwatching paid the bills.
The term "sweet spot" is usually used to describe a shop selling sweets but in optics it generally meant the area in the centre of the field that is relatively free from aberrations.
The experiment of stopping down the objective effectively means you are only using the best part of the objective lens which will reduce aberrations. Nice work!
The experiment of stopping down the objective effectively means you are only using the best part of the objective lens which will reduce aberrations. Nice work!
Alexis Powell
Natural history enthusiast
Sorry Tringa45, you lost me right there with your first sentence. "Of no practical value"?! That's exactly the value they have for me, so your statement is false, incorrect, wrong...
Although I enjoy theoretical arguments very much sometimes, when it comes to birding optics, what has always motivated me has been what I find of practical relevance. That pragmatism is the philosophy behind 99% of my past posts on BirdForum and what motivates this response.
My first decent binocular, which I used for many years, was the Nikon 8x40 DCF Classic Eagle. At first, I didn't know how it differed from other binoculars of the time, but what I now know is that I enjoyed its very flat and low-astigmatism-to-the-edge view. As time went on and the competition improved, I discovered that its view in the center was inferior with respect to brightness, resolution, and especially contrast in comparison to roofs with phase correction coating. Eventually, I switched to the Zeiss 7x42 BGATP as my primary birding bin, and I used it for many years, but I was always bothered by its falloff in image quality off-axis.
I like to dart my eyes around the view (even when not using bins, which is why I always wear modified aviator style glasses, and now that I am old, especially for birding, vastly prefer single-correction or bifocals with the near correction lens set very low in the frame over even the latest and greatest progressive lenses. I do use progressives for indoor/office work). The Nikon 8x40 Classic Eagle allows for darting the eyes around the view, whereas the Zeiss 7x42 BGATP does not. Even today, I can get those bins out of storage and appreciate that difference. It is not a subtle difference! Sure, I move my bins around to look at different objects in different directions, but I nevertheless greatly appreciate a bin that performs well off-axis because it allows me to dart my eyes around the view and because--even when moving the bin to different positions--it allows me to look through the bin "sideways" and still get a good view. I really appreciate a bin that delivers a good view quickly, i.e., even when not perfectly aligned with my eye, not perfectly aligned with the "target" etc. I find that such bins are much more comfortable to use in the course of a long day of rough and tumble birding. Getting the flat-field view of the Nikon with the bright contrasty view of the Zeiss in a single quick-focusing, close-focusing, ergonomic, and robust bin with low CA and low sensitivity to exact eye placement was a long time in coming, but several good choices are available these days. My personal choice, which for me is the perfect all-around birding bin, is the Swarovski 8.5x42 EL SV late production pre-FP. Awesome bin! Love that wide flat field with excellent edge sharpness!
--AP
Alexis Powell
Natural history enthusiast
As someone interested in this topic for decades, I've paid a lot of attention to it and to differences in people who I know well. What I have found is that some people have what you describe--a tendency to move their head and not their eyes--whereas others, like me, move their eyes much more and only move their head comparatively reluctantly. At least those are end points of a wide continuum of behavioral variation. People like you, e.g., adjust to using progressive glasses lenses easily. People like me have a hard time with progressives and require a lot of retraining. When I look at a scene, I look around with my eyes. When I look through a bin, I try to look around with my eyes. If it is well corrected, looking through the bin is like looking through a nice clear window or a hole in a wall. If it is not well corrected, the view gets very soft just as soon as I'm not looking straight through the center axis of the window. For me, it is very unnatural to think that I should have to move the window to see clearly in the direction of my gaze. I certainly don't move the windows in my house to see through them clearly off-axis!
--AP
dorubird
The unskilled mechanic blames his tools!
Wide flat field and edge sharpness an illusion?
It is not an illusion, but I can say that I like small apparent visual fields view as well as large ones, if the following conditions are met:1 To be surrounded by a clearly defined fieldstop (like a knife edge), followed by absolute black
2 Then it doesn't have to be as clear on the edges as it is on the center, but it must have an acceptable clarity on the edges without distracting my peripheral vision with any kind of blurring.
The binoculars that have these are the Habicht 7x42. Here are two places where I wrote about this aesthetic aspect of my Habicht 7x42 small AFOV
At the opposite pole are binoculars with enormous AFOV, but which pay with a fact that you cannot naturally see the entire enormous field at a glance, because anyway the visual angle of attention is about a 60 degrees. What is larger than that is useful for peripheral vision. Precisely for this aspect, I am not attracted to the wide-field astronomy eyepieces of more than 82 degrees. Through these eyepieces you float in unlimited space, the fieldstop of the AFOV is no longer perceived as a clear and obvious delimitation, but is more guessed with peripheral vision. I have to move my head a little from the optical axis to see the edges of the AFOV. Others are crazy about this spatial non-enclosure effect, but I like to see the clear and well-defined fieldstop followed by a black background. It is a purely aesthetic perception! That is exactly why I like the Habicht 7x42 enormously, and the Victory SF 10x42 68 degrees AFOV seems to me to be at the maximum limit, for my pleasure and naturalness to look at the edges of the AFOV
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tenex
reality-based
Yet I've used 10x32s for 20+ years, two quite different designs now (BN,FL), and always felt they had a large sweet spot and liked that. There are good questions here but answers don't seem clear yet...
Do many binoculars today really suffer from significant edge softness? (Edit: for terrestrial viewing.) Those I've seen hardly do, although many from 50 years ago did. And in this case, what is the point of the vaunted flat field?
Does the size of the sweet spot depend more on how well corrected an eyepiece is, or the size of the user's pupil? Surely the former, if only to explain the previous observation?
The practical utility of edge sharpness seems to depend on the ability to look at/near the field edge without blackouts. Does this depend more on the design of a binocular (and if so what aspect), or the anatomy of an individual user?
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Alexis Powell
Natural history enthusiast
Looking at the edge without blackouts depends upon exit pupil size of bin, shape of the image behind the ocular (recall discussions of the Nikon SE), pupil dilation of user, and eye position. Most of the discussion on this seems to presume an ideal user looking toward the edge of the FOV from an eye position on axis. I certainly enjoy using bins that way, which is why I enjoy full-size bins over 2/3 size or smaller bins for their larger exit pupil, which facilitates off-axis viewing as the pupil moves around while everything else remains stationary. But that is not the only value of bins that have a field that is well-corrected to the edge. In the course of birding, in my experience, especially in brush and forest situations, there are many times that I find myself crouching, stooped over, or bending awkwardly this way or that to see birds flitting in/out of view, in which cases I often end up holding my bin in nonstandard ways, sometimes one-handed, such that they are only sloppily aligned to my eyes. In such cases, a bin with good edge performance is nice to have because you will be able to see the bird sharply just as long as you can see it through the bins and the bins are focused, regardless of whether your eye is centered or not and regardless of whether you are taking in the whole FOV at once or not. I call it looking through the binocular sideways and being able to do it is something that I value in a good field instrument.
--AP
Ignatius
Texpat
Let me assure you, I too do not move the windows in my house to look at other parts of the scenery outside (my eyes, head and binos do that for me). But then again comparing houses' windows to binoculars makes comparing apples to oranges seem decidedly sane. But ymmv.
However, I do agree with some other observations you have made about behaviour.
That in a nutshell is why my brother chose his 8.5x42 Swaro - ease of view. He's said on a number of occasions that he also finds a flat field binocular less fatiguing. I've used the same binocular on numerous occasions and agree its ease of view is exceptional, as well as more generally that flat field binoculars do appear to be less fatiguing if you have to scan with them for long periods of time. That said, I think my eyes may be more forgiving than his, as I can quite happily use binoculars that he's uncomfortable with (eg. 4mm exit pupil - though I agree 5mm exit pupil is more comfortable in use) and am happy to use binoculars that don't have a flat field if I think I need their magnification (eg. I sometimes prefer my Nobilem 12x50B to my Nikon 10x42 SE).
I wonder whether it's what one gets accustomed to in the early days - I mainly used old porros until I got a 10x40 Dialyt, whereas my brother used an 8x30 SLC but quite quickly upgraded to the 8.5x42.
In most of my own viewing I have the luxury of being able to get my binoculars lined up perfectly on axis, and with a good grip; often with elbows propped up, to boot. But my experience of less optimal situations very much accords with what you noted here (NB. These sorts of observations underline the difference between those who put binoculars to use in the field, and those who merely own them):
I don't need the FOV to be sharp right to the edge; a large sweet spot will do (funnily enough, when I use the 7x42 Dialyt I find it's sweet spot perfectly acceptable). But, without a doubt, I agree that in situations like this good edge performance really helps. I think edge performance, or perhaps, perceived edge performance, also has a lot to do with how much accommodation your eyes have. When going from the 10x42 SE to the old 8x30 Binuxit I normally grumble a bit at the latter's visibly poorer edge performance (although the nearly 50% wider field of view is very welcome). But after 30 minutes or so I can get along with it. I'd still like it to be better in that respect though!
This is a great thread!
So my question is, which makes/models of binocular have good edge sharpness, decent eye relief for eyeglasses, and minimal flare/glare and blackouts?! The EL?
For many years, I didn't care much about edge sharpness, but I value it now. There are times where I dart my eyes, like Alexis mentioned. Or times where I don't have my eyes centered well for a quick look.
There are also times that I use binoculars on a tripod. I like to let my eyes wander around the field.
I live in an urban/suburban area but we have many greenspaces mixed in. During my walks, I can observe several wetlands with small creeks from the sidewalk, along busy roads.
Typically, I rest my elbows on the fencing along the sidewalk and look upstream or downstream in these wetlands. So I look side to side, plus near and quite far, but I keep the binocular stationary. I don't scan by moving the binocular, as I focus more on detecting movement which requires the view to be stationary. I just move my eyes around.
When I had a Kowa 8x33, I would get blackouts if I darted my eyes. I had an 8x42 SLC for a few years, which I really enjoyed, but the edges were not sharp/flat and I started noticing bad flare/glare unless I positioned it correctly.
Maybe I need to reconsider the EL. I was never impressed with them but maybe I need some more time with one.
So my question is, which makes/models of binocular have good edge sharpness, decent eye relief for eyeglasses, and minimal flare/glare and blackouts?! The EL?
For many years, I didn't care much about edge sharpness, but I value it now. There are times where I dart my eyes, like Alexis mentioned. Or times where I don't have my eyes centered well for a quick look.
There are also times that I use binoculars on a tripod. I like to let my eyes wander around the field.
I live in an urban/suburban area but we have many greenspaces mixed in. During my walks, I can observe several wetlands with small creeks from the sidewalk, along busy roads.
Typically, I rest my elbows on the fencing along the sidewalk and look upstream or downstream in these wetlands. So I look side to side, plus near and quite far, but I keep the binocular stationary. I don't scan by moving the binocular, as I focus more on detecting movement which requires the view to be stationary. I just move my eyes around.
When I had a Kowa 8x33, I would get blackouts if I darted my eyes. I had an 8x42 SLC for a few years, which I really enjoyed, but the edges were not sharp/flat and I started noticing bad flare/glare unless I positioned it correctly.
Maybe I need to reconsider the EL. I was never impressed with them but maybe I need some more time with one.
