Wide flat field and edge sharpness an illusion? (1 Viewer)

[Tringa45]

These attributes combined with adequate eye relief for glasses wearers are an outstanding (and recent) optical achievement, but for hand-held binoculars they are of no practical value.
The fovea of the human eye, where highest visual acuity for 20/20 vision is achieved only has a visual field of 1° and outside this, visual acuity degrades more rapidly than the off-axis resolution of a binocular.
If we wanted to view the field edge of a typical binocular with 60° AFoV we would have to redirect our eyes by about 29°. However, the eye's pupil is at about 11 mm radius from the centre of the eyeball, so a 29° redirection is accompanied by rotation and about 5 mm lateral movement, sufficient to result in a complete blackout in most binoculars!
Of course we just don't do this and simply redirect the binocular to place an object of interest in the field centre.
Should there be any doubts about this, I suggest placing a binocular on a tripod and viewing a detailed plane surface (or bookshelf). To see detail in the left field it is necessary to move one's head to the right and vice versa. This technique is often used by amateur astronomers, whose telescopes are always tripod mounted and where eyepieces with up to 110° AFoV are sometimes used.
It is often asserted that for daylight viewing one does not need more than 2,5-3 mm exit pupils but they result in a loss of viewing comfort.
The complaints of veiling glare in some modern designs can perhaps be attributed to their complexity and the "poor" edge performance of Ultravids, SLCs and FLs, particularly those of lower magnification and large exit pupils is in practice more advantage than detriment.

John

 

[Eric_D]

There have been many discussions on this.

I have considered asking whether a binocular with the highest edge-sharpness is necessarily the best (for hand-held use)?
There is a distinction between resolution and sharpness. Perhaps a high contrast is more important than resolution towards the edge of the FOV.

Photographers use lens MTF (modulation transfer function) or OTF charts (contrast vs. lines/mm on the sensor), since contrast near the maximum spatial frequency is more important than contrast at the maximum spatial frequency ('microcontrast').

Given the eye has a narrow cone of high resolution, it could be argued that contrast is more important than resolution outside that cone. 'Is it sharp if you don't look at it?' is like a Zen 'koan' about trees falling when no-one is listening.

I don't know if anyone has done visual field tests through binoculars. Perhaps they should use a rotatable target, rather than a flashing light. The apparent size of target will be important. Maybe videos of two distinctive birds in flight? (Pigeon/falcon could be tricky?)

You will get different answers on birding and astronomy forums.

There are model-specific threads

• Nikon Monarch 7 8x30 edge sharpness/focus question
• Leitz Ultravid HD edge sharpness
  Astro view
• The End All Zeiss Victory FL Edge Sharpness Thread

and

• What make and type of binoculars are sharpest at the edge?

The related question 'Is curvature of field a good thing?' is now a different thread ...

 

[Binastro]

The best test of edge sharpness is to use a fixed mirror mount and let the stars drift over the field.

You then see how awful edge performance is in many binoculars that are perfectly O.K. in normal use.

Regards,
B.

 

[Maljunulo]

The best test of edge sharpness is to use a fixed mirror mount and let the stars drift over the field.

You then see how awful edge performance is in many binoculars that are perfectly O.K. in normal use.

Regards,
B.

Interesting.

I never would have thought of it.

 

[hopster]

These attributes combined with adequate eye relief for glasses wearers are an outstanding (and recent) optical achievement, but for hand-held binoculars they are of no practical value.
The fovea of the human eye, where highest visual acuity for 20/20 vision is achieved only has a visual field of 1° and outside this, visual acuity degrades more rapidly than the off-axis resolution of a binocular.
If we wanted to view the field edge of a typical binocular with 60° AFoV we would have to redirect our eyes by about 29°. However, the eye's pupil is at about 11 mm radius from the centre of the eyeball, so a 29° redirection is accompanied by rotation and about 5 mm lateral movement, sufficient to result in a complete blackout in most binoculars!
Of course we just don't do this and simply redirect the binocular to place an object of interest in the field centre.
Should there be any doubts about this, I suggest placing a binocular on a tripod and viewing a detailed plane surface (or bookshelf). To see detail in the left field it is necessary to move one's head to the right and vice versa. This technique is often used by amateur astronomers, whose telescopes are always tripod mounted and where eyepieces with up to 110° AFoV are sometimes used.
It is often asserted that for daylight viewing one does not need more than 2,5-3 mm exit pupils but they result in a loss of viewing comfort.
The complaints of veiling glare in some modern designs can perhaps be attributed to their complexity and the "poor" edge performance of Ultravids, SLCs and FLs, particularly those of lower magnification and large exit pupils is in practice more advantage than detriment.

John

Interesting thoughts and to a good extent I agree. I have never found the wide & flat field designs particularly persuasive, and I think that they may introduce other issues in search of this goal (e.g. glare, Absam ring, rolling ball, other?).

I can imagine that if you are using high-magnification on a tripod (for example 12x50 or 15x56) for astronomy or in a hide looking at a fixed position there could be a stronger argument for them than when they are being used hand held.

I have noticed that the lateral CA effects sometimes present on high-contrast edges at the edge of the field in the Leica Noctivid disappear entirely if one moves ones eye slightly in the opposite direction relative to the eyepiece. This is easier to do precisely if the binocular is supported..

M

 

[Eric_D]

Absam ring

See this thread if this term is new to you:

Every binocular I have used with a field flattener has not been truly sharp throughout the FOV - most have areas just outside of the sweetspot [maybe 70% out] that go soft, then sharpen again towards the edge. Brock called this the ''Absam Ring.''

I find this distracting, as the zone of unsharpness is kind of ''hanging'' out, surrounded by sharpness on either side, which makes it more noticeable.

 

[tenex]

If we wanted to view the field edge of a typical binocular with 60° AFoV we would have to redirect our eyes by about 29°. However, the eye's pupil is at about 11 mm radius from the centre of the eyeball, so a 29° redirection is accompanied by rotation and about 5 mm lateral movement, sufficient to result in a complete blackout in most binoculars!

This has certainly been my experience, but there seems to be a common claim here that ELs and NLs (which I don't know well) are actually designed to allow one to do this, at the cost of insufficient baffling against glare -- i.e. assuming that aggressive baffling is what generally restricts the view of the outer field.

On the other hand, I do like to have a wider field for peripheral vision, and am sorry that the full-flat thing took over first. I'd love to add another 5° AFOV to some bins without worrying about edge sharpness or complicating ease of view.

 

[Patudo]

Speaking from my own personal experience only, I definitely notice when going from a flat field binocular to one without, how unsharp the edges of the latter are in comparison. My eye accommodation can adjust to this fairly effectively (as of right now anyway ... this might not be the case in 10 or 20 years) and I'm quite happy to use binoculars that don't have a flat field, but I wouldn't say that poor edge performance is an advantage. Especially with binoculars that have a narrower field of view - the tighter the FOV, the more important it is that the sweet spot be as large as possible. That's my preference anyway.

 

[jring]

Hi,

the Absam ring in the form of a less crisp ring at ca. 70% of the field of view is obviously only visible in some instruments originating from Absam, Tyrolia in Austria...

Other field flatteners just tend to get to get a bit soft right at the edge of the fov... bragging rights are how close to the edge is sharp and whether that softness can be fixed by refocusing (i.e. it's due to the inevitable field curvature) or not (because it is due to real aberrations - usually astigmatism).

Rolling ball or the illusion of barrel distortion when panning is normal for any kind of eyepiece unless the designer has chosen to counteract it with some pincushion distortion.

There is another kind of distortion called angular distortion which has to be balanced against rectilinear (barrel or pincshion) distortion in eyepiece design and the sum of both types will get higher for wider angle EPs (regardless of field flattening). Since most sports optics manufacturers want a specific amount of pincushion to combat rolling ball effect, the angular distortion of wide angle binoculars can get a bit high for some astro users...

Also there is the question whether the field stop is in focus or not... some like it when it is, some don't care. But for field flattening purposes we don't care whether the field stop is in focus, just whether the image of the object is crisp...

Joachim

 

[tenex]

Speaking from my own personal experience only, I definitely notice when going from a flat field binocular to one without, how unsharp the edges of the latter are in comparison.

The question asked here then is whether you're physically able to do anything with those sharp edges (distinguish subtleties of plumage at the field edge?) or this is just an aesthetic preference, where taste seems to vary:
(1) don't care about edges, that's not where I look
(2) don't mind modest softness near the edge, but smeary astigmatism distracts my attention (that's me)
(3) sharp edges are obviously superior, why not insist on them

 

[Maljunulo]

I like sharp edges.

It is disconcerting to me to have the edges of the field out of focus.

 

[Patudo]

@tenex - I don't think the OP posed the question in your post #10 as such, but made a couple of statements, one of which (the one you quoted in your post #7) I cannot say agrees with my own experience, and the second debatable to say the least (the "poor" edge performance of Ultravids, SLCs and FLs, particularly those of lower magnification and large exit pupils is in practice more advantage than detriment). I'm able to look around (mainly moving my eyes, much like one would read a book, rather than my entire head) the field of view of the ELs I've tried, and even wide field binoculars like the WX 10x50, without suffering blackouts. In my own experience, and I gladly accept others may differ, blackouts aren't a problem if the binocular is at the right distance from my eyes.

As for poor edge performance being some kind of advantage, I would (with the greatest of respect) have to disagree. We all agree that a sharp image is better than an unsharp one, and most of us would rather have a large sweet spot (more of the image being sharp) than a smaller one. Given this, I cannot see how poor edge performance could possibly be considered to be an advantage. Unless the argument is that the steps taken to improve edge performance compromise other aspects in some way (glare etc) - but do we know for sure that they do? Some flat field binoculars like the Nikon EDG are considered to perform well against glare.

With regard to your own question (whether you're physically able to do anything with those sharp edges (distinguish subtleties of plumage at the field edge?) or this is just an aesthetic preference) - I think it's both. The sharper the image is across the field (the larger the sweet spot) the better your situational awareness is. I believe it reduces fatigue to some extent, as your eye doesn't need to accommodate as much (or possibly not at all) when checking something away from the centre. And I like seeing a sharp image more than an unsharp one - and the more of the image is sharp (towards the edge as well as in the centre), the better. To my eyes, anyway.

 

[Maljunulo]

If you take a photo through the binocular, and it shows soft edges, I don’t see how you can call it an illusion.

Of course you need to photograph something flat (at a 90° angle) to eliminate the camera.

 

[Tringa45]

@Eric_D: Resolution of binoculars and scopes is measured on a strongly illuminated test chart (e.g. USAF 1951) and is the ability to separate line pairs at the highest spatial frequency. Its theoretical limit is defined by the objective aperture. An instrument with superior resolution would also show better contrast at lower spatial frequencies.

John

 

[Tringa45]

If a detailed view of an object outside centre field is required then one either has to redirect the instrument or move one's head.
The latter option would require unnecessary contortions if the binocular were hand-held!
AFAIK no binocular prior to the introduction of the Swarovski EL Swarovision attained such a high level of edge correction. It has since become a marketing feature.
I have a 10x42 SV and, at an inland stretch of water not immediately accessible, have set it up on a tripod next to my scope. Moving my head from side to side I have an almost equally sharp view across the full 6,4° FoV and, as Hopster mentioned, can also reduce lateral CA. It is aso very good on a tripod on the night sky, but hand-held I perceive no advantage.
I did not mean to imply that binoculars with a less well corrected field edge were per se better, merely that the additional optical complexity of edge correction brought disadvantages.
In a subjective comparison of veiling glare my EL was marginally the worst of my binoculars and it seems that the EL and NL are the two that receive most criticism in this respect.

John

 

[tenex]

I'm able to look around (mainly moving my eyes, much like one would read a book, rather than my entire head) the field of view of the ELs I've tried, and even wide field binoculars like the WX 10x50, without suffering blackouts. In my own experience, and I gladly accept others may differ, blackouts aren't a problem if the binocular is at the right distance from my eyes.

I wonder how this difference is to be explained... can there be some degree of anatomical variation involved? I don't think I've ever been able to look directly at the field edge in any binocular (certainly no recent ones) without decentering my eyes, as I would never normally do in the field.

In a subjective comparison of veiling glare my EL was marginally the worst of my binoculars and it seems that the EL and NL are the two that receive most criticism in this respect.

Is it clear how the optical design of EL/NL eyepieces would increase susceptibility to glare?

 

[William Lewis]

I don't tend to get blackouts when I look at the field edge but I only tend to look right to the edge of the field so see if it's flat, check the distortion profile or compare the field of view between 2 optics - it doesn't happen very often! When I do look at the field edge it's a conscious effort so I assume I must be moving the binoculars slightly. Not to do with birdwatching but I definitely do have to decentre my eye to see the field edge clearly in my 68° and 82° eye pieces.

Having a flat field binocular to hand hasn't made any difference in the amount of birds I've seen though, I'm pretty sure of that.

I like a field of view around 65° apparent, any more is wasted on me, I think I only have a 65° attention span. I'm happy to work with 50° and like to be able to see the field stop when looking straight ahead.

Having a wider field of view I think does make a difference to the amount of birds I see, it's a little like getting to a lake or estuary and going straight to the scope without scanning with the bins first, once you go to them you often find things you'd missed with the scope or are gliding by silently out of the scopes field of view. The same thing happens every time you put your binoculars up to your eyes so a little more width does help - whenever I can take my e2's birding I do!

 

[Tringa45]

If you take a photo through the binocular, and it shows soft edges, I don’t see how you can call it an illusion.

A good 50 mm standard lens achieves acceptable sharpness across its 46° FoV. The fovea is limited to about 1°.

John

 

[Tringa45]

Is it clear how the optical design of EL/NL eyepieces would increase susceptibility to glare?

I can only speculate that it might be related to the number of air/glass surfaces (field flatteners etc.).
Among amateur astronomers who observe planets with low variations in surface contrast, there seems to be a preference for simple eyepiece designs such as Plössls or orthoscopics. And they are not confronted with strong light sources outside the FoV as are birders.

John

 

[Maljunulo]

A good 50 mm standard lens achieves acceptable sharpness across its 46° FoV. The fovea is limited to about 1°.

John

Therefore?