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"Edge Performance"

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#1 KennyJ


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Posted 07 January 2004 - 05:23 PM

Since I became a "serial follower" of binocular reviews and articles I have probably been as guilty as anyone of consciously or subconsciously "copying" descriptive methods I have seen and / or admired elsewhere when attempting to write a binocular review.

I am sure neither when nor by whom the custom of estimating relative on and off -axis properties such as resolution , "curviture" , "softness" et al in terms of direct percentage figures was first introduced , but the practice seems to me to have become far more commonplace than it used to be prior to the the period ,about four years ago from this time of writing, when Todd Gross first published his oft -quoted but rarely updated " Astro Binocular review page " on the WWW.

Whether completely accurate or otherwise , Todd's particular style of concise summarising in easy - to -follow format had me amongst it's very first admirers ,and I was very quick to tell him so.

Todd is after all an extremely knowledgeable person in the field of optics and astronomy --and very modest with it.

When reading the hundreds of binocular reviews that I have since , I sometimes wonder to myself if by some strange quirk of fate , it has somehow come about that every pair of binoculars ever made, apart from those I happened to have looked through ,have been made with some kind of built -in concentric rings not dissimilar to crosshairs found in rifle and finder scopes , clearly marked in 5 per -cent increments , by which to inform the user of the relative radii of vision through which he or she is looking.

So many reviewers suddenly seem quite at home quoting figures of anything between 60% to 95% to describe precisely in their view at which point in the circle of vision anything varying from "slight softening" to unbearable "curviture" "distortion" or any other undesireable property , and all stations in between ,begins to rear it's ugly face, without so much as clearly explaining whether they are talking about percentages of radius , diameter or even AREA.

I suspect these quoted "percentage figures" are open to much misinterpretation , possibly by as many as the majority of the readership.

Let me use a couple of examples , first of how such a figure could conceivable be vastly UNDERATED ,and secondly, and in my humble opinion , more importantly , how such a figure can be taken as being of far more significant consequence that it ever need be , or indeed actually is.

Let's take my Swift Audubon Kestrel 10 x 50 with it's 7 degree TFOV ( 70 degree AFOV )

Although I use unorthodox units , the actual AREA of Field of View wth this particular binocular can quite justifiably be expressed as being of 38.5 square degrees.

Why ? --How ?

Area of a circle is calculated by Pi x radius SQUARED.

In this case , 3.142 (pi) x 3.5 ( radius in degrees ) x 3.5
3.142 x 12. 25 = 38.5 "square degrees"

If for example , a typical "flat -field fanatic" decided that he or she considered "unnaceptable degradation" to commence at "from 80 per cent out" -- then presuming the reviewer is speaking in terms of "diameter" then 20 per cent of the instrument designer's TFOV is considered to be "surplus to requirements"

In this case , the TFOV being 7 degrees , then 1.4 degrees would fall into that "wasted space" category.

This would leave a "useable" TFOV of 5.6 degrees.

If we apply our old friend Pi R squared to our newly arrived upon "diameter" then the actual AREA of useable TFOV has been quite dramatically reduced from 38.5 square degrees to 24.6 square degrees !

Now 24.6 is only 64 per cent of 38.5

We have "lost" a whopping 36 per cent of our "useable AREA"!

This sounds pretty serious !

If I go out of my way , deliberately taking my eyes OFF the subject I am looking at -- to look for this --I will notice it -- in fact I can then adjust the centre focus of the binocular to compensate for it --making the object in the very edge of field of view become clearly focussed -- at the expense of the centred object being out of focus-- but even when I do THAT -- I no longer NEED to look at the centred object.

Now --take the two largest books you have in your possession and place them side by side - about six feet from your naked eyes.

Focus your eyes on any ONE of the front covers.

Can you see the book at it's side ? -- Yes ! --you can SEE it's there -- but when you are looking at one book --can you read the title of the book by it's side ? -- NO !

Does the presence of the book out of focus in any way interfere with your sharply focussed vision of the book in focus ? -- NO !

Think about how close these books are together --and how tiny a proportion of your ENTIRE field of view they actually occupy -- depending on their size --possibly as small a "percentage" as FIVE PER CENT -- the book you are actually focussing on may only occupy TWO PER CENT !

Does this matter ? -- NO !

Now find yourself the narrowest length of tubing through which you can still see just one of the books -- you'll probably find the inside of standard biro pen will suffice !

O.K you can still see the book as clearly as you could with both eyes wide open and no obstruction -- but be honest -- which situation of the two would YOU prefer to be the "norm"?

Human eyes were designed to look at objects within a far wider TFOV than is actually necessary -- and there are reasons for that.

Why this feature of human visual apparatus should present a "problem" when looking binoculars is because people who I call "flat field fetishists" would seem to think that an artificially "two -dimensional" image of a desired object seen without it's perfectly natural "curved" or "distorted" surroundings is somehow a more noble way of looking at things.

"Flat field technology" has become something of a "buzz word", or more precisely , group of words.

If the designer of the superb Zeiss classic 7 x 42 binocular for example , had so wished , he could have made it as having as flat a field of view as any other 7x binocular ever made -- except with probably superior brightness and resolution than his competitors.

All he needed to have done was use eye -pieces with FOVs as ridiculously narrow as those utilised in many Pentax binoculars.

But he didn't.

And I for one am delighted that he didn't.

Clear skies -- Kenny.

#2 brocknroller


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Posted 07 January 2004 - 11:15 PM


Great post! This topic is worthy of it's own thread so I hope it doesn't get buried here. I know exactly what you mean about the edge performance percentages, even though I never figured out the math. For example, Edz estimated the clear field on the Minolta 7X35 WP FP to be about 70%, another Minolta owner judged them to be 80+%. I'd trust Ed's figure more, because from reading his articles and posts, it's obvious that he does meticulous observations and measurements, and like yourself, knows how to calculate these figures to lend objectivity to his observations (as much as that's possible, given the subjective nature of esthetics, which play a large role in how "pleasing" an image looks to any given individual).

The difference in his measurement and the other Minolta owners measurement may be due to evaluating edge performance day and night. The objects you look at during the day are generally pretty large, even a hummingbird at your feeder is a giant compared to a star. So you might move the entire bird to the edge before declaring it "out of focus" at 80% while a star just passing into the zone where the views start to defocus is noticeable at 70%. I remember Barry Simon's review of the new Audubon on Bino-net (also posted on other groups). During the day he thought the edges looked very good, but at night, he found them not as good as he thought.

Given the scarcity of clear nights in Central PA (when it's above 20*F), I've taken to selecting bins (for the most part) that can do double duty for stargazing or nature study, so they don't wind up in the closet along with the APO refractor, 15X70s (out of collimation anyway), and skywindow.

Because of this, I like bins that can be handheld, have a wide FOV, and *decent" edges for astronomy. The 6X30 FMTR-SX was very sharp almost to the edge of the wide 8.3* FOV, and was a great finder bin for my telescope and also great for scanning the Milky Way. But useless for birding, because of the IF EPs and not so close focus, so it sat in the closet for weeks at a time during cold or inclement weather. After much deliberation, I sold it to a boater who's making better use of it.

I like good edges for birding or astronomy (no less than 66% clear field from center on my very subjective scale) because bins that "fuzz out" less than 2/3 from the center bother my eyes. I got eyestrain from my Celestron 8X32 Ultima and 9.5X44 ED, both of which had fuzzy edges. They appeared to be in collimation, but somehow, perhaps my eyes' FOV exceeded the clear FOV of the bins?, I experienced eyestrain after using them for a while. Same with my 9X63 roofs with poor edges. I simply cannot use bins with poor edges due to this "peculiarity" with my eyes (I'm also very sensitive to miscollimation and when one EP is not perfectly focused with the other).

I know people who do, in fact, set the right diopter just once and use the bin for both close and distance observing. Me, I got to fiddle, and the shallower the depth of field, the more I have to fiddle with the diopter to make both eyes match. These (over)sensitivities have make me more aware of many of the critical issues we talk about on this site, for better or for worse.


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Posted 07 January 2004 - 11:34 PM

:bigshock: WOW!... What a post Kenny!

The days of "Poor", "Good", and "Great" view would certainly have been much easier for my decision making process...

This has, in part, been the reason for my most recent shift in my "pursuit of binocular knowledge" :question:

I've all but given up on specifics for image sharpness, % of FOV good for astronomy, etc and am now seeking feedback on "view" instead...

How does the "view" in the Swift look? I know there is loss of quality due to the wide field, but I also now know that your eye (and brain) doesn't focus on the whole image. You focus on the centre field and your eye simply takes note of the periphery vision to let the brain know that there is something there (a point of light, movement of a bird, etc). If I focus my eye on the central view of a wider angle binocular I see the whole FOV, but don't really notice the distortion that much unless I shift my view away from the centre and "look" for it...

If you make a wide field bino (especially with an extra wide AFOV like the Swift) the view is similar to the naked eye (vast surround of visual image, with a smaller central field that takes your focus at any point in time). So the Swift performs this role, and as such any distortion on the periphery of your vision is largely unnoticed while focusing on the central view provided by the binocular... In Theory (since I've never looked through a pair of the Swift binos :smirk:).

Maybe this is why flat field in say the Pentax binos is so important... By its design, the Pentax has a narrow FOV and subsequently a narrow AFOV. This brings the image you see into a smaller and narrower context visually, and as such your eye is capable of taking more of this image into "view", thus making it more critical that the image is sharper across a wider field than would be necessary with a wider angle view...

Am I on the right track?

I hope that all made sense... :confused:

It would be interesting to know what the FOV of the eye actually is... Not the entire FOV, but the "sharp" or "usable" field that we actually visually use or consider to be in focus. As you outlined Kenny, if you keep your visual focus on an object but take note of the visual surroundings around this object without moving your focus, the surround is not clear, and for that matter even loses colour toward the limits of your visual field. Apply this directly to a wide binocular field, and at what point (FOV) does the eye no longer notice distortion or loss of image quality without actually looking at it? Hmmmm... I wonder :question:

#4 EdZ


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Posted 08 January 2004 - 06:25 AM


Great post! This topic is worthy of it's own thread, Brock

I coudn't agree more! Excellent post Kenny.

I believe it is generally accepted that human vision can see clearly out to a field of view of about 60°.


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Posted 08 January 2004 - 11:32 AM


Sounds good in print. You can have a 6* FOV in a bino, but if your distortion factor out from the center is 65%-70%, as an example limiting your ACTUAL USABLE FOV to 3.9* to 4.2*, whats the advantage?

I would rather have a flat, distortion free FOV, as those found in the Pentax as an example, albeit satisfactory for viewing many, many objects in the heavens. I agree with Ed Z., we should have a thread on Usable FOV's.

Ray A.

#6 brocknroller


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Posted 08 January 2004 - 12:21 PM

Thanks, Ed, this may explain the eyestrain I experience with poor edge bins, which blurr inside my field of vision. The Ultimas began to blur at about 50% out, the EDs were a bit better. This may also explain, at least in part, why the views through my 8X32 SE seem so natural on the sky (AFOV 60*). The views are not altogether "natural" for terrestrial use as I explained in another post (field curvature tends to give terrestrial objects viewed on-axis a more 3-D look while flatter field bins make them look 2-D).

This also begs the question if 60* is the limiting view of the human eye, do we need bins with larger clear fields? For handheld use, one can always move the binocular to see what's fuzzy in the outer edges beyond 60*. Since I tend to experience black outs with wide field bins when I look at the field edge, I tend to do this anyway. With mounted bins you'd have to move the whole assembly, but you can simply refocus the edges as Kenny suggested (if the blurriness is caused by field curvature, some bins also have astigmatism, which can't be refocused).

How many DSOs span more than 60* in the sky? Not sure about the Veil Nebula, but you can't see that in most bins anyway, and even those that can would need a filter. So I wonder then, why amateur astronomers would want bins with larger clear FOVs than 60*? Personally, I'd rather have a wide FOV bin like the Audubon, with exceptional on-axis resoution but 20-25% soft on the outer edges so I can find objects more easily, then a narrow FOV bin like the 20X60 PCF V where I could not even find Jupiter without looking over the top of the bins to line it up. Obviously, I'm comparing apples and oranges because of the differences in powers, but the general idea applies. Others would rather have a smaller FOV sharp to the edge like the Pentaxes.

So it's a matter of personal preference, esthetics really, that determines which is more "pleasing" to the eye. What's interesting is that the abstract concept of esthetics, which is usually associated with the "mind", may actually be determined by the limits and pecularities of one's eyes, as I think is true in my case.

#7 EdZ


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Posted 08 January 2004 - 12:40 PM

Of all the binoculars I own, the ones that "FILL" my eye's field of view comfortably are

Fujinon 16x70 - 4.0°Tfov/64°Afov
Oberwerk 15x70 - 4.3°Tfov/65°Afov
Pentax 12x50 - 4.15°Tfov/50°Afov

One other binocular I own does not quite fit into the "fills my eye" category. The field is so small it only shows one half or less the area of the sky as the others above.

Pentax 16x60 - 2.8°Tfov/45°Afov

The field of view in all these binoculars is clearly sharp (% center to edge) right out to the edge or almost to the edge. The entire view can be taken in by the eye at once.

Of the other binoculars I own in the 6° Tfov to 9° Tfov range, my eyes cannot take in the entire Tfov all at once. Some do have similarly nice sharp fields. Some others do not. For instance the 8x42 Swift Ultralite has a 6.5° field of view, also sharp out to near the edge. My 10x50 Orion Ultraview has a 6.0° field of view but clear sharp view falls off about 60% out from center.

Based on testing recorded in my last Limiting Magnitude effort, the 10x50 Orions by 70% to 80% out lost 1 full magnitude. The other five binoculars mentioned above lost at most only about 0.5 magnitude by 80% out from center.

Drop off in sharpness in not just making objects a little fuzzy towards the edges. In every case, objects are lost from view and in some cases, as in the Orion 10x50, more objects are lost from view and they are lost even closer in towards the center.

I no longer find the need for wide field binocs for finding objects. I still use my wide field binocs. But I prefer the image matches the field of view of my eyes. The narrow 2.8° field of the Pentax 16x60 does not fall into that comfort zone.

I usually have a pair of hand held wide field binocs on the table with me whenever I have a scope out. But also, I usually have a mounted pair of 15x or 16x binoculars with me whenever I am at the scope.

Because my preference seems to be a binocular in the 4°Tfov range, critical sharpness increases in importance for me. Although there are some brands that exceed normal performance, it is common for lower power wide field binoculars to have edge sharpness fall off. Below 10x, they are what I would call scanning binoculars. In these, image scale is so small that it usually doesn't make that much difference as these lower power models are not used for critical observations.


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Posted 09 January 2004 - 12:10 PM

Hello Ed,

Great insight from you and all the gang! We all learn a great deal from you and eachother. If everyone had the same opinions, then there wouldn't be a need for Cloudy Nights!

What is you opinion regarding the Minolta Activa 12x50Wp.Fp?
They have a 5.50* angle of view at 66*. They are on my shopping list also. I highly value your opinions.
Thank You,
Ray A.

#9 EdZ


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Posted 09 January 2004 - 01:28 PM

Hi Ray,

Thank you.

I have not used that one, but

I have 7x35 Minolta Activas. Mine are water resistant, not waterproof. The mechanical functions are excellent. The optical quality is very good.

5.5° fov in a 12x50 sounds nice. They may even be hand holdable for short periods. For any type of use except hiking, any binoc will benefit from stable mounting.


#10 jmoore


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Posted 09 January 2004 - 06:54 PM

I'm relatively new to astronomy, but have been using binos and spotting scopes for years in terrestrial settings (I'm a birder).

There are 2 common occasions when edge-to-edge sharpness is really nice. First, when panning across the sky (or landscape). If the edges are blurry or fish-bowl distorted, it's less comfortable and more distracting to scan, harder to pick out an object you're looking for, and harder to enjoy the moving view. Second, I commonly look at objects at the edge of my field. I might center a particular object, but then I'll scan around that view by moving my eye rather than my scope. Maybe there are 2 objects to look at...both at opposite edges of the view...just barely fitting into the field. You want them both there, and so you look at them while they're near the edges.

I think these are 2 important advantages of having a sharp flat field from edge to edge. And in my experience, I'd rather have a slightly narrower FOV with clarity and flatness throughout, than a wide FOV with a significant portion of it considered unuseable.

Just a matter of preference, I think....


#11 KennyJ


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Posted 09 January 2004 - 07:52 PM

"Edge performance" is in itself a rather inadequate title to cover what I consider to be an attempt to categorise what could encompass a whole range of possibly quite different and unrelated properties of an optical system.

This is why there was a deliberate ( although apparantly not universally acknowledged )irony in my selection of this topic heading in the first place.

There is a world of difference between a poorly and cheaply construction binocular assembled in certain parts of the world ( which I will refrain from specifying out of respect for political correctness )that suffers from a multitude of
optical inadequacies that include more than one aberration that could be classified as belonging to "poor edge performance" categorisation , -- and the subtle differences between a binocular made in highly developed optical communities that can only qualify as being of "highly desireable quality" out of consideration for it's relative "flatness of field" alone , when compared with binoculars of an even higher standard of design , componentry , glass quality and coating technology that need not be deliberately "cut -down" so as to be restricted to having a minimal field of view simply to enable the product to be deemed "worthy of serious consideration" in any true binocular admirer's list of "top drawer" instruments.

To put it in a VERY politically INCORRECT way -- I probably have more reason than most members of this cyber community to harbour sympathy for anti-nazi protestors past and present , but that in no way harnesses my opinion that in spite of relatively modern developments and no little political propaganda , the central Europeans continue to rule the roost when it comes to optics.

How many Leica, Swarovski and Zeiss binocular designers have
had to resort to "flat -field tactics" to produce a world -class binocular ?

I struggle to think of a single example of ANY of their fine products having a AFOV of less than 50 degrees !

Perhaps it is therefore hardly surprising that in
"binocular astronomy circles" such manufacturers would seem to receive disproportionately scant praise for their products in a world within a world where "flatness is god" almost to the exclusion of all the many other properties that contribute to making a binocular enjoyable to use.

I can only presume that most , if not all the criticisms I read about the "poor edge performances" of so called wide -field ( over 5O degree AFOV ? )binoculars come not from people who actually own and use Leica , Swarovski or Zeiss binoculars !

Clear skies -- Kenny.

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Posted 10 January 2004 - 06:22 PM

Have not had a chance to chat in quite a while. Your contribution (paper) on edge performance was "spot on". Thank you and nice work.


#13 edcannon


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Posted 14 January 2004 - 11:56 PM

My new 8x42 and my old 10x50 both have apparent field of view of about 65 degrees, TFOV of 8.2° and 6.5° respectively.

To the extent that I've been able to test so far (only a couple of okay evenings since I got the new ones, presumably due to bad cloud karma) I see more "edge softness" in the 8x42 and am wondering if their slightly larger exit pupils might have anything to do with it -- getting some additional distortion from the outside edges of my eye pupils. The 10x50 cost more -- and that was four years ago (but I probably overpaid as I didn't know much at all then) -- though they appear to be BK-7 rather than BaK-4 prisms.

Can just the wider TFOV explain more edge softness in the 8x42? It's harder to test the difference in daylight, it seems -- no doubt due to my eye pupils being more contracted. I guess the bottom line question is whether wider TFOV is inherently blurrier around the edges, even if AFOV is the same.

Ed Cannon - ecannon@mail.utexas.edu - Austin, Texas, USA

#14 KennyJ


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Posted 15 January 2004 - 02:22 AM

Hi Ed,

I think the basic answer to your bottom line question is YES

Binoculars with a wider TFOV DO tend to show more optical defficiencies than ones with narrower TFOV , per se ,GIVEN other qualities being "equal".

It is a question that could be addressed in a very long and detailed way --but I am not the person to do it !

Not all of these aberrations are the same ,and certainly not as noticeable in a top of the range wide -field bino such as the Zeiss 7 x 42 --although even these great binos do suffer , and a wider field is certainly not the ONLY factor that can cause "poor edge performance"

Of those other factors that can , a larger exit pupil in itself is hardly likely to be one of them.

One does not read , for example , of troublesome "edge performance" with the Fujinon 10 x 70 FMT SX !

What a larger exit pupil WILL do however , is emphasise your OWN astigmatism.

Another seldom mentioned factor is that exit -pupil itself can suffer from spherrical abberation , which is the reason why one or two contributors recently have mentioned "black-outs" with Nikon Superior Es and E2s in daylight.

Regards --Kenny.

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