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FOV at 7-8x (again, still...)

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Old Wednesday 18th September 2019, 18:14   #1
tenex
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FOV at 7-8x (again, still...)

There's still something here I'm not quite understanding. I like and am accustomed to about a 68* AFOV in a good 10x bino (BN or UV 32).

If I want 68* in a quality 8x, there's only... the Zeiss SF, at $2500+. (Yes I know about the Nikon EII, but that's not really at the same level of performance.)

If I want 68* in a quality 7x, there's only... the Nikon WX at $6000+, and 5+ pounds!

As surprising as the first result seems, the second is even more so. Why is the technical challenge of providing FOV so great at these lower powers?
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Old Wednesday 18th September 2019, 18:43   #2
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There's still something here I'm not quite understanding. I like and am accustomed to about a 68* AFOV in a good 10x bino (BN or UV 32).

If I want 68* in a quality 8x, there's only... the Zeiss SF, at $2500+. (Yes I know about the Nikon EII, but that's not really at the same level of performance.)

If I want 68* in a quality 7x, there's only... the Nikon WX at $6000+, and 5+ pounds!

As surprising as the first result seems, the second is even more so. Why is the technical challenge of providing FOV so great at these lower powers?
It's consumer, not technology-driven.

BC
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Old Wednesday 18th September 2019, 18:51   #3
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If I want 68* in a quality 8x, there's only... the Zeiss SF,
Hello,

The Zeiss SF has "only" 64 AFOV and the SF 10x42 65 AFOV!

Andreas
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Old Wednesday 18th September 2019, 20:41   #4
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...Why is the technical challenge of providing FOV so great at these lower powers?
I think it is because for a low power bin to have a big AFOV, it has to capture and deliver to the eye a big TFOV, which is optically more challenging to do well (dealing, e.g. w/field curvature) and which requires bigger prisms etc. A high power bin need only magnify the small TFOV more at the eyepiece to achieve a big AFOV. The former can be done, but it results in a very bulky, or else a short eye relief bin.

--AP

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Old Thursday 19th September 2019, 00:09   #5
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Alexis and Bill are both right.

In telescope eyepieces as you go to lower power with a given telescope you use a longer focal length eyepiece. As the focal length of the eyepiece increases, eventually the apparent field of view (AFOV) is limited by the diameter of the eyepiece barrel or other field stop within the barrel. For instance with a 1.25" diameter barrel (popular in amateur telescopes) a Plossl has a 50 degree AFOV up until a focal length of around 32 mm, but beyond that the apparent field is reduced. For a Panoptic with 68 degree AFOV the max focal length in 1.25" barrel is 24mm, and for Nagler with 82 degree AFOV the max focal length is 16mm. Amateur astronomers use a wider barrel size (often 2") to get wide AFOV with longer focal lengths. These eyepieces tend to be larger, heavier, and more expensive, and you need a 2" focuser and perhaps 2" diagonal, etc..

Now back to binoculars. Say you have a 10x50 binocular, and maybe the F ratio of the objective is 4 (just to make it simple). Then your objective focal length is 200mm and your eyepiece focal length is 20. Sounds like you are still in the realm of a Panoptic (68 degrees) if your eyepiece field stop is at least 1.25". If you use the same objective to build a 7x binocular, now your eyepiece focal length needs to be more like 29mm and you will not be able to use a longer version of the same eyepiece, so maybe you go to a 50 degree AFOV eyepiece. Note that actual field of view in the example above is 6.8 degrees at 10x and 7.1 degrees at 7x, so you are not giving up actual field, but you do not get that wide AFOV experience. Of course as Bill suggested, given the money and/or willingness to change other design requirements such a thing could be made.

Alan
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Old Thursday 19th September 2019, 23:25   #6
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Yes, I suppose my question must involve consumer demand (or manufacturer perception of that) as well as technology. So, manufacturers take note: I demand a wider field in your 7-8x binoculars! Should I start a poll?

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The Zeiss SF has "only" 64 AFOV and the SF 10x42 65 AFOV!
How do you figure that? Working from a TFOV of 147.4m for the 8x SF, I get 67 or 67.5* AFOV using the simple approximation or the ISO formula. That's close enough to 68* for the purpose. (Or are other factors like field flatteners reducing the expected AFOV of the SF? In that case there is no quality 8x with a 68* AFOV at all.)
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Old Friday 20th September 2019, 00:19   #7
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Yes, I suppose my question must involve consumer demand (or manufacturer perception of that) as well as technology. So, manufacturers take note: I demand a wider field in your 7-8x binoculars! Should I start a poll?


How do you figure that? Working from a TFOV of 147.4m for the 8x SF, I get 67 or 67.5* AFOV using the simple approximation or the ISO formula. That's close enough to 68* for the purpose. (Or are other factors like field flatteners reducing the expected AFOV of the SF? In that case there is no quality 8x with a 68* AFOV at all.)
Hi Guys'

If you hold an objective lens in front of you, you may notice that it has a FOV of 2, maybe 3 feet, and you can see that it will form an image. But would it be an image you would want magnified?

In optics, the footbone is connected to the headbone. The engineer has to design what the consumer will buy. All that an objective CAN do will not fit into an observer’s eye. It can be made better. But then, the observer will bellyache about the cost of what it would take to keep them from bellyaching about the field. Wouldn’t it be great if folks spent 1/20th as much time learning about the realities involved with optical design as they do in trying to reinvent someone else’s wheel?

The reality here is that if some ad didn’t TELL the observer what to believe—and they are often wrong*—they bloody-well wouldn’t know the difference. And to head off the ultracrepidarians, yes, I know they could test. More BB stacking.

* Attached is a Fujinon ad in which the binocular is clearly shown to have an APPARENT field of “7 degrees and 30 minutes." IT DOES NOT! That’s its REAL FIELD! Obviously, the ad guy didn’t know the story and Fujifilm didn’t waste their time proofing the ad.

“The truth is incontrovertible. Malice may attack it and ignorance may deride it. But, in the end, there it is.” — Winston Churchill

Back in my hole,

Bill
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Old Friday 20th September 2019, 10:07   #8
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Originally Posted by tenex View Post
How do you figure that? Working from a TFOV of 147.4m for the 8x SF, I get 67 or 67.5* AFOV using the simple approximation or the ISO formula. That's close enough to 68* for the purpose. (Or are other factors like field flatteners reducing the expected AFOV of the SF? In that case there is no quality 8x with a 68* AFOV at all.)
Hello,

it has been described x times, this simple formula is not good for measuring the correct AFOV, the distortion is not included!
Most manufacturers make it easy and use this simple formula, the consumer will eat it already and, lack of knowledge, take over!
Only Swarovski and the Zeiss SF are the correct AFOV specify.
https://www.zeiss.de/sports-optics/d...r.html#modelle

They go to (ffnen) open there is the data.
See "Subjektiver Sehwinkel"...SF 8x42 64 Grad, SF 10x42 65 Grad!


Using their simple formal as the basis for the AFOV results in a partial 5 degree AFOV difference that Swarovski 12x50 would actually have "only" 63 AFOV after measuring it above 68 AFOV.

Andreas

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Old Friday 20th September 2019, 11:59   #9
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Yes, I suppose my question must involve consumer demand (or manufacturer perception of that) as well as technology. So, manufacturers take note: I demand a wider field in your 7-8x binoculars! Should I start a poll?
I would be astonished if your demands or a poll result will make mother nature change physics. As Alan has explained, a wider apparent field means a wider true field and thus eyepiece and prisms need to be larger (and heavier). Most of the times that is a no-no for customers.

I wish you much fun with your new pair of Nikon WX 7x50!

Joachim
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Old Saturday 21st September 2019, 04:31   #10
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Only Swarovski and the Zeiss SF are the correct AFOV specify.
Presumably because they're the ones using field flatteners that make the calculation inaccurate. For the same reason Nikon also states a measured AFOV of 67* for the 7x WX, whereas one would calculate 75* from its stated field of view.

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I would be astonished if your demands or a poll result will make mother nature change physics. As Alan has explained, a wider apparent field means a wider true field and thus eyepiece and prisms need to be larger (and heavier). Most of the times that is a no-no for customers.
I wish you much fun with your new pair of Nikon WX 7x50!
Change physics? As you seem aware yourself, the issue I was just addressing is consumer preferences, which vary. I'd accept a heavier bino myself to get a wider true field. (It needn't be a 50mm glass either; 7x35 would do fine.) I'd be very interested to see what's possible under the budget of the WX.

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Old Saturday 21st September 2019, 10:07   #11
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Presumably because they're the ones using field flatteners that make the calculation inaccurate. For the same reason Nikon also states a measured AFOV of 67* for the 7x WX, whereas one would calculate 75* from its stated field of view.
No, Swarovski also gives the correct values ​​for the hawk and the SLC and they have no flat fields!
Holger Merlitz says in a German forum the Nikon is the only manufacturer that specifies too low values ​​for its binoculars!
Here's a clip from the discussion ...

Quote Mr. Merlitz
"The confusion arises from the contradictory claims of the manufacturers. While the objective visual angle can be converted exactly into the visual field, a calculation of the subjective visual angle requires knowledge of the distortion. Unfortunately, the manufacturers do not indicate the distortion of their optics. Alternatively, they could at least publish the actual subjective visual angle, but only a few do (Swarovski, and Zeiss only in his SF series, but not in the other models!). In all other cases, they use standard conversion formulas that either overestimate or underestimate the subjective visual angle.
The question of why manufacturers calculate their visual angles with inaccurate formulas (rather than simply specifying the laboratory values), I have often asked myself and never received a convincing answer. As long as you still consistently used the angle condition, the subjective visual angle was usually overestimated - so it is obvious that marketing could live well with this approximation. The fact that Nikon systematically underestimates its visual angle with the ISO formula, is no longer understandable. For the 10x50 WX, they thus determine a 76.4 visual angle, while the actual value should be 83 . My suggestion, but simply to publish the measured data, they have not accepted."

Andreas
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Old Saturday 21st September 2019, 18:46   #12
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No, Swarovski also gives the correct values ​​for the hawk and the SLC and they have no flat fields!
Holger Merlitz says in a German forum the Nikon is the only manufacturer that specifies too low values ​​for its binoculars!
Here's a clip from the discussion ...
Actually the SLC does have a relatively flat field, at least the 56mm I'm familiar with; I have no experience with the 42. But surely you're right that the Habicht doesn't. I should have said that nearly all Swarovskis use field flattening, which they've made a sort of trademark of and therefore need to adjust their apparent-field claims (downward) for.

Thanks for Holger's comments on the WX, which I don't know what to make of. As I said, the usual calculations from true FOV give a higher value than Nikon states, so I can't imagine what other calculation they could have used instead, and assumed they just measured it. Of course I've never seen a WX.

I suspect that manufacturers actually want us to be confused about AFOV, so simple comparisons are impossible and no one will feel certain enough to complain about the compromise made.

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Old Sunday 22nd September 2019, 16:51   #13
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As I said, the usual calculations from true FOV give a higher value than Nikon states, so I can't imagine what other calculation they could have used instead, and assumed they just measured it. Of course I've never seen a WX.

I suspect that manufacturers actually want us to be confused about AFOV, so simple comparisons are impossible and no one will feel certain enough to complain about the compromise made.
The ISO standard calculation used by Nikon, Canon and probably a few others can be found here:

https://imaging.nikon.com/lineup/spo...8.htm#basic002

The ISO standard is more accurate than the simple calculation for binoculars with low pincushion, but of course actual measurement of the AFOV is better than either approximation. Measuring the true AFOV is easy to do by sighting backwards through the objective lens of a binocular mounted on a tripod head with a panning scale in degrees. Just move a tiny distant object across the eyepiece field stop aperture from 3:00 to 9:00 and note the change in degrees on the scale.

The problem with assigning a single value to the distortion profiles of the Swarovski SVs, Zeiss SFs or other binoculars with "mustache" distortion is that their rectilinear distortion profiles result from a combination of pincushion distortion increasing across the inner part of the field and then at some point reversing in the outer part of the field from the application of a compensating barrel distortion. Different amounts of pincushion and barrel can average out to the same value even though the appearance of the distortion profiles may be quite different.

Last edited by henry link : Monday 23rd September 2019 at 02:46.
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