[dbuss]

I own the Swift Audubon 820 ED binocular and I am very happy with it. I am especially impressed with the brightness and 3D effects after spending time using a roof prism binocular. However, my question is this: Since it is still possible to find a Nikon 10x42 SE porro on the market, would I notice any significant gain in magnification with the Nikon over the Swift Audubon? I understand that most would regard the Nikons to be a higher quality binocular but I am wondering whether the extra magnification justifies the $800-900 cost.

[ceasar]

Originally Posted by dbuss

........................... but I am wondering whether the extra magnification justifies the $800-900 cost.

Don't know about the magnification, but the view certainly does!
Bob

[elkcub]

No, I'd stick with your 820ED and save the money. It's a fine binocular, even by comparison with the more expensive Nikon SE series — which frankly doesn't impress me. 10x is no big thing by comparison to 8.5x, and depth perception is aided by a binocular with lower magnification and greater depth of field.

Just my opinion, of course.

Ed

[richt]

Hi

The SE is a very pure view and almost edge sharp however the field of view is 6 degress against the Swift 8.2 degrees and eye placement is more critical
Its purity against ease of view as many others have documented
The Swift Audubon has extremely high centre field resoloution and is the one of a tiny amount of binoculars i have looked through that equals the SE in the centre
The Swifts edge field is not as defined as the SE but you see more on the fringes due to its wider field of view
The 10 x SE will pull in slightly more detail perhaps shore birding but thats only if you can hold the increased magnification steady

Regards
Rich T

Both great instruments either way

[brocknroller]

Originally Posted by dbuss

I own the Swift Audubon 820 ED binocular and I am very happy with it. I am especially impressed with the brightness and 3D effects after spending time using a roof prism binocular. However, my question is this: Since it is still possible to find a Nikon 10x42 SE porro on the market, would I notice any significant gain in magnification with the Nikon over the Swift Audubon? I understand that most would regard the Nikons to be a higher quality binocular but I am wondering whether the extra magnification justifies the $800-900 cost.

I have compared both bins side by side, and it's apples and oranges. It depends on your application and personal preferences.

The eyecups on the 820s didn't allow me to see the entire FOV because their large size wouldn't fit into my deep-eye sockets. I probably could see about 6.5-7* with them, still a gain over the SEs, but I lost DOF.

The focuser bridge also flexed on my 820. My solution was to buy the older 804 model for the backyard, and the 8x32 SE, which is lighter and more compact, for carrying in the field.

I found the 6* FOV of the 10x42 SE too restricting.

According to Steve Ingraham, formerly of BVD, the 10x SE and 8.5X Audubon have nearly the same resolution.

An optics "expert" on Cloudy Nights recently called Ingraham's methodology "flawed" so this throws doubt on the boosted resolution numbers Ingraham posted (3.5 arc seconds for the Audubon, 3.58 arc seconds for the SE).

However, my own eyeball comparisons agreed with Ingraham's and RichT's. The two bins look very close in resolution. At longer distances, the larger image scale of the SE would probably be more pleasing to the eye.

But for all day birding, the 8.5x is less fatiguing, and as Ingraham stated, over time you see more detail with a 8x bin, because the shakes from a 10x can negate its extra reach or in this case, larger image scale.

You don't know what you're missing until you do a side by side test. A few months back, another member of this forum and I tested my 8x32 SE against his 10x42 SE to read a warning sign posted on a transformer at the top of a telephone pole.

We both could read the sign much better with the 8x32 even though the 10x has a higher resolving power. And the more you use your binoculars during an outing, the more your arms fatigue, and the more shakes rob detail.

So I consider the 10x42 a specialty bin. Good for special circumstances, but not as my primary birding binoculars.

However, if you have exceptionally steady hands or you are willing to mount the SEs, and have a need to look at targets a distance often, the SE could be "Superior."

A better comparison would be between the Audubon and 8x32 SE. The Audubon's resolution is better, but the binocular is rather bulky and even with the reduced weight of the 820 model, it still weighs more than the SE.

The SE is not WP like the Audubon, but it's well sealed and if you don't dunk them or use them in the pouring rain, they will hold up fine.

Brock

[edz]

quote [An optics "expert" on Cloudy Nights recently called Ingraham's methodology "flawed" so this throws doubt on the boosted resolution numbers Ingraham posted (3.5 arc seconds for the Audubon, 3.58 arc seconds for the SE). ]

Actually, to correct you, what I said about Ingraham's boosted resolution values is that he has not posted the magnification at which they were recorded, therefore other boosted resolution values cannot be compared to them. Nor can they be compared to anyone elses tests.

If three testers, all with the exact same ability testing the same binocular use three different boosted magnification, for example, 50x 60x and 70x, they will get three different , significantly different, non-comparable results.

edz

[Surveyor]

Originally Posted by edz

If three testers, all with the exact same ability testing the same binocular use three different boosted magnification, for example, 50x 60x and 70x, they will get three different , significantly different, non-comparable results.

edz

Hi Edz;

I am not sure which method you may be referring to here, but in the case of resolution chart methods, I will make some differing observations/opinions.

I have a visual acuity of about 75 arc seconds/150 arc seconds/lp (20/25 vision uncorrected). If we have a lens or system that is truly diffraction limited at 4 arc seconds, then I need to boost the image at least 50x to be able to see at the 4” level so I would choose about 60x for testing. If the optics are truly the limiting factor then further magnification will yield no better results.

The only limitation I am aware of on the “booster limits” are from the ISO procedure for resolution measurements in ISO 14490-7-2005 and stated as:

“The magnification of the auxiliary telescope shall not reduce the diameter of the exit pupil of the whole system below 0,8 mm.
[a] The auxiliary telescope should not be used if the angular limit of resolution behind the eyepiece of the test specimen is worse than 2' to 3' and the diameter of the exit pupil is below 1 mm.”

I routinely use 5x, 6x and 8x boosters and find that I cannot, generally, change a bad resolution figure by increasing the magnification. This applies to small objective instruments like binoculars. I do not know how it would apply to larger objectives and double star or similar methods.

Have a good day.
Ron

[edz]

Hi Ron,

In practice here's some real results.

out of a large number of optics
the best two in each size
all tested on line pairs
boosted by approx 6.2x

12x50 2.56 arcsec
12x50 2.70arcsec
10x50 3.0 arcsec
10x50 3.23 arcsec
7x50 4.3 arcsec
7x50 4.3 arcsec

here's another set
10x42 3.6
10x42 3.6
8x42 4.1
8x42 4.6
8x40 3.8
8x40 4.6


This is just a small sampling. I have enough resolution values recorded both normal and boosted to see the obvious progression of increased resolution as magnification increases from 50x to 70x.

You will approach about 85-90% of the potential resolution when you start getting down to just lower than 1mm-1.25mm exit pupils. Some of these could not be tested with an 8x booster. If you actually approach and pass the limit of power for the lens, resolution goes down. Based on my (extensive) experience with variable power optics (eyepieces), maximum resolution in fine optics is not reached until exit pupil approaches 75x-80x per inch, or 3x per mm of aperture. Few people if any are ever testing their binoculars at magnifications that approach these levels. But I doubt the slow optics of binoculars would test that high, and would expect probably somewhat less than 1.5x-2x per mm for a maximum. However, an 0.8mm exit pupils does not reach maximum resolution, even in these slow optics. The maximums for the binocular apertures used in my tests were reached with an exit pupil of 0.63mm to 0.67mm, powers very close to 1.5x per mm of aperture.

The example you cite of an optic limited to 4 arcsec res using 60x for a test would be perhaps a 28-30mm optic. I've never tested optics that small. 35mm, 40mm, 42mm, 50mm, etc. optics (better and best optics) would require much higher than 60x if you want to see all the potential resolution.

Mediocre/poor optics peter out well before that. I cannot improve a bad resolution reading by increasing much beyond 1.2x per mm, or about a 0.85mm exit pupil. bad optics show up quickly and do not improve.

So I reiterate, regardless of what power allows you to reach the limit, that is not the argument here. In order for the information to be useful for comparison to any other test or tester, the magnification at which the test was conducted must be known. People who think they can test their 8x binocular with a 6x booster and can then compare that to some other test done with a 7x or 8x booster are grossly mistaken.

edz

[henry link]

I have to agree with Ron about this. Boost the magnification high enough, using a standard bar target in daylight or very bright artificial light, and you will clearly see the native resolution the binocular, typically at some magnification between 1mm and 0.5mm exit pupil. More magnification changes nothing. I've boosted the magnification much higher without seeing any increase in resolution. Of course, measuring resolution this way is a pointless exercise if the boosted magnification is too low to show the true resolution of the binocular.

[edz]

And if you read a variety of test results from different testers, how do you know what magnification was used to obtain the result? Will you arbitrarily assume it has been tested high enough? Or, do you think it better to know the magnification at which the test was performed?

Do you realize that given the magnification, it is not necessary to have tested the optic at maximum magnification to compare results between powers or tests or testers? Go ahead multiply out the magnification x result for all the optics I've reported above. Watch what happens. There is NO DIFFERENCE between a 50mm optic tested at 60x versus 70x. Likewise there is little to no difference in a 42mm optic tested at 60x versus 50x.

Hence, the most useful piece of data, the magnification, really gives the factor that allows you to compare.

edz

[henry link]

Ed,

Stephen Ingraham used a 5x scope for his boosted resolution measurements. For my eyes that would not to be not quite enough for low power binoculars with large objective lenses, but it could be sufficient for someone with higher eyesight acuity. The pattern in your measurements suggests to me that you are also not using enough magnification for accurate measurements of binoculars like 7x50s. As a group they should show similar true resolution to 10-12x50's. After all the objective size and its aberrations are what limit the resolution. Why should 10/12x50's show consistently higher resolution than 7x50s unless the boosted magnification of the 7x50's is too low for an accurate measurement.

Henry

[edz]

Henry,

did you do the exercise i suggest in my last post??? if you had, I wouldn't expect you would have just made that comment. if you do, you will find the 7x50s have exactly the same resoltuion as the 12x50s and 10x50s. That's the point I've been trying to drive home. it really can't be that hard that it's been missed, can it? Actually Henry, you and I have had this discussion before, at great length, if you recall. So this 'apparent resoltuion' calculation should be nothing new to you.

all tested on line pairs
boosted by approx 6.2x

12x50 2.56 arcsec = 190 arcsec apparent
12x50 2.70arcsec = 201
10x50 3.0 arcsec = 186
10x50 3.23 arcsec = 200
7x50 4.3 arcsec = 187
7x50 4.3 arcsec = 187

edz

[henry link]

Ed,

I've been responding to your unedited posts. I think I'll wait for your edits to settle down before I respond again.

Henry

[Surveyor]

Ed;

For me, what you call apparent resolution, would be my visual acuity and average 150-160 with insufficient boosting. I do not know what your eyesight is like but would assume these figures made by someone with about 20/30 visual acuity or, more likely, insufficient lighting or done outside with atmospheric interference.

Have a good day.
Ron

Example: If your visual acuity is 190" and you want to measure to the 3" level, you would need at least 64x total magnification.

[edz]

Originally Posted by Surveyor

Ed;

For me, what you call apparent resolution, would be my visual acuity and average 150-160 with insufficient boosting. I do not know what your eyesight is like but would assume these figures made by someone with about 20/30 visual acuity or, more likely, insufficient lighting or done outside with atmospheric interference.

Have a good day.
Ron

Example: If your visual acuity is 190" and you want to measure to the 3" level, you would need at least 64x total magnification.

Ron, your visual acuity on line pairs in daylight is closer to 70-90 arcseconds. Most of these same binoculars, when tested at normal power, give results in the range of 76 to 85 arcseconds. (This falls right in line with the pattern of results obtained in high power optics when testing resolution on point sources. It always requires far more magnification to reach the limits of resolution of the optics than it does to test the low limit level of resolution. Typical results for example for an 80mm Apo scope would be a lowest limit of resolution (best achieved at low power) achieved at approx 140-160 arcsec and a highest possible (best absolute maximum) resolution achieved at 300-400 arcsec apparent).

So the argument that you think this boosted power is testing vision is I think is incorrect. That argument would fail to explain that the best reading for a 12x optic is achieved at a power factor of 74x, 1.5x/mm or an exit pupil of 0.67mm, well into the highest powers needed for testing, and yet all the other lower powered tests on 50mm optics achieve the same, or nearly the same, apparent resolution. In fact sometimes the lower powered reading is even better. Actually all this data has been published for nearly a year, so of it for much longer. Many of the discussions regarding these results and this method took part on this forum, although Henry also took part in lengthy discussions on CN.

To test this for yourself, I encourage you to take some of your data points, and given that you know at what powers the readings were taken, extended the data. You should see the same consistency in your own data. Best binoculars will cluster around a low apparent value. The ones that don't will be recognized as not of equal best resolution. Frankly, this is why I don't even compare raw readings as resolution. I've been reporting all resolution comparison data in terms of apparent resolution, for both normal power and boosted power, for a few years now.

edz

[Surveyor]

Good morning Ed;

Uh-oh, sounds like we might be into the old discussion of line pairs versus lines or point resolution. You are correct about my visual acuity being 75” for engineering purposes.

First, let me make a point about my measurements. When doing government work, both prior and post survey, I have to submit reports of instrument performance and show they meet published specs. I also have to do this annually for state licensing purposes. For this and most other work, I use the engineering practice of lines of resolution or points for both resolving power and resolution (rms values). I only use line pairs on BF since that appeared what the preference was.

Instead of discussing this I thought I would run a check this morning on a bino and show you how I arrive at my figures. If you spot a error, please let me know.

I took a 10x bino (advertised as 12, but that would make the situation worse) and with a 6x booster was able to resolve group 7 element 3 at 400 mm. Without the booster I was able to resolve group 5 element 2, almost 3, but not quiet. The attachments are at 1x for resolution, at 10x to show the group 5 data unboosted and at 60x for the apparent resolution. Note the red columns for resolution in arc seconds for line pairs and one for the engineering type lines or dots.

Note that the data for group 5 element 2 shows 140" apparent for line pairs and 71" for lines or points.

Have a good day.
Ron

[henry link]

Originally Posted by edz

Henry,

did you do the exercise i suggest in my last post??? if you had, I wouldn't expect you would have just made that comment. if you do, you will find the 7x50s have exactly the same resoltuion as the 12x50s and 10x50s. That's the point I've been trying to drive home. it really can't be that hard that it's been missed, can it? Actually Henry, you and I have had this discussion before, at great length, if you recall. So this 'apparent resoltuion' calculation should be nothing new to you.

all tested on line pairs
boosted by approx 6.2x

12x50 2.56 arcsec = 190 arcsec apparent
12x50 2.70arcsec = 201
10x50 3.0 arcsec = 186
10x50 3.23 arcsec = 200
7x50 4.3 arcsec = 187
7x50 4.3 arcsec = 187

edz

Ed,

I confess I didn't perform the exercise you assigned, but that's because I knew what the result would be. Naturally these "resolution" measurements lead to the same "apparent resolution" figure, but not because all these binoculars actually have the same resolution. It's because eyesight acuity is the limiting factor until you reach enough magnification to see the true resolution. Any magnification below that point will inevitably show the same "apparent resolution" because that is the limit your eyesight imposes. Above that point using higher magnification won't change the binocular's resolution measurement, but your "apparent resolution" figure will begin to climb because the binocular's resolution becomes the limiting factor. Frankly, I think "apparent resolution" is a spurious concept. It tells us nothing that we don't already know from the measurements themselves.

What I see when I look at the list above is an effort to measure the resolution of six 50mm telescopes compromised by the failure to use a consistent magnification for all the telescopes. Two of the telescopes are measured at 74.4X, two at 62X and two at 43.4X. I'm not exactly surprised to see that the binoculars measured at the lowest magnification show the lowest "resolution". Why insist that 43.4X is enough when it's so easy to check that assumption? Simply substitute a 10X scope to increase the magnification of the 7x50's to 70x, essentially matching the test magnification of the 12x50's. Those particular 7x50's might still measure no better than 4.3. I've measured 50mm binoculars with worse resolution than that, but I've also measured 7x50's with better than 3 arcsec resolution, just not at such low magnification.

I wouldn't make any hard predictions about what you can see in these tests based on your normal daylight eyesight acuity. When you're testing telescopes at very small exit pupils the image is dimmer than daylight and the size of the exit pupil becomes the diffraction limiting aperture. I find that I usually need higher than expected magnification to see the true resolution of the instrument unless it's very poor.

Henry

[edz]

[Any magnification below that point will inevitably show the same "apparent resolution" because that is the limit your eyesight imposes ]

Sorry Henry, that's not what occurs. At magnifications below the limit apparent resolution is much smaller. In fact if you start with all these and no supllemental boost power, the apparent resolution is about 80 arcseconds. As you start adding magnification boost, the apparent res starts to climb. For instance, if you reference out discussions from 2005, you will find that I reported a series of tests with a 2.5x booster. All the apparent res in those tests was about 120-130 arcseconds. With a 5x booster it climbed even higher. However, with this 6x booster, especially for the 12x binocular it peaked.

You may recall the series of powers I used in a megrez80 from 20x to 100x to show the progression of apparent resolution. That graphic is attached to our 2005 discussions and is available in my CN gallery. From lowest power to maximum power, apparent resolution climbs from your eyes lowest apparent reading,(for me that's about 80) to the highest allowed by the optic (in the Megrez that was about 150). Every power gives a different reading.

Of note in those same tests you and I conducted was you reading for the Prostar 7x50. You got 2.9 arcsec at 80x. included in my data listed above is the Prostar. I got 4.3 arcsec at 62x. Compare the two and actually you will find that I got a better reading for the Prostar than you did. Not by much, but better.
Henry prostar 2.9" at 80x = 232 arcsec apparent
EdZ 4.3" at 62x = 187 arcsec apparent

these are your notes from 2005
nikon prostar 7x50
normal power 7x = 13.64" = 95 arcsec apparent
boosted to 42x = 3.4" = 143 arcsec apparent
boosted to 80x 2.9" = 232 arcsec apparent

here are my notes on a Prostar 7x50
normal power 11.5 arcsec = 80 arcsec apparent
boosted to 18x = 7.65" = 134 arcsec apparent
boosted to 143 = 4.3" = 185 arcsec apparent

Also referenced in the same discussions you and I conducted back in 2005, I tested my maximum with a fine scope, just as you did. I found a maximum that I could reach with an Megrez 80mm scope was 1.5 arcsec at 100 power. I tested that same scope at 125x and could not exceed 1.5 arcsec. My best reading then was marked at 152 arcsec apparent.

You used a Tak FC-50 at 40x next to the Prostar. Could see 3.04" fairly easily with 2.7" suspected. 2.58" resolved at 80x. At 40x, your apparent res was only 103, very near normal apparent res. So your best was 206 arcsec apparent.

A few things we may conclude from this; you may have exceeded the maximum power for testing the Prostar and the FC-50, or your limit of resoluition requires you to use magnifications that result in exceeding 200 arcseconds apparent.

At any rate both your data and mine show that your assumption would not hold for these data sets. that leads me to conclude there was ample magnificcation used for the test.

This has veered considerably for the beginning scope of this discussion. I never expected this conversation to get into the same old back and forth about power as we have discussed that numerous times going on several years now. The point is that any resolution test, without giving the magnification cannot be compared to any other. Our data clearly shows that. howver, given the magnification, comparisons can be made, not only between different testers, but between different binoculars by the same tester.

I would suggest you reassess you understanding of apparent resolution. As I suggested earlier, To test this for yourself, I encourage you to take some of your best data points fr a selection of binoculars, and given that you know at what powers the readings were taken, extended the data. You should see the same consistency in your own data.


edited to add Prostar data points
Out of curiosity, I added more high powered boost reading to the Prostar.
I tried reading it with a fine Fujinon 10x binocular behind it for 70x and the readings were all blurry, best res i could read clear was 4.3 arcsec. I suspected 3.85". (Are you sure you tested the Prostar at 80x!) I then stepped down and used my Pentax 8x40 behind it. I could easily see 3.85" and could still clearly read 3.41 arcsec, but no better. At 56x that gives me 3.41" for 188". Almost the same as above. Sorry I don't have 9x to get in between these two, but i could not see better at 70x than at 56x. And as it turns out the apparent res at 56x is equal to the apparent res at 44x. I conclude I've reached the plateau.

edz

[henry link]

Ed,

Yes, sorry my mistake. Of course you're quite right about the so called "apparent resolution " increasing with magnification, but that doesn't actually change my objections at all. The concept is still simply a convoluted way to express the level of detail visible at a particular magnification, as limited by eyesight acuity, until the magnification is high enough to show the true resolution of the telescope. If the actual resolution is what you want to measure then "apparent resolution" at some insufficient magnification is completely useless and at high magnification it becomes misleading as in your last post when you use it to conclude that a Prostar with 4.3 arcsec resolution at 62x gives a "better reading" than a Prostar with 2.9 arcsec resolution at 80x because the "apparent resolution" figure is better.

[elkcub]

EdZ,

I believe there are typos in the numbers pertaining to your Prostar, which may be complicating matters.

Ed

[edz]

{The concept is still simply a convoluted way to express the level of detail visible at a particular magnification, }

Sorry Henry, I disagree. While you may find it convoluted, since you don't commonly use the term, it is a normal term of discussion in high powered resolution in astronomy. Since I've been invloved with resolution discussions for many years with many other people, some of whom taught me a thing or two about resolution, it is a normal term in discussions, and it does provide for a way to make comparisons.

As you've seen for example, in all normal power resolution tests it provides a way to compare any one size binocular to another. All it takes is a review of the reams of data I've posted on that subject to see the relevance to that effect.

And here at high powers, whereas a 50mm binocular may be found to reach max at ~70x, a 40mm binocular may be found to reach max at ~50x. Comparing these two in any way would be difficult unless you choose a common denominator.

It works well for both those instances. This is not a means to find the max. It provides a means to compare one max to the other across a variety of binocular powers. I thought you would have understood that by now from what I've written and the discussions we've had in the past, or perhaps you have never pulled that out of what I've written. At any rate, you will find that in all the detailed explanation I've posted on this subject.


edz

[edz]

Originally Posted by elkcub

EdZ,

I believe there are typos in the numbers pertaining to your Prostar, which may be complicating matters.

Ed

sorry about that

the line
boosted to 143 = 4.3" = 185 arcsec apparent

should have been properly written as
boosted to 43 = 4.3" = 185 arcsec apparent

So my two best readings with the Prostar 7x50 were
4.3" boosted to 43x = 185 arcsec apparent
3.4" boosted to 56x = 190 arcsec apparent.
both quite comparable,
no clear reading at 70x.


edz

[edz]

[ Frankly, I think "apparent resolution" is a spurious concept. It tells us nothing that we don't already know from the measurements themselves. ]

On the contrary, it most certainly does. Lets put ANY two same size instruments side by side and assume we see 3 arcseconds resolution through both of them. Now lets check and find we see one of them achieved 3" with only 60x power, and to see 3" the other required 80x power. I know which instrument I'd want to own. The lower apparent resolution shows it very nicely. We see this kind of result all the time if comparing a fine apochromat or ED optic to most standard achromats.

edz

[elkcub]

I'm just trying to understand.

Assuming that the booster were successively increased in power, e.g., 1, 2, ... 6, 7, ..., would there be a point where chart resolution no longer seems to improve? If so, is that the "true" resolution of the optic?

If two observers had very different acuity, would the same "true" resolution be attained, but perhaps at different booster powers?

If those two answers are affirmative, then the product of resolution and power (= apparent resolution) seems to reflect acuity differences. No? Yes? ... Help.

Please correct or expand on my misconceptions.

Ed

[edz]

Originally Posted by elkcub

I'm just trying to understand.

[1] Assuming that the booster were successively increased in power, e.g., 1, 2, ... 6, 7, ..., would there be a point where chart resolution no longer seems to improve? If so, is that the "true" resolution of the optic?

[2] If two observers had very different acuity, would the same "true" resolution be attained, but perhaps at different booster powers?

[3] If those two answers are affirmative, then the product of resolution and power (= apparent resolution) seems to reflect acuity differences. No? Yes? ... Help.

Please correct or expand on my misconceptions.

Ed

1st answer yes. Actually I just tested that today in three different optics. You hit an upper limit of power, and then there is no gain. In some instances the view gets worse. In my 80mm scope I saw exactly the same resolution limit at four powers from 125x to 175x. In several binoculars I could see no more resolution at 70x and in fact it in one it got worse at 80x.

2nd, perhaps yes. generally a lower acuity can see the same object when a higher power is applied.

3rd, yes, but only in that scenario. if they both use one of the same binocular and get very different results for power to reach the limit, then that would clearly be showing the difference in acuity between the two observers. But, that is just one scenario.

Suppose there is only one observer and we follow thru #1. And let's also suppose the observer has two binoculars, same size, but different quality. At same power limit they show very different res results. That's a completely different scenario than anything you mentioned, and it's not measuring acuity.

Or suppose two observers have the same acuity and compare two different quality binoculars and get different results. What then? Certainly not acuity.

a fourth scenario. ONE observer uses two different brands binoculars, but both same size, and both used at numerous powers. For sake of discussion they both do very well, they both see 3". But the first one can see 3" at both 60x and 65x and shows no further increase. the second one does not see 3" until you reach 70x. beyond 70x, there is no further increase and it is fuzzy. This is not a test of acuity. The first binocular reaches the resolution limit at a lower power, has a lower apparent res and is better.

these examples provided specifically to show in these scenarios, beyond your example, it is not testing acuity.

edz