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Exit Pupil Constraints

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Old Friday 6th September 2019, 11:29   #1
Tringa45
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Exit Pupil Constraints

A local saying goes that every fool is different and my folly, as far as scopes are concerned, is the aesthetics of the view.
While out birding yesterday I was able to view a Wood Sandpiper (Tringa glareola) at about 50 m and at 25x through my Kowa 883. As far as I'm concerned, birds don't get any more beautiful than this.

However, scopes are very useful for distant identifications and this is the raison d'etre for many users. It was dull and overcast yesterday and the conditions paradoxically allowed the use of higher magnifications, as the air was stable. I had to crank up the magnification to 60x to id a distant Common Sandpiper in an awkward pose. While zooming back to lower magnifications I passed a point, where there was a fast and significant increase in brightness.

Back home under similar conditions at 6 p.m. I repeated this, zooming back from 60x and consistently reached the point of "enlightenment" around 40x, i.e. 2,2 mm exit pupil.
Coincidentally, this is also the exit pupil size of the 30x W on my ATM 65HD and would roughly correspond to the perceived brightness in the BTX 85 and BTX 95 with their beam splitters at fixed magnifications of 30x and 35x respectively.

I would not dispute the usefulness of exit pupils around 1-1,3 mm and their attendant magnifications, but think perhaps that 2,2 mm is the bottom limit for maintaining terrestrial image "quality".

Comments welcome.

John
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Old Friday 6th September 2019, 13:53   #2
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Interesting thoughts. For relaxed viewing I also much prefer exit pupils >2 mm. Larger exit pupils are always better though, not just with binoculars but also with scopes. My ED82 is *really* nice at 25x (exit pupil 3.3 mm), and the difference between it and my EDIIIA at 25x (exit pupil 2.4 mm) is quite obvious in a direct comparison, even though both scopes are of very similar quality. I find the differences quite striking in use, even in bright light. The same applies to comparisons between the EDIIIA and the ED50 at 25x. Size does matter with scopes ...

If I need to I also use my scopes at high magnifications, of course. My personal limit there is about 1-1.5 mm. Exit pupils below ~1.5 mm make for pretty uncomortable viewing IME; I only really use them when absolutely necessary.

BTW, I personally find the magnification as important as the size of the exit pupil. I normally keep the magnification on all my scopes to about 25x. 20x is usually too little magnification, and 30x is a bit too much, mainly because I don't like the shallow depth of field at 30x. 25x seems me to be my personal sweet spot when using scopes.

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Old Friday 6th September 2019, 13:54   #3
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Hi,

I have to say I am fine with a bit smaller exit pupils - the 1.5mm I get with my TSN-3 and the SDLv2 at its maximum magnification of 52x are very nice unless it's dawn or dusk - therefore and due to the cable tie trick that zoom is seldom set to anything lower.

1mm like with the 613 at 60x is indeed a bit dark unless the subject is really out in bright sunlight... but still enough to see good detail.

Joachim
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Old Sunday 8th September 2019, 13:34   #4
Tringa45
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Hermann,

In retrospect the 40x magnification I achieved as the upper limit for a bright image was probably dictated by my own pupil size under those ambient conditions, but I was reminded of your reference to König/Köhler on the "Practical Magnifications" thread, where they considered 40x to be the upper limit for terrestrial magnification.
Could this have been with the Zeiss Jena 80 mm Aspectem scope? Despite the allegedly superb qualities of the 12,5 mm ultra WA eyepieces later used on the Docter/Noblex 40x80 Aspectem binocular, I think a 2 mm EP is critically small for a binocular. The BTX 95 (2,7 mmEP) is truly excellent, but when I tried one at my dealer's it was rather difficult to find the correct IPD. I recall reading a "Cloudy Nights" thread in which someone converted an Aspectem to take 24 mm Panoptics for 21x magnification.

John
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Old Sunday 8th September 2019, 17:33   #5
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You must be joking.
I have no problem with 80x terrestrially with a well collimated 80mm binocular.
I don't wear glasses with binoculars.

With telescopes I have gone much higher terrestrially in suitable locations and good conditions.
During daylight or at night.
When young or old.

Maybe I am living in a parallel universe (:

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B.

Last edited by Binastro : Sunday 8th September 2019 at 17:35.
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Old Sunday 8th September 2019, 20:16   #6
Tringa45
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Quote:
Originally Posted by Binastro View Post
You must be joking.
I have no problem with 80x terrestrially with a well collimated 80mm binocular.
I don't wear glasses with binoculars.

With telescopes I have gone much higher terrestrially in suitable locations and good conditions.
During daylight or at night.
When young or old.

Maybe I am living in a parallel universe (:

Regards,
B.
David,

We're talking about birding applications at or near ground level with birding scopes, which almost invariably have objective diameters less than 100 mm. Even Henry's use of 100x involves an astronomical refractor in an elevated position shortly after dawn or shortly before sunset. That might help getting IDs at long distances, but it's not going to give a pleasant view.

My 88 mm Kowa is seldom used above 50x. If the scope is 20 m from a shoreline on a sunny day, high magnifications are pointless.
I suspect that the older König/Köhler 40x reference applied to military spotters. AFAIK military spotters, even today, don't go above 60x. I ran into a birder recently using a dark grey 62 mm Leica Televid, which he said was ex-Bundeswehr. That would have a maximum magnification of 45x.

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John
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Old Sunday 8th September 2019, 21:33   #7
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Hi John,
I suppose the basic difference is that birdwatchers generally don't choose the place and time, and observe even if it is raining.

If the conditions are poor I just don't bother to observe unless I have no choice.
I automatically read the weather and look at the conditions with unaided eyes.

Here the chimney pots where crows, magpies and gulls perch are 124 metres distance.
The perch is 25 metres high, 80ft.
My position is about 18ft above the street level, about 240ft above sea level.
Even in summer there is rarely turbulent air between me and the chimney pots.
Mixture of house, gardens, grass and trees between.
The only time it isn't crystal clear is if it is foggy or raining or very heavy cloud cover.
I have no problem at all using 120x with a good scope. A 90mm Maksutov or small refractor. One just needs good light.

The use of high powers with scopes, microscopes and sometimes binoculars is to see fine detail.
If a wider general view is used that is fine.

I can usually identify the basic birds above even without my glasses at 124m although my distance glasses are better.

My general binoculars are 8x32BA, 8.5x44 Swift HR/5 and Canon 18x50 IS for aircraft and Jupiter's moons at night.
But I use anything from 2x opera glasses to 20x60, 20x70, 20x80 or 30x50 binoculars.
I don't like higher power binoculars or a binocular viewer, I use a scope and usually if it is a 20-60 zoom I use 60x.
I don't particularly like spotting scopes because 60x is just too low to see fine detail.

Incidentally, Horace Dall's camera obscura 108mm f/30 used a table magnifier giving 135x for exquisite detail of distant flowers, grass or dogs walking a mile away.

I also use an old Kowa 20x50 and MM2 13x-40x hand held braced on window ledge etc.

If one uses 50x only on a Kowa 88mm it just needs to be good or very good but not the very best.

The military Questars are used at 60x or 100x if conditions permit.

Regards,
B

Last edited by Binastro : Sunday 8th September 2019 at 21:37.
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Old Sunday 8th September 2019, 23:00   #8
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.......
If one uses 50x only on a Kowa 88mm it just needs to be good or very good but not the very best........
Even if an 88mm scope was optically perfect, those with excellent eyesight it would find it diffraction limited well before they got to 50x. Even with my aging eyes, anything above about 55x would result in a significant reduction in detail. At 120x, at the very least, the level of discernable detail would be, at the very least, 2.4x worse than at 50x. Just basic optics.

David
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Old Sunday 8th September 2019, 23:22   #9
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I disagree.
Looking at a known object such as a test chart, one would possibly see differences at 50x.

However, discovering hidden detail takes higher powers, in my experience at least 60% more to discover things.

As I have said before, discovering something in astronomy may take a 16 inch telescope.
The next person, knowing its existence will see it easily in a 10 inch scope.
A short time later it will be seen with a 6 inch scope.
Then finally someone like George Alcock will see it certainly with a 4 inch scope.

This sequence of events has repeated over the last 400 years of telescopes.
Once something has been discovered everyone says 'Why didn't I see this, it is obvious'.
Well it isn't obvious until it is discovered. Then it is obvious.

The real world is not test charts.

If I doctored a test chart, with hidden changes, I will give good odds that those with excellent eyesight will not see the changes with the minimum necessary magnification to see that detail.

In addition, many of us do not have excellent eyesight of 20/10, 20/8 or better.
And even those that do, do not always see that finely if the light is not optimum or if they are tired.

B.
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Old Monday 9th September 2019, 07:18   #10
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It's true that both visual acuity and perceptive skills are pretty variable, and I have no doubt that is reflected in an individuals preferred magnification level for a given situation.

The Kowa 883 Tringa referred to is a scope I know reasonably well, but I most certainly have not looked at a USAF chart through one. It's regarded as being amongst the best spotting scopes on the market. I've see no cause to disagree, but I still find there are very obvious magnification limitation caused by resolution and exit pupil as there are with any scope. Those limits were tighter when I was younger and no doubt will be relaxed as I get older still. I don't now how perceptive others are to those change is image quality. Of course it would depend on viewing conditions at the time, but I would expect at 120x, most here would notice pehaps a 90% drop in image brightness and contrast, and possibly a 2 or 3 fold drop in their apparent acuity. I'm sure there are some exceptions.

I learned something new recently that might be valuable to others here.

I know my eyesight is gradually deterioating and changed opticians earlier this year in the hope the new guy could delay the inevitable. This optometrist is rather better qualified than most, and I thought, took a great deal of care with the testing. Unfortunately when I got the new glasses a quick look out of the shop window was quite enough to tell me they were not as good as my previous pair. I protested, but was persuaded to take them home to "see if I got used to them". I didn't. Turns out there was about a 7 arcsecond reduction in acuity across a wide range in luminance compared to the old ones. When I went back, the optometrist repeated the examination, but just a little more slowly. At the end he explained that every time you blink the cornea deforms slighty and then take time to recover. He just waited 10 seconds before asking "better or worse". The only difference with my new prescription was a 10° change in the angle of astigmatism correction. Not quite the miracle I was hoping for, but a 10 arcsecond improvement over his first attempt was definitely welcome. He did point out that it was rather unusual for patients to be able to detect a small difference like this, but I imagine the forum might have a good number of more astute observers.

David

Last edited by typo : Monday 9th September 2019 at 07:23.
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Old Monday 9th September 2019, 16:20   #11
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David,
I agree that that the first look through a really top telescope can be shockingly good even with lower magnifications.
But it takes a lot of time to test it properly.

Telescopes that have shocked me were the £40 AE 6 inch f/8 Newtonian with very poorly coated mirrors.
But Jim Hysom's optics are usually shockingly good.

The c. 1950 Russian 4 inch Maksutov is unbelievably good. But I did push the magnification high.

The 16x56 Hensoldt binocular despite no phase coating is amazingly high quality.

The 3 Zeiss 75cm f/6.3 survey lenses c. 1930 from the Zeiss RMK? 30cm square format cameras.
These seem to be made with money unlimited.

The Vivitar 600mm f/8 solid Cat lens designed by a designer from Argentina in the U.S.

A Schneider T.V. lens 240mm fl??

I listed about 50 cherries that I remember. Usually the experience is memorable.

The reason I saw so many lemons is that many were worn out WW2 lenses that had very heavy use and were poor in the first place, and dreadful Chinese cheapies.
Unfortunately I have no access to top end Kowa, Swarovski etc.

Regarding test charts.
The dead centre eye view is used to seek the finest detail.
Yet the whole test chart is not seen in high resolution.
Even as the eye scans a doctered version will not be sussed out at minimum magnification for best resolution.
Even if the alteration is seen the brain is unprepared and overlooks any inconsistency.

Both Paul Doherty whose eyes were probably better than 20/8 and myself with 20/15 or maybe 20/13 or 20/14 at age 30, used a standard magnification 30% above the minimum needed.
In my case I then went to 100% over minimum for fine detail.

As to eye tests, even now I can repeatedly see 2.5 degree changes in astigmatism, back and forth quickly.
I don't need 10 seconds rest.
10 degrees is a huge change even with small astigmatism.

Until about three years ago I repeatedly could see changes of 1/8 dioptre in lens prescription.
My optician uses actual lenses and does this all by hand as I observe the test chart or the O on the test chart.
One eye is now 20/16 and the other 20/18.
I test them separately.
When I am tired the resolution drops a lot. I need at least ten minutes with rested eyes to recover.

Regards,
B.
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Old Monday 9th September 2019, 17:00   #12
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Re. opticians.

My eye tests take 1.5 hours.
Maybe not many opticians are willing to take this time.
I understand that most opticians don't prescribe to 1/8 dioptre or 2.5 degrees. Yet their equipment is capable of this.
If a Navy pilot can discern half these increments, then the opticians need to work to these standards.

A lab technician needs to be found who is prepared to do accurate 1/8 dioptre and 2.5 degree lenses.
In my case the lab boss.

Regards,
B.
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Old Monday 9th September 2019, 17:58   #13
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I discussed tolerances with my optometrist and he agrees with the British guidlines of 1/4d increments. Apparently studies have shown that drinking a glass of water with induce a 1/8d change and a cup of coffee 1/4d, so the prevailing view was that a 1/8d accuracy was an excuse to increase cost without any practical benefit (I'm sure there are other opinions on the subject). He also made the point that while the UK spectacle manufacturing tolerances were rather more stringent than some countries, including the US, the permitted error for sphere was still +/- 0.12d for sphere and cylinder and as much as +/-7° for axis for my prescription. He assured me that the manufacturer of my lenses worked to a tighter specification, but many didn't.

David
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Old Monday 9th September 2019, 18:15   #14
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The paper on U.S. Navy pilots made it clear that the tolerances were not good enough.

The optical trade is quite capable of higher standards, but costs are as usual the determining factor.

+/-7 degrees is far too high.

Presumably, drinking a cup of coffee affects both eyes unless one drinks it out of the corner of the mouth (:

Per instructions, I haven't drunk a cup of coffee for several years now.

B.
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Old Monday 9th September 2019, 19:21   #15
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Those US Navy studies reported that while the requirement to enter the fast jet program was 20/20 vision, the demands of the selection program resulted in the typical graduate having 20/8 vision. I should point out that the absense of any single eye data from these reports may mean these acuities are actually for binocular vision. That would mean the equivalent single eye values would be more like 20/23 and 20/10 respectively.

David
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Old Monday 9th September 2019, 23:33   #16
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Just to clarify: the magnifications I use on the Kowa 883 are often limited by the atmospheric conditions, not by the scope itself. I have measured mine on a USAF chart at 1,41", where the Dawes limit is 1,32". The next element on the chart would have been 1,25", i.e. not feasible. The measurement procedure was somewhat strenuous and I was quite satisfied with 1,41" and did not feel inclined to shift the scope incrementally.

John
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Old Wednesday 11th September 2019, 20:23   #17
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Originally Posted by Tringa45 View Post
In retrospect the 40x magnification I achieved as the upper limit for a bright image was probably dictated by my own pupil size under those ambient conditions, but I was reminded of your reference to König/Köhler on the "Practical Magnifications" thread, where they considered 40x to be the upper limit for terrestrial magnification.
Could this have been with the Zeiss Jena 80 mm Aspectem scope? Despite the allegedly superb qualities of the 12,5 mm ultra WA eyepieces later used on the Docter/Noblex 40x80 Aspectem binocular, I think a 2 mm EP is critically small for a binocular.
Not really, I think they referred specifically to monoculars, not binoculars:

"Zu der hier behandelten Gattung von Fernrohren gehören auch die Aussichtsfernrohre. Man wird hier bestrebt sein, eine möglichst hohe Vergrößerung zu wählen. Hierbei ist eine Obergrenze durch die Luftunruhe und die partielle Refraktion gegeben. Auch unter günstigen atmosphärischen Bedingungen ist eine höhere Vergrößerung als 40mal nicht möglich. In vielen Fällen wird man sogar nur eine 25fach Vergrößerung anwenden können." (König & Köhler 1959: 201-202, my emphasis)

In some ways they're right: On "normal" days higher magnifications simply don't work all that well for terrestrial viewing, simply because of heat haze and so on. On a typical day in spring or autumn under bright skies there are really only a few hours when higher magnifications (>40x) give you more detail, e.g. from about two to three hours before sunset until about half an hour before sunset when the light begins to fade. In the summer that "window of opportunity" is often even shorter.

However, I find their statement too pessimistic. Sure, on warm, hazy days 25x-30x is more often than not the limit, but there are still quite a few situations where far higher magnifications are possible. IME when the seeing is *really* good it's possible to go a lot higher than 40x to get more detail, provided, of course, the scope is good enough optically. I sometimes do that by using the Zeiss tripler. That fits well into the eyecups of my Nikon scopes, and e.g. with the EDIIIA (60mm) I use magnifications up to about 80-90x from time to time, and *still* get more detail than at 60x, the highest native magnification of the EDIIIA. On such days I also happily use the 75x of the ED82 no problem.

In fact, there were several occasions when I got an ID I would not have got otherwise. That at such small exit pupils [~0.6-0.75mm on the EDIIIA) the scope becomes very difficult to use and more often than not just allows brief glimpses of the bird goes without saying. And on windy days smaller exit pupils become almost impossible to use, even with a (very) heavy tripod.

As an aside, just in case someone believes the improvements in scopes makes it possible to use higher magnifications nowadays: This is IMO not really the case. Sure, modern coatings do make a difference, especially transmission and contrast are better, but compared to the influence of haze and distortions their influence is rather minor at high magnifications. I had a chance to look through quite a few old scopes, both monocular and binocular, including a well-preserved 25x100 binocular dating back to the Second World War, and these old optics are astoundingly sharp, at least in the centre.

One last interesting snippet: König & Köhler state elsewhere in their book that an exit pupil of 4 mm is the ideal compromise for binoculars. They also state quite categorically that magnifications above 10x make no sense in handheld binoculars.

I tend to agree with them on both counts ...

Hermann
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Old Wednesday 11th September 2019, 23:04   #18
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Thank you, Hermann. I think that's a very realistic assessment.

IIRC Zeiss-Jena or maybe even Docter used to produce a scope in 60 mm and 80 mm versions that looked very much like half an Aspectem. It seems though that the new owners, Noblex have discontinued the Aspectem.

John
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Old Thursday 12th September 2019, 00:46   #19
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Even if an 88mm scope was optically perfect, those with excellent eyesight it would find it diffraction limited well before they got to 50x. Even with my aging eyes, anything above about 55x would result in a significant reduction in detail. At 120x, at the very least, the level of discernable detail would be, at the very least, 2.4x worse than at 50x. Just basic optics.

David
An 88 mm scope is diffraction limited at 1.3 arcseconds. at 50x magnification that's 65 arcseconds. The people who can see deterioration "well before they get to it", that is who see details below an arcminute, are one in aillion prodigies. Maybe on a carefully crafted and perfectly illuminated pattern or something with extreme contrast like sunspots, but on a typical messy texture, human eye is nowhere as good.

If you indeed see this effect at these levels, then either you have literally won the genetic lottery or what limits you is not diffraction, but heat haze, eyepiece quality, residual chromatic aberation of telescope or whatever else is in you way.
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Old Thursday 12th September 2019, 07:15   #20
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However, I find their statement too pessimistic. Sure, on warm, hazy days 25x-30x is more often than not the limit, but there are still quite a few situations where far higher magnifications are possible. IME when the seeing is *really* good it's possible to go a lot higher than 40x to get more detail, provided, of course, the scope is good enough optically.
Hi Herrmann,

remember that the lines from König & Köhler were written at a time when fluorite loaded ED glass was not yet discovered and the only way to get ok colour correction in a fast refractor (faster than f15 or so - nobody in their right mind would have tried todays f5.5 back then) was to use a triplet objective with expensive Sonderglas - which were pretty much reserved to astro telescopes.

See http://www.monocular.info/cz_asiola.htm for what was still a very good spotting scope back then...

Joachim

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Old Thursday 12th September 2019, 20:21   #21
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An 88 mm scope is diffraction limited at 1.3 arcseconds. at 50x magnification that's 65 arcseconds. The people who can see deterioration "well before they get to it", that is who see details below an arcminute, are one in aillion prodigies. Maybe on a carefully crafted and perfectly illuminated pattern or something with extreme contrast like sunspots, but on a typical messy texture, human eye is nowhere as good.

If you indeed see this effect at these levels, then either you have literally won the genetic lottery or what limits you is not diffraction, but heat haze, eyepiece quality, residual chromatic aberation of telescope or whatever else is in you way.
Opisska,

My left eye acuity is 66", my right 72", and 62" with both, but these days my vision is on the decline, and I need the aid of a very good optometrist.

It could be genetics. My mothers eyesight was extremely good, and last time I checked, my son's binocular acuity was between 5" and 10" better than mine.

My preferred natural targets for testing are high contrast twigs against the sky at somewhere around 150 to 250m, but I can get pretty close with messy ones if the light is good. My son is better than me on those. My optometrist tells me it's not only my acuity, but my perception that is is rather unusual too, but I know very well there are others that can better me.

David

Last edited by typo : Thursday 12th September 2019 at 20:36.
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Old Thursday 12th September 2019, 22:01   #22
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remember that the lines from König & Köhler were written at a time when fluorite loaded ED glass was not yet discovered and the only way to get ok colour correction in a fast refractor (faster than f15 or so - nobody in their right mind would have tried todays f5.5 back then) was to use a triplet objective with expensive Sonderglas - which were pretty much reserved to astro telescopes.

See http://www.monocular.info/cz_asiola.htm for what was still a very good spotting scope back then...
Funny you should mention the Asiola. I had one myself for quite some time, and with modern coatings that would still be pretty competitive nowadays ... :-) CA was pretty well controlled in the Asiola, magnifcations up to 42x were no problem at all.

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Old Friday 13th September 2019, 08:53   #23
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Hi Herrmann,

I believe that the Asiola works well up to 42x (not yet been able to try one) - I'm quite sure Zeiss wouldn't have sold it that way if it didn't. It probably wouldn't fare so well at 60x or beyond (as some tests on astro pages show...).

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Old Friday 13th September 2019, 11:28   #24
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IIRC Zeiss-Jena or maybe even Docter used to produce a scope in 60 mm and 80 mm versions that looked very much like half an Aspectem. It seems though that the new owners, Noblex have discontinued the Aspectem.
Hermann & Joachim,

On that monocular info site you linked, there is also a reference to the Docter Aspectem 60/375 and 80/500 spotting scopes.

John
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Old Friday 13th September 2019, 21:51   #25
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Hi,

I found a 60mm Aspectem with the legendary 12.5mm EP on german eBay and luckily it was straight... otherwise I would have been very tempted...

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Will I need exit pupil of 7mm in a concert? baofeng Binoculars 11 Monday 5th October 2009 15:16
Magnification vs. Exit Pupil 01Foreman400 Binoculars 107 Saturday 19th September 2009 19:03
Relation of age and exit pupil Raybo Nikon 9 Sunday 29th May 2005 20:42

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