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Distant limits to optics? (1 Viewer)

E

edwincjones

Well-known member
Smaller binoculars are good for closed in areas,
larger (15x) are better for distant birds,
spotting scopes increases the reach, but how far?

What are the limits, in distance, for reasonable use of spotting scopes?

Are telescopes useful for further distances?

just asking for member's opinions-thanks
edj
 
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edwincjones said:
Smaller binoculars are good for closed in areas,
larger (15x) are better for distant birds,
spotting scopes increases the reach, but how far?

What are the limits, in distance, for reasonable use of spotting scopes?

Are telescopes useful for further distances?

just asking for member's opinions-thanks
edj
Click to expand...

Good question. It depends on two things, distance and atmosphere. I know: that sounds obvious. But a scope on a bird feeder at 30 feet can be a revelation (good lord, so that's what a Bluejay is all about!) and a scope at the shore, where a half mile can make all the difference between an ID and an "Ah well, I can live without. That will be another whatever. I'll keep trying."

Big difference, and mostly uphill. Give me a scope.

Mark
 
For birding? Atmospheric conditions and low light are the limiting factors at large distances. I use a scope with max. 50x, and in about 50 % of the situations, that much is already not useable any more. 90-100x is probably only useable 10 % or less of the time.

I guess you ask the question in relation to the discussion of the big binoculars for Costa Rica? http://www.birdforum.net/showthread.php?t=318155

I also wondered about the suggestions of big astro-telescopes. I don't know anything about this type of telscopes. Are they suggested because they allow higher magnifications? I don't think that anything above 100x is useable at all for Long-range terrestric observations, and 50-70x would be the upper limit for most of the time.

As for distance? I guess with 50x I can call divers and such about 3-5 km out in the lake, if conditions are good. Mags. of near 100x would perhaps add 1-2 km.
 
It depends on the steadiness of the atmosphere, and clarity. And the time of day and temperature conditions.
I have used 250x often at 3am across built up areas at a distance of 4.7 miles, resolving about an inch or less on a large clock face.
Using 120mm astro refractor of high quality.

During the day I have observed beautiful detail across water at 5 miles with a 150mm Maksutov at 95x.

With a world class 110mm f/30 refractor, corrected for 4 colours, photographs on film in the 1960s 0r 70s clearly show church guttering at 17 miles. Had a bird been sitting there it would have been shown.
A 16 inch compound scope has photos of the planet Mercury using an effective focal length of 1100 metres, from memory. An atmospheric prism corrector for false colour was used.

But mostly birding seems to be done in less than optimal conditions.

From space resolutions of 4 inches on the ground are shown at 200 miles using 70 inch aperture mirror optics. And aircraft photos show golf balls from 80,000 ft. With image motion compensation incredible detail is shown from 250ft at 500 knots.

The Nikon D5 with up to 3,200,000 ISO should bring in a whole new era of ridiculously unusual images. In video or stills.
 
edwincjones said:
Smaller binoculars are good for closed in areas,
larger (15x) are better for distant birds,
spotting scopes increases the reach, but how far?

What are the limits, in distance, for reasonable use of spotting scopes?

Are telescopes useful for further distances?

just asking for member's opinions-thanks
edj
Click to expand...

Hi Ed et. al.:

You have already heard about atmosphere and lightening. [The more info-starved among you should investigate thermal cells.] However, you can make things simpler for yourself by taking “distance” out of the equation. We can all see Saturn with our naked eye, although it is ~800,000,000 miles away. What you’re looking for is resolution?

The following is from my “ethereal” book, that I’m back to working on.

“ . . . The first thing one must decide is just what constitutes “good,” “better,” or “best.” Is it light grasp, aberration control, weight, glare suppression, color rendition, watertight integrity, near perfect collimation, or any one of a handful of other considerations?

Example: Two observers might ask if a certain binocular will provide a “good view” of Mount Rainier (It’s a Seattle kind ‘a thing). To the first observer, the question means, “Can I see mountain goats from my office on the 21st floor of the Columbia Center?” To the second it means: “Can I get a view of the mountain with at least 10 or so miles on either side?”

Both versions of “good” are valid and may represent the exact goal the observer has in mind. However, while one shopper has realistic expectations, the other expects a level of magnification and resolution that is unrealistic for a handheld binocular.” :cat:

Cheers,

Bill
 
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I'll skip all practical limitations and most of the ifs and buts and just relate objective diameter to visual acuity in theoretically perfect conditions.

If your eyesight is 20/10 you will not see any more detail beyond about 25x only reduce the field of view. With 20/20 vision you would need 50x to see the same level of detail. Star-gazing acuity is very variable but some would need to push the magnification to 200x to see equivalent detail if that was possible.

With an 82mm scope the corresponding figures would be 42x, 84x and 340x.

Just theoretical as I said.

David
 
typo said:
I'll skip all practical limitations and most of the ifs and buts and just relate objective diameter to visual acuity in theoretically perfect conditions.

If your eyesight is 20/10 you will not see any more detail beyond about 25x only reduce the field of view. With 20/20 vision you would need 50x to see the same level of detail. Star-gazing acuity is very variable but some would need to push the magnification to 200x to see equivalent detail if that was possible.

With an 82mm scope the corresponding figures would be 42x, 84x and 340x.

Just theoretical as I said.

David
Click to expand...



Hi David:

Theories have their theorists. Who do you believe? Rayleigh, Dawes, Sparrow, others. Why do so many stick with the math and ignore the visual acuity of the observer!? I prefer the in-depth studies of Mickey Mouse. I read it often while on my way the park bench to enjoy the tranquility of the birds. :cat:
 
As to actual birds.
I see Cormorants at 1 mile with 10x25 Docter.
Buzzards at 1/2 mile unaided eyes either flying fast or sometimes circling. They sometimes seem to be adults and smaller birds.
Buzzards at 2 or 3 miles with 18x50 IS.

With specially filtered H Alpha telescope at 32x, with over 2,000 days observations, I often see birds crossing the face of the Sun. They are simply inverted but that doesn't matter.
Herring gulls, I think, as we have many, are seen at maybe 2 miles or more.
There are single other birds, or several, and sometimes V formations of water birds?
These birds are probably seen up to 5 miles distant.
Looking up the dimensions if identified, one can get reasonably accurate distances and height as one knows the Sun's diameter very accurately and also the Sun's angular height above the horizon.

There are also many aircraft seen crossing the Sun. Mostly twin engine things like 737 and Airbus A320, but also 767s and others. It is fairly easy to get accurate distance and height here once identified.
 
WJC said:
Hi David:

Theories have their theorists. Who do you believe? Rayleigh, Dawes, Sparrow, others. Why do so many stick with the math and ignore the visual acuity of the observer!? I prefer the in-depth studies of Mickey Mouse. I read it often while on my way the park bench to enjoy the tranquility of the birds. :cat:
Click to expand...

Bill,

Someone on the forum once told me that binoculars always exceed the resolution of the eye and any differences I thought I might be seeing were due to other factors. Turned out to be total c**p and it was Dawes that helped me figure it out....while waiting for the birds to come along. ;)

David
 
typo said:
Bill,

Someone on the forum once told me that binoculars always exceed the resolution of the eye and any differences I thought I might be seeing were due to other factors. Turned out to be total c**p and it was Dawes that helped me figure it out....while waiting for the birds to come along. ;)

David
Click to expand...

Hi (comma) again, Dave:

I've seen enough of your posts to know you are a pretty sharp cookie when it comes to optics. I must say, however, that what your friend said was really not crap.

We can SEE more than we can RESOLVE! There are factors in play that are frankly out of the league of most to understand. World War II fighter pilot, and later test pilot, Chuck Yeager, claimed to be able to SEE enemies at 50 miles. I believe that was true. Mathematically, he couldn't resolve the planes, but he DID SEE them—because of those other factors.

Recently, I saw more evidence on an engineering list I frequent. The discussion centered around being able to see guy wires on a tower near an observatory. ALL recognized that, based on Dawes Limit, the wires were impossible to resolve. However, more than one of those luminaries admitted to having SEEN them at the given distance.

Long after we’re taking our dirt nap, someone—now in the cradle—will probably be able to explain exactly why (at least until the next generation proves him wrong) certain impossibilities are true. :cat:

Cheers,

Bill
 
Wires against a clear sky are typically seen when they have a diameter of less than one arcsecond.
Keen eyed observers have seen them with a diameter of less than 0.5 arcsecond.

Dawes limit applies to equal 6th magnitude double stars seen at high magnification with a high quality telescope about 4.5 inch aperture and a keen eyed observer. Nothing else.

All these limits of seeing basically stem from empirical data and vary enormously depending on the object.
Wires, black spots on white backgrounds, white spots on black backgrounds, Black lines on white backgrounds. These also depend on the eye, crowding etc. etc. Whether the eye is at 7mm or 2.5mm etc.

I go with actual observations by experienced observers and not other limits which vary from 0.5 arcseconds to several arcminutes.
 
Bill,

Even an apparently 'big' star like Betelguse is about 0.05 arcseconds when viewed by the unaided eye or about 1/12000th of the eye's acuity limit and we can certainly see objects way beyond that. That's not resololution as you well know (and is actually easy to explain). If your eyesight is very good many binoculars will be effectively resolution limiting and it's quite possible to spot it without the use of test charts. For most users the eye will be limiting and it will be necessary to stop down the objective to produce a 2.5mm exit pupil and do a boosted resolution test to determine those differences.

David
 
Not being a birdwatcher, when I first saw the birds flying over I thought they were herons, cranes or maybe Pterodactyls.
From my photos I was told that they are cormorants.
 
Binastro said:
Not being a birdwatcher, when I first saw the birds flying over I thought they were herons, cranes or maybe Pterodactyls.
From my photos I was told that they are cormorants.
Click to expand...

Oh good lord, and I listen to you, or Bill?? For optical advice? Reality just smacks you sideways compared to that. ;)
 
perterra said:
They are all the same anyway, no sweat.
Click to expand...

No they aren't.

I think birders might have different requirements for a scope than nonbirders. When birding at long distances birders rely on GISS, which I guess is a military term ("general impression of size and shape") spotters relied on to quickly ID WWII aircraft. But maybe birders actually came up with it. I've seen reference of 1920, and it was the cornerstone of Peterson's 1934 field guide. Anyway, with birds it's a gestalt thing, and it takes a long time to be any good at it.

Point being, the absolute resolution is maybe less important than just getting a good enough view. At closer distances you'll be looking for more than just an ID so that's where a better scope is most enjoyable.

Mark
 
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dalat said:
For birding? Atmospheric conditions and low light are the limiting factors at large distances. I use a scope with max. 50x, and in about 50 % of the situations, that much is already not useable any more. 90-100x is probably only useable 10 % or less of the time.

I guess you ask the question in relation to the discussion of the big binoculars for Costa Rica? http://www.birdforum.net/showthread.php?t=318155

I also wondered about the suggestions of big astro-telescopes. I don't know anything about this type of telscopes. Are they suggested because they allow higher magnifications? I don't think that anything above 100x is useable at all for Long-range terrestric observations, and 50-70x would be the upper limit for most of the time.

As for distance? I guess with 50x I can call divers and such about 3-5 km out in the lake, if conditions are good. Mags. of near 100x would perhaps add 1-2 km.
Click to expand...

Yes-you are right as my ? a spinoff of the Costa Rica thread.

The astro telescope is suggested due to it ability to use higher mag and greater light gathering due to size.

To ask another way, if a bird (not other objects) is too far away/too poor air conditions for a spotting scope; does a larger astro telescope help?

From the above responses the answer seems to be -- maybe a little but not much???

edj
 
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