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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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I've always been a bit uneasy when I'm asked how one of my binoculars rates on edge sharpness, because I've found it so difficult to quantify. Describing how a star image looks is very subjective, and systematic double star measurements just isn't for me.
Exactly one lunation ago, I was struck at how clearly unsharp the Full Moon is when put at the edge of field. Here was an object that enabled a much more clearcut distinction between "sharp" and "unsharp", and additionally it has a built-in scale since it measures almost exactly half a degree across. So today, with the full Moon conveniently located in an otherwise clear sky, I decided to do a little test.
My 8,5x44, mounted on a tripod and aimed at the test target, quickly revealed that the outer 1 degree of the 8.2 degree field is quite unsharp, especially the outer half degree which also shows a significant distortion - at the top and bottom, the Moon if flattened, and at the right and left edges it is narrower - is this astigmatism? The area between 1 and 1.5 degrees away from the edge is sharp, though less so than at the center. If I had to summarize the result in just one figure, I'd say that 75% of the field is sharp.
Next up was my 20x80 which has a 3.5 degree TFOV. The apparent, subjective level of unsharpness at the edge is decidedly less than in the 8,5x44. There was significantly less distortion at the edge too, though there was still a very slight amount of it. I found that about 75-80% of the field is sharp; when put right at the edge, some part of the Moon was still sharp. When put one lunar diameter away from the edge, it is quite sharp, just a touch less so than in the center.
I ended my evening by wondering, again, as I have for the past 25 years or so, at how much more pleasing the stars look when a poor limiting magnitude is caused by moonlight rather than haze or dust.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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Mark ,
I read this with great and special interest .
What you describe is very similar to what I have seen through a variety of binoculars , and akin to something I posted to this very forum exactly one lunar month ago , when I described our closest celestial as being " at it's worst at the moment " .
Over the month which seems to have passed so swiftly since , when skies have been clear , I have , on occasion , tried to re - check " edge performance " of the little collection of optical instruments I have acquired or had access to .
One of various points I made mental notes of , was that in ALL cases , edge performance appears more agreeable when using stars as criteria , rather than a brightly shining moon .
My gut feeling is , this is probably a result of effects of sunlight , but clearly , more detailed studies and explanations from those better qualified would be more beneficial to the majority of readers .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Quote:
One of various points I made mental notes of , was that in ALL cases , edge performance appears more agreeable when using stars as criteria , rather than a brightly shining moon .
Kenny, I would have to agree with this completely.
Quote:
My gut feeling is , this is probably a result of effects of sunlight , but clearly , more detailed studies and explanations from those better qualified would be more beneficial to the majority of readers .
Do you mean sunlight (reflected of course) as opposed to starlight? Hmm, doesn't inspire any explanation with me, or else I'm completely not understanding what you mean. If I would have to guess for an explanation, it would be:
1) when you want to look at a star, you normally put it in the center of view. It also takes some effort on the eye to look at the edge - more so for wider AFOV. So we're not that aware of stars at the edge as those in the center.
2) with all but the most grossly distorted stars, the brain is still telling us "that's a star" and interpreting it closer to how it should look than how it actually does. With the Moon, we're normally looking for craters and other small detail, and when those suddenly go missing it really stands out.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Several things are going on when you use the moon to test for sharpness at the edge of field. And I can understand why one would say "edge performance appears more agreeable when using stars as criteria , rather than a brightly shining moon".
For one, you are seeing a much larger area in your binocular filed of view taken up by the distorted target near the edge. Maybe on the order of 100x more moon is seen distroted than a single star distorted.
Also, the moon is so very bright that not only are you dealing with seeing lens edge distortions that cause a loss of sharpness, but also you are seeing the affects of chromatic aberration. Even in a binocular that has decent edge sharpness, you are bound to see some affects of CA when placing the moon near the edge to view sharpness.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Quote:
the moon is so very bright that not only are you dealing with seeing lens edge distortions that cause a loss of sharpness, but also you are seeing the affects of chromatic aberration. Even in a binocular that has decent edge sharpness, you are bound to see some affects of CA when placing the moon near the edge to view sharpness.
Ed, thanks for your input. There certainly was more CA at the edge than in the center, but not that much. The level of smearing of detail was orders of magnitude larger than any color fringing I saw. In the 8,5x44 the outer half degree was so fuzzy it was as if I was looking at the Moon through a sheet of polythene. I do have to say that part of this, but not the larger part, was due to field curvature as it improved upon refocussing.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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< Do you mean sunlight (reflected of course) as opposed to starlight >
Yes Mark , the reflected sunlight , of course .
And I think both yourself Mark , and Ed , have helped to explain at least part of the phenomena ( if phenomena is the correct classification for this )
Another thing I notice is that these lunar image distortions are variable , depending on phase ( shape , size and effective brightness ) of the moon , it's relative proximity to earth , it's declination , the time of year and last but certainly not least , sky and seeing conditions .
For presumably a combination of most of these reasons , the full moon of a month ago ( 19 August 2005 ) as seen from MY LOCATION just after sunset , presented the WORST case I can recall .
So alarmingly bad was it , that I was actually WORRYING most of the night after about the quality of my Zeiss scope , Zeiss binoculars and even about the normally astronomically peerless ( amongst my modest , motley collection ) Captain's Helmsman .
I would say Ed's estimation of " 100x more moon disorted than a single star distorted " is very conservative to say the least , considering even a 10x binocular increases apparant area by 100x , and the full moon occupies roughly 0.2 square degrees to the naked eye , whilst stars are immeasurably tiny points of light without the glare and optical aberration factors added , but , with advanced apologies for the obvious puny pun , I will say point very well taken .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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DJB
Pooh-Bah
Reged: 02/23/05
Posts: 1269
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Hey Kenny,
Lost my post, so I'll do it again. I am having trouble duplicating the 0.2-SD for the moon with the naked eye.
A degree (or 1/2 degree) is still a degree at 1X (eyes), 10X, 100X, etc.
Perhaps I missed your point entirely.
Thanks Kenny,
Dave.
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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Dave ,
Sorry if my point was misunderstood , or poorly described .
What I OUGHT to have said , was that when an object is magnified by a factor of 10x -- the viewed object APPEARS to occupy an area which would be 100x that of the same object viewed naked eye .
For example , let's image we were looking through a 10 x 50 binocular at something really interesting , such as a house brick in a chimney stack , measuring 9 inches x 3 inches in reality , from a distance of 25 yards .
In reality , the area of that brick is 27 square inches .
But when seen through the binocular , the brick APPEARS to measure 90 inches by 30 inches ( ten times longer and ten times wider ) , which gives an area of 2700 square inches .
The APPARANT area seen is therefore 100x larger than the actual area , not 10x " larger " as some people seem to think .
THAT was the point I was trying to make .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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johnno
scholastic sledgehammer
Reged: 05/03/04
Posts: 807
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Kenny,
That is a very good explaination,
unless I have misunderstood what you are saying,it would explain why we cant fit 10,.5 deg moons in a pair of 5 deg fov binoculars.which in theory should be possible.
Please correct me if I am wrong.
Regards.John
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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John ,
I appreciate your commendation , but we should STILL be able to actually fit " about ten " full moons into a binocular TFOV of 5 degrees .
I say " about ten " because the actual diameter of the full moon is slightly in excess of 0.5 degrees MOST of the time.
All I am saying is that those ten ( or 9.5 or whatever APPARANT full moons ) will actually APPEAR to be 100x larger in terms of area .
If you look through just the left eyepiece with your right eye , and look at the actual moon naked eye with your left eye at the same time , you ought to see what I mean .
Regards , Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Quote:
< Do you mean sunlight (reflected of course) as opposed to starlight >
Yes Mark , the reflected sunlight , of course .
And I think both yourself Mark , and Ed , have helped to explain at least part of the phenomena ( if phenomena is the correct classification for this )
sorry Kenny, despite having helped to explain this phenomenon to you, I actually still don't have a clue what it is you're trying to say about reflected sunlight...
Quote:
Another thing I notice is that these lunar image distortions are variable , depending on phase ( shape , size and effective brightness ) of the moon , it's relative proximity to earth , it's declination , the time of year and last but certainly not least , sky and seeing conditions .
This begs for a follow-up project, Kenny! Go for it! 
I personally selected the full Moon because then at least it is round, which helps in repeatability of the test along different position angles along the edge of view. And I did my test when the Moon was low in the sky (about 15 degrees altitude) because then with my tripod-mounted binoculars it is easiest to look straight through them rather than slightly looking up.
Quote:
For presumably a combination of most of these reasons , the full moon of a month ago ( 19 August 2005 ) as seen from MY LOCATION just after sunset , presented the WORST case I can recall .
Well, from memory I would have to agree that the August full Moon looked worse than what I tested this weekend. But for lack of actual test results I'll never be able to compare them now.
Quote:
I would say Ed's estimation of " 100x more moon disorted than a single star distorted " is very conservative to say the least
Sure, you're quite right. But for me the Full Moon Test cannot strictly be compared to looking at images of stars. I'm sure if we could scientifically measure the distortions using both methods, they would give the same result. I'm also quite sure Ed's double star measurements give a much more meaningful result since you get a curve rather than a yes/no answer. But the full Moon test to me excells in ease of the procedure (this especially opposed to double star measurements) and relatvie freedom of the result from interpretation (this latter especially opposed to estimating the sharpness of a star).
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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< I actually still don't have a clue what it is you're trying to say about reflected sunlight... >
Don't worry about this Mark , you're not alone .
Neither do I :-)
Regards , Kenny
--------------------
If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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DJB
Pooh-Bah
Reged: 02/23/05
Posts: 1269
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Hi Kenny,
Thanks for the replay. Sorry, but I must admit that I entirely missed the point that you were driving home.
Point is well taken, and the same holds true in microscopy as well, as a friend of mine and I used to have frequent discussions about it on our walks home from Jr. High. We didn't have school buses in West Virginia back then.
Regards and take care,
Dave.
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johnno
scholastic sledgehammer
Reged: 05/03/04
Posts: 807
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Hi all,
Kenny,
Thanks for the further explaination,I will try your little experiment with the moon later tonight,(if the clouds clear),I seem to remember a discussion as to how many moons will fit a given fov,some time back.
I have often thought though,theres no way could I fit 9 moons in my 4.5 deg Binos,when holding a full moon in the centre fov,should be an interesting test.
Regards.John
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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John, it would be interesting to know how your subjective impression is when the Moon is not in the center, but at the edge of the fov - and even then it might make a difference depending on where on the edge you put it.
Have you checked that this is not due to the real TFOV being smaller than advertised?
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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johnno
scholastic sledgehammer
Reged: 05/03/04
Posts: 807
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Hi Kenny,
I tried your little test tonight,with a little bit of difficulty.
I tried the Russian 20x60s with a claimed fov of 3.5 deg,only problem is,the Moon is blocked TO MY LEFT EYE,due to the length of the barrel,same problem with the 15x70 Barska's.
I then used a pair of 10x42 roofies,with a claimed 6 deg fov,and could then see the Moon through the left barrel,with the right eye,AND the Almost full moon with the left eye,the size difference is very apparent,as you say.
It still SEEMS,to ME,that I could NEVER, fit 12 Moons in the 6 deg fov,it looked like I could fit 6 easily,but then, it could be my eyes playing tricks,or simply a quirk of,optic trickery,or maybe I am just thick,(my dear wife agrees with this one).
The one thing I did learn to my amazement during your little test,that it is a really good way to test EDGE SHARPNESS.
With the Moon in your right eye through the left Barrel,and moving the binoculars to the right,(so I can see Moon NAKED, with my left eye,really shows the edge sharpness dropping off as you move the Binos across.
I had allways tried this test,with both eyes through both barrels,and due to blackouts when trying to look left or right,it was never quite an accurate test.
So,if,I have learn't anything else,I have at least learnt a good method for testing edge sharpness.
No Doubt the CN NAUGHTY WORD GNOMES will edit this post due to the disgusting word NAKED,which is WORSE than the word I used in an earlier post,which was a derivative,of a hole in the ground,used to collect water for cattle.
Please forgive me again.
Regards.Kenny
John
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johnno
scholastic sledgehammer
Reged: 05/03/04
Posts: 807
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Hi Mark,
I do agree,often the advertised fov,can be a little inflated,but as you can see in my reply to
Kenny,I think it is a quirk of Optic/eye trickery.
If I look through a pair of binoculars,of a given fov,I SHOULD see approx twice the amount of moons as the fov SUGGESTS,however I have allways found,if I move the Binoculars to put the Full moon right on the edge of the fov,in my left eye,and calculated how many more moons OF THIS SIZE, it would take to fill the view,to the edge of the vision in my right eye,it NEVER appears,to fit in with the binoculars supposed fov.
I hope I haven't confused you Mark,its a very difficult thing to describe.
Regards.John
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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John,
I was playing around doing exactly the same thing just two nights ago, comparing the view thru one eye at the binocular to one eye unaided. It naturally followed that I tried to estimate the number of moons it would take to fill the view. Any attempt to use the image to measure the field of view is at best I would say an extremely difficult method of guesstimation. I would put probable error at maybe 10%-20%. Although this started out as laid back armchair viewing, knowing the true field of view, I could not even get within 10% with a fine tuned slow motion adapter. So it's not the variation in the FOV (although we know ture vs published differs), it's the human behind the eyepiece trying to guesstimate the number of small items that will fit across a large space.
This little proof provides sufficient reason to question most observers estimates of distance out from center or distance from edge. Values typically reported as 60% and 70%, if not measured but estimated, may vary from actual by quite a bit. At least Mark's method of using the full moon provides a means of placing a distinct form of measurement in the field of view.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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Johnno ,
Thanks for your posts and for at least TRYING a little test which I suggested .
I've tried to get members of forums to try little two minute tests for themselves before , but very few ever seem to take the trouble , so I really appreciate that .
By the way John , you are not alone if you feel you might be a victim of optical trickery .
Until about 3 years ago , I was CONVINCED that a 10x binocular did NOT actually magnify the full moon by a factor of TEN ! --
Yes I KNEW it did magnify 10x ( or thereabouts -- it is not always a precise figure actually ) , but if I hadn't , and had never looked through a binocular before , and had no idea what magnification it was supposed to have , without confirming with the " one eye naked " test , I would have SWORN it was only making the full moon about 3 or 4 times BIGGER !
Perhaps it's a family trait . At the time , I asked one of my daughters ( then aged 20 ) to ESTIMATE how many times bigger the moon looked through through a 10 x 50 binocular than with naked eye , after taking just a quick glance through the binouclar .
I remember my daughter saying " about 4 or 5 times bigger " !
Details of the good old " tape measure test " are to be found somewhere amongst the CN bino archives , so perhaps EdZ can dig it out , or re - iterate .
That said , because of Johnno's impressions about how many moons he thinks he could fit into the field of view , this has become a triple - issue thread now .
All three interesting , but not necessarily linked .
Issue 1. Does using the FULL moon as a test target affect impressions , estimations or indeed actual RESULTS of edge performance , compared with using stars ?
Issue 2 . Why does an object when magnified by a binocular to say 10x it's linear dimensions , not actually appear to do so unless or until one measures it ?
Issue 3 . Why does a field of view which OUGHT to be capable of " containing " a certain number of spherical objects , such as a full moon , not APPEAR to be of sufficient area to do so ?
By the way , if anyone is doubting my statement about a 10x binocular magnifying the area of a rectangle by 100x , just try using the gable end of a large building as the target , and with one eye naked ( or with eyeglasses worn ) and the other looking through one eyepiece of a binocular , now just imagine if it would take exactly ONE tin of paint to paint the gable end if it were the size as seen by the naked eye , then how many tins of paint do you think it would take to paint the gable end if it were the size as seen through the binocular ?
Ten -- or one hundred ?
Regards , Kenny
--------------------
If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Quote:
This little proof provides sufficient reason to question most observers estimates of distance out from center or distance from edge. Values typically reported as 60% and 70%, if not measured but estimated, may vary from actual by quite a bit.
This is the single most important reason why till now I have refrained from giving quality descriptors for my binoculars as a percentage of field of view. I just know I'm not good at estimating distances, and the fact that the FOV is round doesn't help.
Quote:
At least Mark's method of using the full moon provides a means of placing a distinct form of measurement in the field of view.
Exactly, and I also think that when one is satisfied with a sharp/unsharp distinction - as in "this binocular is sharp over XX% of the FOV" - the Moon is an easier test object than a star.
--------------------
Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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