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Practical magnifications

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Old Monday 26th February 2018, 22:44   #1
aeajr
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Cool Practical magnifications

I see 50 mm spotting scopes with 20-60 zooms and wonder if 50 mm is enough aperture to handle 60X. My wife uses a Nikon 10-22X 50 mm binocular and at 22X the image is still good but I can already feel it dimming.

I am now wondering if 60 mm can really handle 60X and deliver a reasonably good image.

I am shopping for a scope to handle 60X for long distance (1 KM or more) for bird identification and wondering if I need to go to an 80 mm to have enough light to support 60X.

What has been your experience?
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Old Tuesday 27th February 2018, 09:19   #2
Hauksen
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Hi Aeajr,

Quote:
Originally Posted by aeajr View Post
What has been your experience?
I've been using a Nikon ED50 with the 13x - 40x zoom eyepiece, and the 40x magnification, while usable, really gives a somewhat dim picture that's not entirely enjoyable unless there's plenty of light. I'd not expect 60x on a 50 mm scope to be of much value, but I haven't actually tried it yet.

From comparison to the Svarowski 65 mm and Leica 65 mm scopes, they are really much better at their highest magnifications than the ED50 at 40x. The even bigger Kowa 883 is yet another step up, but not as big as the first one.

Regards,

Henning
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Old Tuesday 27th February 2018, 10:04   #3
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Hi,

a good example of an ED doublet should be fine with a 1mm exit pupil (eg. 60x in a 60mm instrument). A normal achro will probably be soft due to longitudinal CA (and maybe other aberrations). Eye placement will take some more care than binoculars unless you habitually use telescopes.

Of course good light is needed, on a dark overcast winter day or in the shadow you will have to dial down the magnification a bit...

Joachim
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Old Tuesday 27th February 2018, 11:51   #4
aeajr
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Thanks for your comments. I look forward to input from others as well.
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Old Tuesday 27th February 2018, 14:24   #5
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Some of my friends use minus blue filters on their 80mm f/5 refractors or fringe killers, but probably not good for bird watchers.

The Nikon 10-22x50 has a lot of false colour at 22x, but I can just see Jupiter's equatorial belts and the gap between Saturn's rings and globe at 22x.
I just ignore the false colour.

A long focus refractor, for instance an old Japanese Royal 60mm f/12, showed 4 Saturn moons plus Iapetus.
It could easily be used above 120x.

I use the Acuter ordinary doublet 80mm spotter at 95x through window glass and have seen 4 Jupiter belts.

It depends how one tolerates aberrations and some loss of image quality.

Of course ED scopes and fluoride scopes are usually better.
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Old Tuesday 27th February 2018, 15:07   #6
Binastro
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I used my 123mm f/5.1 Jaegers doublet refractor at all powers from 16x to 210x.
However, my usual magnification was 80x with RKE eyepiece. It was also good at 105x with an Ortho and also 145x with Swift orthos.
I almost never used filters although I tried light pollution, selective wavelength, and contrast enhancing filters.

But trying to make intensity estimates on Saturn was difficult because of the aberrations.
It could be done, but not as reliably as with long focus instruments or mirror scopes.

The David Hinds doublets on the 120mm f/8.3 refractors could be used at any magnification, usually 250x or 160x.

I was brought up on refractors, my slightly younger colleagues on Newtonians.
They mainly hate refractors because of false colour.

I can use any type of scope and go with the flow as long as they are well made.
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Old Tuesday 27th February 2018, 15:39   #7
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If using 60x mag a lot in variable light levels, I would get a 85mm or larger quality scope. (Swaro or Kowa). Most comfortable scope at 60x is probably Kowa 883 or Swaro ATX 95.

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Old Tuesday 27th February 2018, 16:25   #8
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Personally I would recommend 30x max with 50mm and 40x max with 65mm but a lot of this is personal preference.

Lee
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Old Tuesday 27th February 2018, 17:32   #9
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What I want from the highest magnification is an image size large enough to allow an observer to see the smallest details the scope can resolve under ideal conditions. For a diffraction limited scope and an observer with 20/20 vision in daylight that will be a magnification corresponding to an exit pupil of about 1mm, or about 60x for a 60mm scope. In the real world that number will be lower if the observer has better than 20/20 vision (someone with the best human acuity of 20/10 would need only 30x) or if the scope specimen is not diffraction limited (almost certain in an under $300 spotting scope).
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Old Tuesday 27th February 2018, 17:55   #10
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You want not only detect a bird with 60x at a distance of 1km and you want identify it - good luck… The magnification is mostly influenced by the air-column between your scope (maybe a prime scopel or not), so that in only 'not many' days (and times of the light-Level) you can successful use these magnifications. E.g. in winter, when there's very cold air above a 'warmer' waterside sometimes you can use only 20x - similiar heated air above a street in summer.
And: if you'll use 60x, there has to be a very sturdy tripod with a very good panhead under your optic - enough exit-pupil or not.

best regards
Manfred (Zeiss DiaScope85 + MM4-60 and some Gitzo-stuff)
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Old Tuesday 27th February 2018, 17:59   #11
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I don't know many birders who like to use exit pupils under 1.5 mm except under special circumstances. I use a 27x fixed wide angle on my 50 mm, and a 30x on my 78 (or 82 mm scope). I have a 50x fixed wide that use on the 80 mm sometimes for higher power viewing. I also have a zoom that goes to 75x on the 82 mm, but I rarely find the magnification-for-brightness tradeoff worthwhile above ~50 or 60x. Frankly, as you know from my other posts, I'm not much a fan of high magnification, and in my birding experience, it is often the case that atmospheric limitations make magnifications above ~40x useless anyway.

You've gotten a fair number of replies from astro oriented telescope folks. In my experience, small exit pupils are less useful for birding than they are for astro applications (with which I have a fair amount of experience via my father, who has a passion for astronomy) because birding often doesn't allow for protracted observations of the exact same object/view (where the brain has time to "composite" details seen in a dim but usually fairly stable and sometimes even high contrast object). When birding, the subject is often moving relative to the background, changing shape, head orientation etc, and is often darker than or same contrast range as the rest of the scene. For that kind of viewing, I find that my eye/brain greatly benefits from the brightness of a more generous exit pupil, which seems to allow me more information or the information fast enough, to allow me to make sense of what I am seeing when dealing with a subject that is either only in view for brief moments or that is constantly moving.

--AP

Last edited by Alexis Powell : Tuesday 27th February 2018 at 18:02.
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Old Tuesday 27th February 2018, 18:56   #12
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In most things I agree with Alexis, but in this case I'm an enthusiastic user of "high" magnifications between 75x and 100x with my 92mm scope. In many cases those magnifications have made the difference between an ID or no ID of a distant duck, grebe, loon or gull on our local reservoirs. You just have to pay some attention to the air and water temperatures and try to time very long range scoping sessions for the favorable periods when air and water temperatures are close to equal.
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Old Tuesday 27th February 2018, 19:00   #13
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Alexis, I agree.

For me, with birds, I mostly look at crows that sit still on a chimney pot at 120m.
Also herring gulls.
They don't move for ages.
With the 90mm Skywatcher Maksutov the best power was 125x, the eyes are large and the birds huge. But it needed bright sunshine.

I have little experience of chasing moving birds with a scope, although I sometimes see Buzzards at night with the 18x50 IS.

I just don't observe in poor Seeing.
I learnt long ago not to bother if the Seeing is poor for astronomy.
I suppose birdwatchers are forced to observe in poor atmospheric conditions.

Also the strain of using the absolute minimum magnification for my eyes is not restful.
It has to a bit more for me.
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Old Tuesday 27th February 2018, 19:14   #14
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Lots of good advice here already, and I agree with it.

Water does make a difference. It allows for greater mag in most cases.

Like Alexis and Lee I think a 50mm will top out at 30x--for birding. Anything more and the loss of brightness and exit pupil will probably be a hindrance.

For 60mm, probably 45x.

80mm+ can easily go to 60x if the scope is good, but for birding you have to consider how much tripod you're willing to carry. Weight matters too and if you want to travel light...

Most of my birding with a top-notch 82mm scope (Meopta S2 30-60x) is between 30-45X. But if I want to see the rings of Saturn it goes to 60x just fine. Pretty cool. Anything more and it's probably time for a telescope and a big honking tripod.

Last edited by Kammerdiner : Tuesday 27th February 2018 at 19:28.
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Old Tuesday 27th February 2018, 21:34   #15
aeajr
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Cool

So many excellent comments. I am so grateful for your insights.

As noted, I have experience with astronomy. Exit pupils, atmospheric conditions, seeing, and the like are known to me as they apply to stars and clusters, the moon and planets. But my terrestrial experience is limited to binoculars of 8X32, 10X50 and 15X70, all moderate priced instruments. My wife uses 10-22 50 mm zoom binoculars.


So, drawing some generalized benchmarks from this discussion. Reasonable expectations for top useable mag per aperture could run:
  • 50 mm/30X
  • 60 mm/45X
  • 80 mm/60X

None of this is hard and fast. I understand that image quality for bird identification at distance will vary by conditions. I am just trying to level set my general expectations so I know what to buy to meet my Wife's expectations.

Personally, I don't worry about identifying that group of birds on the opposite shore of the bay at 1 KM away. But it drives her crazy if she can see they are there but can't identify them. Often we can't hike or drive over to the other shore. So, we will address this with optics.

Weight - While I want to keep weight reasonable so my Wife can use this on her own, we do most of our bird watching together, near the car, so I am not concerned about carrying a heavy scope or tripod through the woods. This spotting scope will be used within 50 meters of the car most of the time.

Thanks again to all for your posts. And I welcome hearing from others.
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Last edited by aeajr : Tuesday 27th February 2018 at 21:42.
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Old Wednesday 28th February 2018, 20:36   #16
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I pushed one particular unit of the many ED50 i have owned to ridiculous magnification using an 8x pair of binos as a booster of the 27x and he results were incredible,..sure the image was dim,and started to break,but was not a mush by any means..Not really remember But conditions were probably pefect ,with cold still air and perfect sun location ...this of course was a test and not something i would use to ID birds,not could my eyes go through the task for a long time,but is my experience to share...With the ED50 though,in the field,The 27x was already letting me down in dimm days ,and more than that , the real FOV was too small for my birding..I prefer less magnification in that particular scope and a brighter image/more field of view..with my current scope,a really good fluorite 82 mm specimen(has to be diffaction limited with the image it delivers)I use the max magnification of my zoom all the time,and i tend to forget i am there(56x)..I also used a nikon 7mm wide eyepiece ,that delivered 67x and loved it,but my adapter was worned out and decided to sell the eyepiece..i am now looking for a 5mm astronomy eyepiece(bst explorer?) to get 90x
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Old Wednesday 28th February 2018, 21:03   #17
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Quote:
Originally Posted by mayoayo View Post
..with my current scope,a really good fluorite 82 mm specimen(has to be diffaction limited with the image it delivers)I use the max magnification of my zoom all the time,and i tend to forget i am there(56x)..I also used a nikon 7mm wide eyepiece ,that delivered 67x and loved it,but my adapter was worned out and decided to sell the eyepiece..i am now looking for a 5mm astronomy eyepiece(bst explorer?) to get 90x
I don't doubt that a cherry TSN-823/4 will do a very nice 90x. The single best corrected spotting scope that has come my way was a TSN-823 that was around 1/8 wave, better than diffraction limited. I've yet to personally see or hear of a TSN-883/4 specimen quite as good as these occasional ultra-cherry 823/4s.
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Old Thursday 1st March 2018, 21:57   #18
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I was thinking to get a 6mm eyepiece to go to 75X, but having reached 67X with ease,I want to go for the 90X to try .
I dont know if my unit is super cherry,but it very well might be..It is not a case of luck either that i ended with a good unit, since I have bought and sold at least four units of the 820 prominar. The one I had for the last few years was a pretty nice scope,no problems i could detect in the star test ,and excellent image quality.Then i bought this other unit and it was pretty obvious that it was a better scope..Star test looked similar to my casual approach ,perhaps better defined rings outside focus..But testing resolution in small print and street signs at 75 to 150 meters ,the advantage was clear...Then in the field ,in a more subjective and intuitive way I clearly see the extra amount of detail that I was missing . This unit is older and the coatings are a bit different..The inside lens coating is green/purple,just like the other unit,but the outside lens has a blue coating reminiscent of the TSN·3 series..the newer unit had a pink salmon outside coating..I dont see any difference in contrast, but if the coatings were upgraded they were probably improved somehow...
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Old Thursday 1st March 2018, 22:32   #19
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I would expect to be able to use 160x with such a scope on suitable targets in bright light.
And 120x routinely.
Of course I would choose to observe in good Seeing conditions.

I think that any decent scope around 80mm should take 90x with ease.
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Old Friday 2nd March 2018, 15:37   #20
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The last test i did was at 75m aprox..about 250 feet..I was looking at a bulletin board in a corner of a street,from my living room,..There are adds,and public announcements that the city post there ,Small to very small print..I had two scopes of the exact same model,bot more than decent ,magnification 67x ..performance was very similar and most of the print that i was able to read with the best unit I was also able to read with the other,but it was that point,that limit,with very small print,when one scope just didnt cut it..no matter how much i concentrated or focussed back and forth. One of the scopes had reached its resolution limit there,while the other was still capable to resolve...Maybe at 90x i could have read the print with both scopes,but the image of the best scope is always going to be sharper and more clear..This is the reason I am now planning to get more magnification,but i really didnt feel i needed it with the other unit.
High magnification is going to show the true potential of a good scope but is also going to show the limitations of a "decent" scope.I also wanna add that this two possibilities (potential/limitation),in my view,are not necessarily extremes far apart ,just a small improvement of quite similar results
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Old Friday 2nd March 2018, 21:13   #21
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I find I normally keep the exit pupil at ~1.5mm or larger for normal observations. However, I find I can use my Nikon Fieldscopes at 1 mm and see more detail than at 1.5mm, provided the atmospheric conditions allow the use of high magnifications. In fact, I find even higher magnifications than 60x on my EDIIIA or 75x on my ED82 can be useful to clinch an ID.

I should perhaps add that my Fieldscopes all seem to be cherries. But the old Nikon Fieldscopes seem me to show less variation than many other scopes anyhow. I've got access to 5 60mm Fieldscopes, and while one seems to be a bit weaker at 60x and one just a tiny bit better than the others at that magnification, they're all remarkably similar and definitely useable at 60x. That's more than I can say of most other makes.

Hermann

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Old Friday 2nd March 2018, 23:22   #22
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Ed,

See if http://www.pt-ducks.com/cr-telescopes.htm helps you. My goal is to read codes of duck nasal saddles at highest distance possible...
I add that I'm enjoying the Swaro X95 with the extender - will update the site when will have time.
From memory, I remember that some years ago I looked through a Nikon EDII 60 at 60x and noticed the image dim (although high quality one...), that never noticed much with the ED78 at 75x or the Optolyth 100 at 90x.
So if weight and cost is no problem get a X95 with the extender...
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Old Sunday 4th March 2018, 08:58   #23
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Another factor to take into account when using higher mags is stablity; this can either translate into the quality of the head and tripod, or the weather, particularly wind. At 60x and above, it takes very little movement to spoil the image.
Although i have the extender for the Kowa 883, i tend to take it out now only on the stillest of days; the starting point of 40x is often too much. As time has gone on, i've become much more a fan of the lower mags when using a zoom, as 25x to 35x is often enough, except when trying to read rings etc.
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Old Wednesday 7th March 2018, 14:58   #24
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I have a 20-60X 80 mm Celestron Trailseeker coming Friday so we will see how it works. Will let you know my experience with this scope.
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