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Light transmission of spotting scopes

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Old Sunday 8th March 2020, 16:38   #1
BoldenEagle
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Light transmission of spotting scopes

Hi to all,

is there data of light transmission measurements for top quality spotting scopes anywhere available?

Often manufacturers announce light transmissions for binoculars (at least if it is >90%...) but rarely for spotting scopes.

This may be stupid question but I was just wondering; which would be more important with spotting scopes with objective lens 82mm or more; having very good light transmission or just bigger aperture (by bigger objective)? For example; are there so big differences between light transmission performances with alpha brand scopes, that it could turn scope with smaller aperture to have brighter image than scope with bigger aperture (using same magnifications)?

Juhani
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Old Sunday 8th March 2020, 16:59   #2
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Yes Juhani, House of outdoors has transmission values for some scopes. Swarovksi seems to do very well.

Perhaps the most important thing with a spotter is getting a good sample. Even with expensive brands there is variation.

Your best bet is to try and star test a decent brand and go from there. Bigger objectives will be brighter and have more resolution. It all depends what you want the scope for.

https://www.houseofoutdoor.com/verre...n-vergelijken/
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Old Sunday 8th March 2020, 22:00   #3
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Hi,

with spotting scopes the transmission is in my opinion not so important, because aperture always wins. Transmission would be interesting at high magnifications, when the exit pupil is smaller than your eyes pupil. But then 5mm more aperture will beat any advantage of better coatings.

Also the best coatings won't help if the instrument is not able to deliver a sharp image at high magnification - which is unfortunately not unheard of even for alpha brands. So the most important advice is to personally test the example you buy. It should at least deliver a sharp image with an easy to find best focus at its maximum magnification - a star test with an artificial star at 30m or above will of course be better.

Joachim
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Old Monday 9th March 2020, 08:05   #4
BoldenEagle
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with spotting scopes the transmission is in my opinion not so important, because aperture always wins. Transmission would be interesting at high magnifications, when the exit pupil is smaller than your eyes pupil. But then 5mm more aperture will beat any advantage of better coatings.
I was wondering partly because I'm still considering new scope to replace my Leica Apo Televid82 but increasing the aperture comes with the fact that you have to pay a lot more money for it. So that basically (and also with some ergonomic properties) leaves Swarovski ATX95 and Zeiss Harpia95 out of question. And as I have read on this forum, Harpia's aperture shrinks in the wide end of zoom so that bigger objective advantage is partly lost. So judging by that, Harpia doesn't feel a good choice for me.

So, the choice between scopes like 82 mm objective (mainly Meopta S2 82, Nikon Monarch82) and Kowa Prominar 883 is at the peresent most relevant. Of course the Kowa has the aperture advantage but is that kind of difference (82mm objective at 60x mag. 1.37mm aperture vs 88mm objective at 60x mag. 1.47mm aperture) so significant that one can ignore the transmission differences between those scopes?

For what I found, Kowa Prominar has not so good light transmission when compared to Meopta S2 82, which seems to have really stunning light transmission (about 92% !) with the 30-60x WA eyepiece. I didn't find light transmission values for Kowa's newer 25-60x eyepiece, does it have better performance than the older 20-60x eyepiece, which had light transmission value just about 72%? (Thank you "dipped" for the link!).

Of course the difference between aperture is bigger when Kowa zoom is 25x mag. vs Meopta 30x mag. because Kowa also has the bigger objective lens. 30x minimum mag. feels quite big for my use (birding 99%) and about 20x would be ideal (there are days when even 25x mag. feels too much because of heat vibrations in the air when watching distant raptors).

I subjectively tested the Meopta S2 82 comparing with my Leica Apo Televid 82 and there was no clear difference in image brightness between them. The Meopta was with 20-70x eyepiece and that combination seems to have about the same transmission value as Leica so that would make sense. I definitely have to check the Meopta again with the 30-60x WA eyepiece. I haven't found Nikon Monarch82 transmission values anywhere, it would be interesting to know.


Juhani
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Old Monday 9th March 2020, 11:32   #5
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There's a simple mathematical way to predict how much higher light transmission is required from a smaller scope to equal the image brightness of a larger one. Square the diameters of the scope apertures and then find the percentage difference between them. For 82mm vs 88mm that would be about 15% (6,724 sq. mm vs 7,744 sq. mm). That's how much higher the light transmission of the 82mm scope needs to be above the transmission of the 88mm scope for the images to appear equally bright in both scopes.

Last edited by henry link : Monday 9th March 2020 at 15:16.
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Old Monday 9th March 2020, 12:03   #6
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Hi,

for comparing the amount of energy going through the objective aperture, you need to compare the squares of the radii or diameters...

The Harpia does indeed stop down the objective focal ratio at the low magnification end in order to reach a uniform apparent field of view of 72 degrees. Conventional scopes keep the focal ratio constant and use a significantly narrower apparent field of view at the low magnification end. The loss of light of the Harpia 95 vs an ATX95 was visible on a rainy day just before sundown at magnifications beyond 40x in one test on a german birding website:

https://translate.google.com/transla...vski-atx-30%2F

I think that is a small price to pay in very limited situations for a significantly wider field for all magnifications below the maximum...

Once again, testing the example you want to buy is most important to get a good example. My old TSN-3 body is not even multicoated but has a fairly good figure. We had it out on a bird walk in rainy weather together with a so-so 883. The 883 was of course brighter at the 30x the owner had wisely chosen, but the TSN-3 at 52x was very sharp (maximum with my Opticron zoom) and had people queuing to get a good view.

Joachim

Last edited by jring : Monday 9th March 2020 at 13:41. Reason: speling
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Old Monday 9th March 2020, 13:50   #7
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Moi RohkeaKotka,

Besides the above, the aperture of scopes is not always as marked.
I think that a Swarovski 95 has a 95mm aperture.
But the Harpia may have a 95mm filter thread and the maximum aperture may be 93mm.

I think that the Apo Televid 77 was about 75.5mm aperture.

Additionally there may be internal stopping, although I would hope not with top scopes.

Some 127mm Maksutovs may be 118mm true aperture, besides the light loss due to the secondary.

The convention in the past with astro refractors was that a 3 inch diameter scope or 76.2mm was about 73mm aperture.
But a clear aperture scope was as stated.
The diameter of the objective of a 3 inch diameter objective was 3 inches, but this did not allow for the cell holding the objective.
My 5 1/8 inch Jaeger refractor is actually 123mm clear aperture. Jaeger supplied the cell, and I had it custom made into a scope.

More important is to get a good example of a scope, as stated above.
Then there is the transmission of the eyepieces.

In addition, how the scope behaves with temperature variation, Also the colour of the scope etc.

Maybe a 100mm ED Skywatcher astro refractor would do better or a 120mm ED even more so.

Terveisin,
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Old Monday 9th March 2020, 14:52   #8
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Binastro is right about apertures not always being on spec. I measured 93mm for the Zeiss Harpia's clear aperture and only 80mm for the "85mm" Zeiss Gavia. The Harpia also has the transmission disadvantage of unusually complex optics, with no fewer than 32 glass to air surfaces.

Another problem with loss of clear aperture that affects scopes with moving prism focusing is a gradual loss of aperture as distances decrease below about 30 meters. In some cases the scope may be effectively stopped down 15-20% at nearest focus.

Last edited by henry link : Monday 9th March 2020 at 15:11.
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Old Tuesday 10th March 2020, 10:39   #9
BoldenEagle
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Quote:
Originally Posted by henry link View Post
There's a simple mathematical way to predict how much higher light transmission is required from a smaller scope to equal the image brightness of a larger one. Square the diameters of the scope apertures and then find the percentage difference between them. For 82mm vs 88mm that would be about 15% (6,724 sq. mm vs 7,744 sq. mm). That's how much higher the light transmission of the 82mm scope needs to be above the transmission of the 88mm scope for the images to appear equally bright in both scopes.
If the Kowa 25-60x eyepiece is not clearly brighter than the old one, Meopta with the 30-60x WA eyepiece has 27% better light transmission (about 72% vs about 92%). So judging by that, Meopta should be brighter over Kowa. Have to test that thing also next time I'm comparing them.

I still hesitate getting Meopta because of it's helical focuser. Certainly if I would pay about nearly 4k for a scope (ATX or Harpia), I would like to have dual focuser (and everything else to be about perfect)...The helical focuser of ATX was though better than I expected because it was enough slow and extremely smooth, I'm not that sure of the Meopta but have to check that thing too. Nevertheless Kowa's focusing system is strong advantage for me and if good sample is found, I would probably choose Kowa IF brightness is not that much behind Meopta with the newer zoom eyepiece.


Juhani
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Old Tuesday 10th March 2020, 13:03   #10
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Always keep in mind that these calculations depend on the accuracy of the light transmission measurements you are relying on. Old measurements might not represent current production or they may be wrong for some other reason.

For instance, if you are using Kowa measurements from Gijs 2010 group test notice that all the scopes in that test had relatively low transmission. The test itself might have been off for some reason and/or Kowa's coatings may be better now, ten years later.

I don't think the 25-60x Kowa zoom is likely to have higher transmission than the 20-60x, provided the same coatings are used on both. The 25-60x has 14 glass to air surfaces, the 20-60x only 10. If I were buying a Kowa 883 today I'd get the astro-eyepiece adapter and use a Baader Hyperion Zoom. I compared my Baader zoom to the Kowa 25-60x on my astronomical refractor about 3 years ago and preferred the Baader. I thought it had a brighter image and its highest magnification has the small advantage of being 63X on the Kowa scope instead of 60x.

Last edited by henry link : Tuesday 10th March 2020 at 13:22.
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Old Tuesday 10th March 2020, 13:22   #11
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Always keep in mind that the light transmission measurements you are relying on may not represent current production or may be wrong for some other reason.

If you are using Kowa measurements from Gijs 2010 group test notice that all the scopes in that test had relatively low transmission. The test itself might have been off for some reason and/or Kowa's coatings may be better now, ten years later.

The 25-60x Kowa zoom is unlikely to have higher transmission than the 20-60x, provided the same coatings are used on both. The 25-60x has 14 glass to air surfaces, the 20-60x only 10. If I were buying a Kowa 883 today I'd get the astro-eyepiece adapter and use a Baader Hyperion Zoom. I compared the Baader zoom to the Kowa 25-60x on my astronomical refractor about 3 years ago and preferred the Baader. It had a brighter image and its high magnification would be 63X on the Kowa scope.
Yes I just noticed that in another thread ( https://www.birdforum.net/showthread.php?t=280384 ) there was 85% transmission confirmed by manufaturer for the Meopta S2 82 + 30-60x WA, so the difference with Kowa is not that obvious as I declared on my earlier post. Anyhow, if transmission value of Kowa ( Gijs 2010 test) is to be trusted and related to newer Kowa zoom eyepiece, there is still about 18% transmission difference between Meopta and Kowa, that puts those scopes about equal in image brightness when coupled with aperture effect.

I think that if there is very big transmission difference between those two scopes, it would be obvious when I directly compare the two. If there is not clear difference, probably it won't be that crucial to me.

Juhani
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Old Tuesday 10th March 2020, 15:39   #12
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Quote:
Originally Posted by jring View Post

The loss of light of the Harpia 95 vs an ATX95 was visible on a rainy day just before sundown at magnifications beyond 40x in one test on a german birding website:

https://translate.google.com/transla...vski-atx-30%2F

I think that is a small price to pay in very limited situations for a significantly wider field for all magnifications below the maximum...

Joachim
Hi Joachim

That seems a strange result because isn't Harpia 95 at full aperture from 40x and higher magnifications?

Lee
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Old Tuesday 10th March 2020, 16:27   #13
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It says, brightness equality from 40x upwards.

But the complex Harpia probably doesn't take high powers as well as the Swarovski.

With spotting scopes other factors are how black the inside is, and how good the baffles.

Frankly, I think that with regular top scopes there isn't much difference in brightness.

I would go with a 150mm Maksutov for high powers if one doesn't need to carry it or use it in the rain.
Or a large astro refractor.

B.
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Old Tuesday 10th March 2020, 17:11   #14
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Hi Joachim

That seems a strange result because isn't Harpia 95 at full aperture from 40x and higher magnifications?

Lee
Quote:
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It says, brightness equality from 40x upwards.

B.
Hi,

indeed it does - sorry for reading too fast. But my main point was that just before sundown on a rainy day is not the usual observing situation for most and a little bit more light in the Swaro and thus maybe 5 or 10 minutes less observation time with the Zeiss in that rare situation are a small price to pay for a much wider field of view which you can enjoy every day.

This obviously assumes equally good examples of both scopes - I would certainly not blame the fact that this single review finds the ATX a bit sharper at high magnification on the design - after all a single data point is not quite enough for statistics... And I have seen one really bad ATX 95 too - among a lot of better and one great example.

Joachim
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Old Wednesday 11th March 2020, 14:20   #15
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Hi,

indeed it does - sorry for reading too fast. But my main point was that just before sundown on a rainy day is not the usual observing situation for most and a little bit more light in the Swaro and thus maybe 5 or 10 minutes less observation time with the Zeiss in that rare situation are a small price to pay for a much wider field of view which you can enjoy every day.

Joachim
Thanks for this Joachim and I certainly agree about the wider field of view of Harpia that you can enjoy all day.

Lee
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Old Wednesday 11th March 2020, 16:35   #16
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The effect of the Harpia's aperture stop down at 23x can be easily simulated with any large scope. See below:

https://www.birdforum.net/showpost.p...&postcount=125

Instantaneously changing the exit pupil sizes back and forth makes the difference quite obvious.
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Old Friday 13th March 2020, 15:55   #17
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Originally Posted by BoldenEagle View Post
If the Kowa 25-60x eyepiece is not clearly brighter than the old one, Meopta with the 30-60x WA eyepiece has 27% better light transmission (about 72% vs about 92%). So judging by that, Meopta should be brighter over Kowa. Have to test that thing also next time I'm comparing them.
https://www.meoptasportsoptics.com/e...-angled-10668/
https://www.meoptasportsoptics.com/e...0-60x-wa-7730/
MeoStar S2 82 HD - angled.................Daylight Transmission 95 %...............Twilight Transmission 91 %
MeoStar S2 eyepiece 30-60x WA...........Daylight Transmission 92 %...............Twilight Transmission 89 %

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Old Tuesday 14th April 2020, 00:14   #18
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Maybe I'm the odd one out here....But light transmission while great, isn't always what it's pushed to be. Brighter, higher light transmission doesn't always mean sharper all the time.

I personally find some high end glass too bright...I've actually used a tinted filter fairly often on both my standard and HD 82mm scopes on a bright day.

Anyway, that's just my experience.....that comes with the "non-expert" disclaimer.
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Old Tuesday 14th April 2020, 04:55   #19
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helical

Quote:
Originally Posted by BoldenEagle View Post
If the Kowa 25-60x eyepiece is not clearly brighter than the old one, Meopta with the 30-60x WA eyepiece has 27% better light transmission (about 72% vs about 92%). So judging by that, Meopta should be brighter over Kowa. Have to test that thing also next time I'm comparing them.

I still hesitate getting Meopta because of it's helical focuser. Certainly if I would pay about nearly 4k for a scope (ATX or Harpia), I would like to have dual focuser (and everything else to be about perfect)...The helical focuser of ATX was though better than I expected because it was enough slow and extremely smooth, I'm not that sure of the Meopta but have to check that thing too. Nevertheless Kowa's focusing system is strong advantage for me and if good sample is found, I would probably choose Kowa IF brightness is not that much behind Meopta with the newer zoom eyepiece.


Juhani
I have seen others bring up the Helical wheel and frankly I would prefer the helical over the dual focuser any day. Now there is a difference in helical focus wheels. When I was looking for a spotter a few years back he Meopta was in the running but I found the helical wheel to be stiff. The Swaro was smooth... So I elected to go with the Swaro ATS, not ATX. For me, the money to purchase the ATX was not worth it. So each their own but don't rule out a helical wheel. For me it is faster focusing and since I enjoy digiscoping, having a faster focus was important. jim
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Old Tuesday 14th April 2020, 12:10   #20
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I have seen others bring up the Helical wheel and frankly I would prefer the helical over the dual focuser any day. Now there is a difference in helical focus wheels. When I was looking for a spotter a few years back he Meopta was in the running but I found the helical wheel to be stiff. The Swaro was smooth... So I elected to go with the Swaro ATS, not ATX. For me, the money to purchase the ATX was not worth it. So each their own but don't rule out a helical wheel. For me it is faster focusing and since I enjoy digiscoping, having a faster focus was important. jim
That may well be true that one who is digiscoping, benefits fast helical focuser over dual focuser. I used to have Nikon Fieldscope 82ED and did some digiscoping with it and now as I remember, the fast helical focuser was good feature to have.

In other use, I found Nikon's focusing system just too fast and it caused quite much shaking when tracking flying birds.

In my usual use of scope, I don't need fast focus ability (i.e. to be able to fast focus for example from 300 meters to say below 10 meters) because I mostly need the scope for ID quite distant flying birds say about 300m-10km away from me. In that use it's much more important to have enough slow focusing because one is using high mags. and it's essential to have as little shaking as it is possible. I basically don't do digiscoping at all nowadays. Thats why my preference is dual focuser.

So as I interpreted your post, I totally agree with you, that what focusing system is the best option, depends heavily on what will the primary use of the scope be. And still someone likes helical or dual focuser anyway; it's also a bit individual thing.

Juhani

PS. I finally chose Kowa over Meopta and I'm now enjoying it's dual focuser (but which could be just a bit slower also...).
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Old Tuesday 14th April 2020, 13:48   #21
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Quote:
Originally Posted by BoldenEagle View Post
That may well be true that one who is digiscoping, benefits fast helical focuser over dual focuser. I used to have Nikon Fieldscope 82ED and did some digiscoping with it and now as I remember, the fast helical focuser was good feature to have.

In other use, I found Nikon's focusing system just too fast and it caused quite much shaking when tracking flying birds.

In my usual use of scope, I don't need fast focus ability (i.e. to be able to fast focus for example from 300 meters to say below 10 meters) because I mostly need the scope for ID quite distant flying birds say about 300m-10km away from me. In that use it's much more important to have enough slow focusing because one is using high mags. and it's essential to have as little shaking as it is possible. I basically don't do digiscoping at all nowadays. Thats why my preference is dual focuser.

So as I interpreted your post, I totally agree with you, that what focusing system is the best option, depends heavily on what will the primary use of the scope be. And still someone likes helical or dual focuser anyway; it's also a bit individual thing.

Juhani

PS. I finally chose Kowa over Meopta and I'm now enjoying it's dual focuser (but which could be just a bit slower also...).
Can't go wrong with a Kowa.... good scope. I too think it is a individual preference thing, for helical or dual focus. Each to their own. I did like the Meopta though and was just about ready to purchase it as opposed to the Swaro but I found a rep who was trying out both at the same time in the field and that is when I saw the difference. It could have just been the model of Meotpa he had too, as I was impressed with it, Have fun, jim
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Old Tuesday 14th April 2020, 16:42   #22
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A few scopes with helical focusers (Nikon Monarch ED, Zeiss Gavia, current Vortex Razor and possibly a few more) have very effective variable speed focusers that gradually change from fast focus at close distances where a fast focus is needed to a slower finer focus at longer range.
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