Spotting scopes test (2 Viewers)

[macs]

Hello,

This test ('INVESTIGATION OF TOP QUALITY TELESCOPES' By Dr. Gijs van Ginkel, February 2009) was sent by fellow birder to a Polish yahoo mailing list. I do not know the original source of this write-up.

Meanwhile, please read and comment what you think.

Regards,

Maciej

 

[RJM]

Nice to see some objective measurements being attempted though I feel the scoring is still subjective. Can my Kowa 883 get bonus points for ditching the "required" case in favor of superior camo wrap?8-P

Of course, the most shocking revelation of overall transmission will cause a stir among us optic quant geeks as the results deviate significantly from expectations. I certainly would like to see more investigation into how the 88mm Kowa's 34% larger exit pupil can deliver 10% LESS light than a 80mm scope because I am not buying into the "larger FoV" of the eyepiece and some supernatural AR coating performance causality.

On the otherhand, if you calculated the so called Twilight Factor or "Adler Index" for low-light performance, this result perhaps makes more sense when the scopes are used at the lowest possible magnification BUT complete falls apart when exit pupil is kept constant. Hmmmm....

And no penalty for how prone the Swaro 25-50x eyepiece is to kidney beaning blackouts!? Whassup wit dat?

Rick

 

[mooreorless]

Interesting, but I see no resolution tests, no star tests. Unless I missed it. Overall light transmission taken with a grain of salt by me. They made a big deal out of the Kowa and Zeiss needing a separate mounting adapter for the tripod head. I never used a spotter with a mounting foot that fits right into the head, so maybe I would change my mind if I did.If they added at least a resolution test this would of been a really good test, but still somewhat subjective in the final standings.
Steve

 

[macs]

Hi guys,
Thanks for your comments. I also think this test is subjective.

Is the following statement true?

'The following parameters play a role for the image brightness of telescopes and binoculars: the amount of light entering a telescope is determined by the field of view.'

I thought that the objective lens size determines the amount of light entering the scope. Am I right?

Maciej

 

[henry link]

It's very odd to see such a fundamental misconception about how telescopes work in an apparently informed test like this. The author's misunderstanding renders the light transmission measurements useless except perhaps for the shape of the transmission curves, but even those may be unreliable because a non-linear detector was used.

Light transmission measurements are typically done by applying a narrow beam of light to a small area at the center of the objective lens. I've seen references to 1mm-3mm beam widths in different tests. This is done precisely to avoid any contribution from exit pupil, magnification or FOV, all of which are irrelevant to light transmission.

 

[kristoffer]

Swarovski definitely has the best ergonomics of these. Rubber body, nothing that can get stuck when the scope goes in and out of the car, heliocentric focus being far easier to use and less sensitive to damage. The rest of the test seems more sketchy.

 

[John Russell]

I was surprised that the reviewer was able to view the whole field of the Swarovski with ski goggles (!!!). Surely not with the 25-50x zoom. It has an excellent FOV but its eye relief is rather limited compared to Swaro's 20-60x zoom.

 

[fugl]

Strange review. It continues to be a mystery to me why people go to the considerable trouble of conducting such tests without taking the extra few steps to do them right.

 

[lmans66]

I think the closing statement says it all...."It is advisable to investigate the properties of the different telescopes carefully and compare their performance in accordance with your own demands"...period.

You are the one making the decision and forking out huge dollars for these. All in all, they are probably pretty equal but we each place priorities on our birding needs depending on how we bird and where we bird. Lots of qualities of each. I looked carefully before buying my scope and feel I made the best decision based on my needs and what I could afford. the same holds true for cameras, lens, binos etc....

Good reading though

 

[NWBirder]

Strange review. It continues to be a mystery to me why people go to the considerable trouble of conducting such tests without taking the extra few steps to do them right.

I got the same impression too.

 

[Gijs van Ginkel]

investigation of to quality telescopes

My concept test of top quality telescopes dated February 2009 was published on internet without my approval. The test was send for comments to the telescope manufacturers and at that time not finished. The final version of the test is dated May 2009 and is published with my permission also on internet. It can be found using Google and typing Dr. Gijs van Ginkel telescope test and it then can be read under "home.europa.com" or "www.europa.com" etc. The test was made on request of the Dutch Bird Protection Association. With respect to subjectivity concerns, all data were obtained by measurements, weight with a balance, diameters of exit pupils and objective diameter with appropriate rulers, focussing speed with a stopwatch, light transmission with professional equipment of the Physics Laboratory of Utrecht University. The set-up of it is published in a well known scientific journal. The conclusions about color reproduction of the different telescopes from the transmission spectra were independently confirmed with photography by colleagues from the Bird Protection Association.

 

[mblank]

Gijs,

as to your results. In the first table I can see that Kowa's exit pupil is smaller than Zeiss.
I have alway thought that Kowa exit pupil is 88:20=4.4 and Zeiss is 85mm:20.should be ~4.3 in your table Zeiss is 4.4, while Kowa is 4.3.
Why in your results Leica's exit pupil is smaller than Swaro. Leica is 82mm:25=3.28, Swaro 80:25=3.2.


Regards,
Mariusz

 

[Gijs van Ginkel]

Dear Mariusz,
The published exit pupils are the measured ones and they are the real ones. The values you refer to are calculated ones from the values given by the manufacturer. As is clear from the measurements these data are not exactly the ones as measured. That explains why the 88 mm Kowa at 2x has a smaller exit pupil as the 85 mm Zeiss at 20x. Th same holds for the differences between the 80 mm Swarovski and the 82 mm Leica. Best regards, Gijs

 

[RJM]

Thank you Gijs for your effort in making a detailed report of these optics using objective measurements. While the end results will certainly cause a stir among owners of these scopes, I am more interested in your methods for measuring transmission and your conclusions regarding the influence of the FoV. I feel something has been "lost in translation" because many of us are skeptical of this FoV conclusion as it does not conform to our collective practical experiences.

Let's say we are measuring the quantity of light entering through two scopes (whose only difference is FoV) directed at point light sources of identical brightness surrounded by total darkeness. One light source is on axis in both scopes and the other source is just outside the FoV of the scope with smaller FoV. Your light meter will naturally record 2x more light entering the scope with the wider FoV. BUT the light is still equally bright across the whole field. As Henry has stated, that seems to be why transmission of an optics system is measured by a narrow on-axis beam source.

Perhaps you might be interested in explaining your testing methods and reasoning in more detail either here, and/or on the binocular forum of Cloudy Nights where there are many more optical "experts" that would appreciate your contribution to understanding this phenomena? FYI, I have started a thread there called Transmission and FoV.

Thanks again,
Rick

 

[Gijs van Ginkel]

investigation of top quality telescopes

Dear readers,
I noticed that there is some confusion about image brightness and the total light output measurements under standardized conditions in the telescope test.
First of all: image brightness is fully determined by the size of the exit pupil (= objective diameter divided by magnification) and the instruments light transmission characteristics as measured by the spectral light transmission curves. The light transmisssion curve is an instrument property. For those who are worried about the role of vignetting in our measurements: that is practically ruled out with the spectrograph used.
Moreover we observed the amount of vignetting to be very small in all four telescopes investigated even for the wide-angle eypieces of the Swarovski and Leica telescopes: the designers simply did a good job.
The FOV matter: the amount of light offered to a telescope is confined by the cone of light of the FOV, so one can see this more or less as the lamp for the telescope. Part of that "lamp" light is transmitted and that part is determined by the light transmission curve. The transmission percentage is constant in a good telescope no matter if you change the magnification from 20x to 60x. Changing that magnifiation however does change the exit pupil drastically and that affects the telescope image brightness. I hope this clarifies some concerns.
The measurement of total light output under standardized conditions was meant to show the effect of the light transmission curve at identical exit pupils , but that requires that one takes into account the strenght of the "lamp" c.q. the size of the FOV. To perform that sort of calculations some mathematical excersises are necessary. Therefore I did not include that in the test and I have put in an attachment. It is something for the reader to do on an evening when the doors are immobilized by a 10 foot layer of snow outside or similar circumstances. I hope to have clarified some things. Regards, Gijs

 

[Gijs van Ginkel]

Dear reader,
Some more additional information to the observation telescope test may be helpful. The test came into being upon requests of birders form the Dutch Bird Protection Asociation. The most important criteria for birders with regard to observation telescopes were determined after many talks with birders. We decided not to give weight factors to the different criteria, since these are in fact subjective. Ranking in first place etc. looked much more helpful, since the reader of the test can determine him/herself very well if that particular test criterium is of importance for him/her. With this approach the telescope with the least amount of points scores highest, as you can read in the test.
As far as concern about the measurement of transmission spectra is concerned: we are already for many years spectroscopists and we know of the pittfalls and problems going with it. That does not mean that we can not make mistakes. The reproducibility of the measurements generally is within 0.5% (half a percent), so that is not so bad from a scientific point of view. Tripod platforms fitting into a fast coupling device was considered to be important by many Dutch birders I have talked to. This can be different of course for birders in other countries.
Low weight and focussing speed were also highly valued and that differs quite a bit for the different telescopes. As far as eye relief is concerned. I wear no spectacles, so I use special safety spectacles with ordinary glass as test spectacles. The are like normal spectacles. The ski goggles used are of the spectacle type, the glass is only a little further away from the eye then normal spectacles. There are also ski goggles the glass of which is at a much larger distance away from the eye. I did not use those ones for the test.
I think I have reacted to all questions and comments, if not feel free to react.
Regards, Gijs

 

[fugl]

I guess what bothers me about your test—which I o/w find interesting—is the absence of any discussion of inter-sample variability. Did you attempt to control for this in any way? Where did your test samples come from?

 

[henry link]

I made some photos today to try to demonstrate one of the the problems with the method used to measure "light transmission" in this test. The three images are the eyepiece fieldstops of three telescopes as viewed from the edge of the objective lenses. This is a quick and dirty method to show how much internal obstruction there is between the objective edge and the fieldstop edge. There are better ways to see this from the eyepiece end, but I don't know how to photograph those methods. These images get the idea across even if they are not perfectly accurate.

The scope on the left is an Astro-Physics 92mm Stowaway combined with the Zeiss Diascope zoom eyepiece set at its lowest magnification (24x in this scope). The middle image is a Takahashi 90mm SKY 90 with the same eyepiece (20x in this scope) and the right image is a Kowa 883 with its zoom set at 20x. The Stowaway shows no obstruction at all between the fieldstop edge and the objective edge, which means there is no off-axis vignetting. An object placed at the edge of the field will be just as bright as the same object placed at the center. The other two scopes show obstructions from undersized internal apertures, more severe in the Kowa than the Tak. Those obstructions cause off-axis vignetting which will dim the image toward the edge the field. If the light transmission is measured with a full field white target, as is done in this test, the transmission figures for the Tak and Kowa will be dragged down compared to the Stowaway by the dimmer field edges even if their light transmission is identical or higher in the unobstructed area of the field center. I should add that the Kowa designers didn't make a mistake. They chose to sacrifice some unvignetted FOV for smaller prisms to keep the size and weight of the scope down. Other birding scope designers will have done the same thing, but not necessarily to the same degree.

 

[henry link]

fugl,

Sample variation isn't likely to affect light transmission or color accuracy very much, and those were the only things actually measured in this test. The kinds of defects that turn up in birding scopes, like misalignment, astigmatism, pinching and excessive spherical aberration mostly degrade resolution and contrast.

Jan Meijerink's reviews at http://www.tvwg.nl/ show the effects of sample variation in some of these same scopes. I sadly noticed when I just visited the site the Jan Meijerink has retired from reviewing optics. Hopefully his excellent work will continue to be available.

Henry

 

[Gijs van Ginkel]

Investigation of top quality telescopes

Dear Henry Link,
In your view nothing we did with our telescope test was right and you actually said that we do not understand anything about the working mechanism of telescopes. I will react to that, since I am of the opinion that you are wrong.
We measured a little more then only the light transmission spectra as you will see upon re-reading the test. With regard to vignetting effects: these are very small in telescopes with a field of view of 1-3 degrees, they become serious at wider angle fields of view let us say 60 degrees like in wide angle photographic lenses. If you are not convinced: read optical textooks and on top of that scan the exit pupil of a telescope with a very narrow glass fiber and measure the intensity distribution and you will see how that works out. We have checked non-uniformity of light intensity over the field of view and it was within experimental error. Your statement that we used a non-linear detector bears no ground, so please do not spread unneccessary confusion if you do not have the facts right.
We looked at the results of your photographs and we compared it with our observations on the different telescopes. All of them turn out with uniform illuminated exit pupils without the ring pattern you show in some photographs.
Your remark that Kowa choose a smaller prism suggests that you took the four different telescopes apart and that you could actually show this. Did you really or was it a wild guess?
With regard to our light transmission measurements the following.
1. the light transmission of the four telescopes (different samples) was also measured by a firm, which does these measurements professionally on a daily basis. That firm found the same results as we did with our transmisiion spectra with regard to differences in light transmission as well as color reproduction.
2. A short time after we started (some years ago) our light transmission measurements we did control measurements on different instruments to check our measuring method. The light transmission of these instruments was also measured by a leading binocular/telescope producer which does these measurements professionally. The same instruments were also measured by an official German institute that does optical calibrations for optical companies and guess what. Our light transmission data were spot on the same as the ones measured by the other institutes except in one case a tiny deviation in the wavelenght range 450-490 nm, which we could easily correct.
Exit pupil, FOV or magnification do not in any way influence the light transmission spectra. In a telescope with a zoom eyepiece of for example 20-60x the optical system (lenses, prism, coatings) does not change in any way when changing from 20 to 60x and at both magnifications the light transmission spectrum stays exactly the same. The only reason that the image darkens when turning to 60x is the strong decrease in exit pupil.
Regards,
Gijs