Well, fair enough. Immediately after I posted my comment I began wondering in the vague kind of way I think about optics if that might not be the case. Thanks for taking the trouble to set me straight.
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Spotting scopes test (1 Viewer)
- Thread starter macs
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Well, fair enough. Immediately after I posted my comment I began wondering in the vague kind of way I think about optics if that might not be the case. Thanks for taking the trouble to set me straight.
henry link
Well-known member
Gijs,
Sorry it’s taken some time to reply. I needed to borrow a Kowa 883 for the attached photos.
First, allow me to apologize for the harshness of my first post, which was narrowly directed at the quote contained in the post immediately above mine; a quote, which I believe many people found odd. My use of the term “non-linear” comes from your description of the detector used in the test as having “a spectral response curve very much identical to that of the human eye”. As you know, the spectral response curve of the human eye is non-linear. It appears to me that the spectral transmission curves shown in your light transmission graphs have been adjusted to remove that curve.
The question of vignette in spotting scopes (and binoculars) is the only other part of your post I think I can respond to.
It’s not necessary to disassemble an instrument or use laboratory instruments to determine whether vignette is present and roughly how extensive it is. This can be done by anyone with a sheet of aluminum foil and a needle. Just punch a pinhole in the aluminum foil and stretch the sheet over the front of the objective lens. In a telescope with no vignette the pinhole can be moved to any spot above the surface of the objective and still fully (but very dimly) illuminate the field. In a telescope with vignette the full field will be illuminated by the pinhole only when it’s positioned above some central area of the objective. As the pinhole is moved toward the edge of the objective the shadow of an internal obstruction between the pinhole and the edge of the eyepiece fieldstop will come into view and increasingly impinge on the field as the pinhole is brought closer to the objective edge. Every binocular and spotting scope shows this, but not all to the same extent. Spotting scopes with zoom eyepieces present a special case, since the true field width is not constant over the zoom range. Vignette may be present at the lowest magnification with the widest FOV, but gradually decrease and even disappear at higher magnification as the true field shrinks.
To my surprise I found that it’s possible to photograph the dim images seen through the eyepiece in this test by pointing the scope at a very bright light source. The left photo below shows the foil in place over the objective lens of a Kowa 883. The right slide shows views through the zoom eyepiece at four pinhole positions with the magnification set at 20x. In the upper left view the pinhole is centered and illuminates the full field. In the upper right the pinhole is positioned about 28mm from the center, in the lower left about 35mm from the center and in the lower right about 42mm from the center. If the light throughput from the entire FOV seen through this scope is measured the result will be the mean throughput from both the obstructed and unobstructed areas of the field.
I should mention that the pinhole is also handy for accurately viewing the internal obstruction that causes the vignetting from the objective end. Just place your eye close to the pinhole and sight down the telescope barrel. You’ll see a view of the obstruction like the first photos I posted. The small bright opening you see from the objective end (the white disk in the first photos) is not the exit pupil. It’s the eyepiece fieldstop, so the obstruction causes dimming at the edge of the field, not the edge of the exit pupil.
Henry
Sorry it’s taken some time to reply. I needed to borrow a Kowa 883 for the attached photos.
First, allow me to apologize for the harshness of my first post, which was narrowly directed at the quote contained in the post immediately above mine; a quote, which I believe many people found odd. My use of the term “non-linear” comes from your description of the detector used in the test as having “a spectral response curve very much identical to that of the human eye”. As you know, the spectral response curve of the human eye is non-linear. It appears to me that the spectral transmission curves shown in your light transmission graphs have been adjusted to remove that curve.
The question of vignette in spotting scopes (and binoculars) is the only other part of your post I think I can respond to.
It’s not necessary to disassemble an instrument or use laboratory instruments to determine whether vignette is present and roughly how extensive it is. This can be done by anyone with a sheet of aluminum foil and a needle. Just punch a pinhole in the aluminum foil and stretch the sheet over the front of the objective lens. In a telescope with no vignette the pinhole can be moved to any spot above the surface of the objective and still fully (but very dimly) illuminate the field. In a telescope with vignette the full field will be illuminated by the pinhole only when it’s positioned above some central area of the objective. As the pinhole is moved toward the edge of the objective the shadow of an internal obstruction between the pinhole and the edge of the eyepiece fieldstop will come into view and increasingly impinge on the field as the pinhole is brought closer to the objective edge. Every binocular and spotting scope shows this, but not all to the same extent. Spotting scopes with zoom eyepieces present a special case, since the true field width is not constant over the zoom range. Vignette may be present at the lowest magnification with the widest FOV, but gradually decrease and even disappear at higher magnification as the true field shrinks.
To my surprise I found that it’s possible to photograph the dim images seen through the eyepiece in this test by pointing the scope at a very bright light source. The left photo below shows the foil in place over the objective lens of a Kowa 883. The right slide shows views through the zoom eyepiece at four pinhole positions with the magnification set at 20x. In the upper left view the pinhole is centered and illuminates the full field. In the upper right the pinhole is positioned about 28mm from the center, in the lower left about 35mm from the center and in the lower right about 42mm from the center. If the light throughput from the entire FOV seen through this scope is measured the result will be the mean throughput from both the obstructed and unobstructed areas of the field.
I should mention that the pinhole is also handy for accurately viewing the internal obstruction that causes the vignetting from the objective end. Just place your eye close to the pinhole and sight down the telescope barrel. You’ll see a view of the obstruction like the first photos I posted. The small bright opening you see from the objective end (the white disk in the first photos) is not the exit pupil. It’s the eyepiece fieldstop, so the obstruction causes dimming at the edge of the field, not the edge of the exit pupil.
Henry
Attachments
Gijs van Ginkel
Well-known member
Investigation of top quality telescopes
Dear Henry,
Apologies accepted. The sentence you referred to is about the light meter with a spectral response curve identical to that of the human eye. That light meter is not used for the light transmission measurements, but only for measuring the total amount of light under standardized conditions, which leaves the telescope. That is where the confusion started I think.
The light transmission measurements are done with a spectrograph, which is described in Appplied Spectroscopy, 55 (2001) pages 1005-1012. I have described the spectrometer also on the web-site of cloudy nights, but I will repeat it here. Basically the spectrometer consists of a prism for light dispersion in combination with a state of the art CCD camera. A total number of 400 pixels is available to record a spectrum ranging form 450-680 nm. The acquisition time is in our case 60 seconds. We have measured light transmissions in the exit pupil for different magnifications with the same telescopes and the light transmission spectrum does not change as one can expect. Vignetting effects were not observed and, considering the control measurements with exactly the same instruments by different companies, which can be considered as experts in the field of light transmission measurements, we are confident that we did a good job. Thank you for sharing the pinhole experiments with us. I will give it a try also. It took you probably quite a bit of time and effort.
Best regards,
Gijs
Dear Henry,
Apologies accepted. The sentence you referred to is about the light meter with a spectral response curve identical to that of the human eye. That light meter is not used for the light transmission measurements, but only for measuring the total amount of light under standardized conditions, which leaves the telescope. That is where the confusion started I think.
The light transmission measurements are done with a spectrograph, which is described in Appplied Spectroscopy, 55 (2001) pages 1005-1012. I have described the spectrometer also on the web-site of cloudy nights, but I will repeat it here. Basically the spectrometer consists of a prism for light dispersion in combination with a state of the art CCD camera. A total number of 400 pixels is available to record a spectrum ranging form 450-680 nm. The acquisition time is in our case 60 seconds. We have measured light transmissions in the exit pupil for different magnifications with the same telescopes and the light transmission spectrum does not change as one can expect. Vignetting effects were not observed and, considering the control measurements with exactly the same instruments by different companies, which can be considered as experts in the field of light transmission measurements, we are confident that we did a good job. Thank you for sharing the pinhole experiments with us. I will give it a try also. It took you probably quite a bit of time and effort.
Best regards,
Gijs
henry link
Well-known member
Thanks Rick. Cheapness is the mother of invention. ;-)
Henry
Henry
Gijs van Ginkel
Well-known member
Two new tests (1) telescopes and (2) 42 mm binoculars
Dear all,
I have extended the quality investigation of 80 mm observation telescopes with the Nikon EDG85, the newly designed Kite KSP-80-HD telescope made in Japan. The Swarovski ATM-80-HD was re-investigated because in the first test I had a preproduction model. Some data of the new Zeiss Diascope 85 are also included.
You can find the data on the following WEB-site:
http://home.europa.com/~telscope/Ginkel.2010.six.top.telescopes.pdf
I also finished a test of 42 mm binoculars among which four new Swarovski models. This test can be found on WEB-site:
http://home.europa.com/~telscope/Ginkel.2010.Test.42mm.binoculars.pdf
If there are any questions or remarks, please let me know.
Merry Christmas and a Happy 2011,
Gijs van Ginkel
Dear all,
I have extended the quality investigation of 80 mm observation telescopes with the Nikon EDG85, the newly designed Kite KSP-80-HD telescope made in Japan. The Swarovski ATM-80-HD was re-investigated because in the first test I had a preproduction model. Some data of the new Zeiss Diascope 85 are also included.
You can find the data on the following WEB-site:
http://home.europa.com/~telscope/Ginkel.2010.six.top.telescopes.pdf
I also finished a test of 42 mm binoculars among which four new Swarovski models. This test can be found on WEB-site:
http://home.europa.com/~telscope/Ginkel.2010.Test.42mm.binoculars.pdf
If there are any questions or remarks, please let me know.
Merry Christmas and a Happy 2011,
Gijs van Ginkel
henry link
Well-known member
Gijs,
You might want to post a link to the binocular test on the Binocular Forum also. It should certainly stimulate some discussion there, but might not be noticed here.
Henry
You might want to post a link to the binocular test on the Binocular Forum also. It should certainly stimulate some discussion there, but might not be noticed here.
Henry
John Russell
Well-known member
Gijs,
I don't think the unweighted apportionment of points for various features (1 or 6 for rubber armouring or not, or compatibility of the tripod foot with the Manfrotto RC system, when there are other and better quick release systems) can lead to a serious assessment of the qualities of the scopes tested.
Several portions of the text (ski goggles!!) lead me to question whether you are an experienced scope user. To draw an analogy, it is as if a mechanical engineer had been charged with conducting a track test of supersports cars, when he had passed his driving test the previous week.
This may not be in accordance with the customary politeness on BF, but neither was your disparaging response to a pertinent point raised earlier in this thread.
John
I don't think the unweighted apportionment of points for various features (1 or 6 for rubber armouring or not, or compatibility of the tripod foot with the Manfrotto RC system, when there are other and better quick release systems) can lead to a serious assessment of the qualities of the scopes tested.
Several portions of the text (ski goggles!!) lead me to question whether you are an experienced scope user. To draw an analogy, it is as if a mechanical engineer had been charged with conducting a track test of supersports cars, when he had passed his driving test the previous week.
This may not be in accordance with the customary politeness on BF, but neither was your disparaging response to a pertinent point raised earlier in this thread.
John
Thank you very much Gijs for providing and sharing your results. These kind of information is usually hard to get in the internet and elsewhere. I´m sure this is because of the great amount of work that is needed to do something like this. Your transmittance measurements/graphs are the only ones I ever saw so far dealing with spotting scopes. Very interesting!
Steve
Steve
Gijs,
you have compared some features of scopes. It is interesting. But for me the most important factor is a resolution of a scope. Did you compare scopes side by side what you can recoganize, see in each of them?
I think that weight, armor etc. does not matter much for birders whilst field of view, resolution is the most important factor.
Regards,
Mariusz
you have compared some features of scopes. It is interesting. But for me the most important factor is a resolution of a scope. Did you compare scopes side by side what you can recoganize, see in each of them?
I think that weight, armor etc. does not matter much for birders whilst field of view, resolution is the most important factor.
Regards,
Mariusz
Gijs van Ginkel
Well-known member
Spotting scope test
Dear all,
Through circumstances beyond my control I could not replye earlier to different threads, so I will combine them here:
Dear Henry:
I posted the report on the WEB-site of Peter Abrahams, since the time they were finished coincided with the meeting of the Binocular History Society in Vancouver in 2010. It may be good to post the test also on Birdforum.
Dear John Russell,
I read that you want to teach me a lesson, since you have read somewhere in my treads that I wrote a disparaging response. Since I do not have a clue where you are referring to I can not respond.
With regard to the system of evaluation of the telescopes by ranking them: of course one can make objections to that method. We discussed with birders quite frequently what system would be helpful. This system I used in the test looked in so far useful that we noticed that persons who want to buy a telescope very frequently put different telescopes next to each other for comparison. And the ranking system is based on that way of decision making, but yes, there are of course also other ways to give points to telscope performances.
The ski goggles item: I used a telescope in the mountains covered with snow in sunlight and I had ski goggles on my head. It was then that I noticed that the telescope I was using at that time was not suited for observations with ski goggles. Not many persons will use ski goggles, I know, but if you do..
As far as the fast coupling of a tripod platform is concerned: that certainly is important for many birders in Holland and, yes, there are differences in user comfort of different fast coupling systems, but I did not make that an item in the test. With regard to your worries about my experience with telescopes: it is roughly 50 years or more that I am a frequent user of binoculars and telescopes and I think that I did learn something during these years.
Dear Hinnark,
Thank you for your compliments about the test.
Dear Mariusz,
You raise a good point about telescope resolution. That is not incuded in the test. What we did was: the telescopes investigated were aimed at a test chart under high light conditions and with the eye we could not see differences in resolution between the telescopes. I think that with laboratory measurements differences can be found.
Dear all,
Through circumstances beyond my control I could not replye earlier to different threads, so I will combine them here:
Dear Henry:
I posted the report on the WEB-site of Peter Abrahams, since the time they were finished coincided with the meeting of the Binocular History Society in Vancouver in 2010. It may be good to post the test also on Birdforum.
Dear John Russell,
I read that you want to teach me a lesson, since you have read somewhere in my treads that I wrote a disparaging response. Since I do not have a clue where you are referring to I can not respond.
With regard to the system of evaluation of the telescopes by ranking them: of course one can make objections to that method. We discussed with birders quite frequently what system would be helpful. This system I used in the test looked in so far useful that we noticed that persons who want to buy a telescope very frequently put different telescopes next to each other for comparison. And the ranking system is based on that way of decision making, but yes, there are of course also other ways to give points to telscope performances.
The ski goggles item: I used a telescope in the mountains covered with snow in sunlight and I had ski goggles on my head. It was then that I noticed that the telescope I was using at that time was not suited for observations with ski goggles. Not many persons will use ski goggles, I know, but if you do..
As far as the fast coupling of a tripod platform is concerned: that certainly is important for many birders in Holland and, yes, there are differences in user comfort of different fast coupling systems, but I did not make that an item in the test. With regard to your worries about my experience with telescopes: it is roughly 50 years or more that I am a frequent user of binoculars and telescopes and I think that I did learn something during these years.
Dear Hinnark,
Thank you for your compliments about the test.
Dear Mariusz,
You raise a good point about telescope resolution. That is not incuded in the test. What we did was: the telescopes investigated were aimed at a test chart under high light conditions and with the eye we could not see differences in resolution between the telescopes. I think that with laboratory measurements differences can be found.
henry link
Well-known member
Gijs,
Lab equipment isn't needed to measure resolution. You just need to increase the magnification enough to make the resolution limit visible. That can be done by substituting a shorter focal length eyepiece or by sighting through a small low power telescope placed behind the scope eyepiece.
In my view a star test is also essential for understanding aberrations and identifying sample defects. Even among the "alpha" brand spotting scopes there are significant sample variations both for basic aberrations like spherical aberration and assembly defects like astigmatism, misalignment and pinching.
Henry
Lab equipment isn't needed to measure resolution. You just need to increase the magnification enough to make the resolution limit visible. That can be done by substituting a shorter focal length eyepiece or by sighting through a small low power telescope placed behind the scope eyepiece.
In my view a star test is also essential for understanding aberrations and identifying sample defects. Even among the "alpha" brand spotting scopes there are significant sample variations both for basic aberrations like spherical aberration and assembly defects like astigmatism, misalignment and pinching.
Henry
Steve C
Well-known member
While I enjoyed reading this, I cannot get my head around the idea that fov is responsible for brightness or light transmission. Maybe I'm dense, but that does not compute. I have always been under the impression that fov was basically a function of eye piece design.
It also seems that there is a definite brightness difference in the 20-60x standard zooms and the 25-50 WA. Apparently something caused that. That alone makes this somewhat more of an apples to oranges test than it would have been if the 20-60 eye pieces were used on all scopes.
It also seems that there is a definite brightness difference in the 20-60x standard zooms and the 25-50 WA. Apparently something caused that. That alone makes this somewhat more of an apples to oranges test than it would have been if the 20-60 eye pieces were used on all scopes.
Gijs van Ginkel
Well-known member
Test spotting scopes
Dear all,
To make my life easier I wil reply to your comments in one message.
Henry,
I agree that lab equipment is not needed to measure resolution and your methods works allright. I also agree that a star test is very wortwile. I noticed differences between different telescopes of the same brand, which was also reported by Jan Meijerink and Martin van Venrooy who both investigated a number of Zeiss Diascopes.
What I did for this test also, but not for all telescopes investigated, was: I constructed a tube, so I could use a camera to photograph the same spot of a brick wall under identical light conditions. That also gave a good impression of telescope resolution and moreover of color reproducion and optical aberrations. I think that I read in "Optics for the Hunter" that the author used a 1 dollar bill glued on a piece of wood, placed it in a certain distance from the telescopes and compared resolution observed by eye (the bill has many very small line structures).
Steve,
FOV really does not have any effect on image brightness. I recently investigated a Japan made 500 euro Kite Petrel 10x50 binocular (FOV 89 m/1000m) and a 2000 euro Leica Ultravid 10x50 (FOV 117m/1000m). After adaptation of my eyes for 30 minutes to the existing low light level used I could not see any difference in the observation of small details. The secret was in the identical light transmissions of 85% of both binoculars at 500 nm, the optimum spectral sensitivity of the human eye in the dark (rod vision).
A 20-60x eyepiece has a different optical construction as a wide field 25-50x eypiece. The optical path length through the glass components is probably different (resulting in differences in light transmission) and the number of glass components may be different (also resulting in differences in light transmission). Some telescope makers like Leica and Kite do not even make 20-60x eyepieces anymore, therefore I investigated the telescopes as they are sold.
Dear all,
To make my life easier I wil reply to your comments in one message.
Henry,
I agree that lab equipment is not needed to measure resolution and your methods works allright. I also agree that a star test is very wortwile. I noticed differences between different telescopes of the same brand, which was also reported by Jan Meijerink and Martin van Venrooy who both investigated a number of Zeiss Diascopes.
What I did for this test also, but not for all telescopes investigated, was: I constructed a tube, so I could use a camera to photograph the same spot of a brick wall under identical light conditions. That also gave a good impression of telescope resolution and moreover of color reproducion and optical aberrations. I think that I read in "Optics for the Hunter" that the author used a 1 dollar bill glued on a piece of wood, placed it in a certain distance from the telescopes and compared resolution observed by eye (the bill has many very small line structures).
Steve,
FOV really does not have any effect on image brightness. I recently investigated a Japan made 500 euro Kite Petrel 10x50 binocular (FOV 89 m/1000m) and a 2000 euro Leica Ultravid 10x50 (FOV 117m/1000m). After adaptation of my eyes for 30 minutes to the existing low light level used I could not see any difference in the observation of small details. The secret was in the identical light transmissions of 85% of both binoculars at 500 nm, the optimum spectral sensitivity of the human eye in the dark (rod vision).
A 20-60x eyepiece has a different optical construction as a wide field 25-50x eypiece. The optical path length through the glass components is probably different (resulting in differences in light transmission) and the number of glass components may be different (also resulting in differences in light transmission). Some telescope makers like Leica and Kite do not even make 20-60x eyepieces anymore, therefore I investigated the telescopes as they are sold.
