Why are binocular eye tubes angled inward slightly?

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hans
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Why are binocular eye tubes angled inward slightly?

#1 Post by hans » Wed Aug 12, 2020 9:02 pm

I am wondering about typical upright compound microscopes where a splitter in the head separates the optical path to the left/right eye.

The inward angle is several degrees between the left and right eye tubes on the Reichert 410 "Microstar IV", how common is this across different microscopes and head designs?

As far as I can understand optically, this inward angling of the eye tubes should actually result in a mild stereoscopic effect, sort of like a common main objective stereo microscope but with more in common than just the main objective. Is that correct?

If so, is the stereoscopic effect considered intentional/useful in the design? (I can't really notice much effect visually, but I think it would be hard to tell without comparing side-by-side with a microscope where the views were truly parallel.) Or is the inward angle just done for some more mundane reason like people find viewing more comfortable with their eyes angled inward slightly? Or am I just really confused about what it going on optically?

wabutter
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Re: Why are binocular eye tubes angled inward slightly?

#2 Post by wabutter » Wed Aug 12, 2020 9:45 pm

As far as I remember, the converging eyetubes from the A0 through Leica infinity series compound microscopes was unique to that brand. Although it would seem logical to assume that it would replicate a stereo effect, that is not the case.
The philosophy from the designers was that the image being viewed through the microscope is virtual. A representation of an object at about 250mm in front of the viewers face. Not unlike reading a book, the eyes normally converge slightly at that distance. If was felt that the converging eyetubes would promote greater comfort over long periods of time while using the microscope. Recall that the Series 10 was a dominate market share holder in the field of Pathology and clinical laboratories . Often the Clinician was making notes as they scanned slides. It was felt that the Clinician could easily reference desk top material without refocusing their eyes for the distance.

There were a few limitation in that about 5% of the population is not capable of converging, so those individual were not comfortable with these viewing tubes. Similarly there is about 5% of the population that can not use parallel viewing tubes.

Keep in mind that AO was also a huge player in the field of Ophthalmic instrumentation and eye care products in the same era, so they had resources to support the physiologic and anatomic claims that were offered.

As an added point, it provided American Optical a unique selling proposition when offering it competitive bids.

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Re: Why are binocular eye tubes angled inward slightly?

#3 Post by hans » Thu Aug 13, 2020 1:37 am

Thanks for all the background. It is easy to look at the cartoon ray diagrams in introductory microscopy resources and think you have a pretty good understanding and that most microscopes are basically the same. But trying to map that onto a real microscope, no end in sight, just increasingly subtle and carefully engineered details, trade-offs, cost optimizations... no wonder microscopes are so expensive new.
wabutter wrote:
Wed Aug 12, 2020 9:45 pm
Although it would seem logical to assume that it would replicate a stereo effect, that is not the case.
Is this to say, the optics are intentionally designed to keep the lines-of-sight coaxial with theoretically zero parallax, or just that the parallax resulting from angling the tubes inward for ergonomic reasons is too small to give any perceptible sense of depth?

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Re: Why are binocular eye tubes angled inward slightly?

#4 Post by Zuul » Thu Aug 13, 2020 3:39 am

hans wrote:
Thu Aug 13, 2020 1:37 am
Thanks for all the background. It is easy to look at the cartoon ray diagrams in introductory microscopy resources and think you have a pretty good understanding and that most microscopes are basically the same. But trying to map that onto a real microscope, no end in sight, just increasingly subtle and carefully engineered details, trade-offs, cost optimizations... no wonder microscopes are so expensive new.
wabutter wrote:
Wed Aug 12, 2020 9:45 pm
Although it would seem logical to assume that it would replicate a stereo effect, that is not the case.
Is this to say, the optics are intentionally designed to keep the lines-of-sight coaxial with theoretically zero parallax, or just that the parallax resulting from angling the tubes inward for ergonomic reasons is too small to give any perceptible sense of depth?
The exact same image is being split 2 ways. No amount of angle in the tubes will change the image. Stereo scopes use 2 distinct optical paths to achieve that.

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Re: Why are binocular eye tubes angled inward slightly?

#5 Post by hans » Thu Aug 13, 2020 5:32 am

Zuul wrote:
Thu Aug 13, 2020 3:39 am
The exact same image is being split 2 ways. No amount of angle in the tubes will change the image.
This was the mental model I had, without having thought about it too carefully, until earlier today. But now I think there is no "exact same image" and that is why I brought up the possibility of parallax. I think there would only be an "exact same image" if there were something like a frosted glass plate in a conjugate of the specimen plane prior to the splitter (ignoring that no such plane exists in the 410) which preserved only the amplitude information present in the light field.
Zuul wrote:
Thu Aug 13, 2020 3:39 am
Stereo scopes use 2 distinct optical paths to achieve that.
What about common main objective designs I mentioned in the first post? Another example of a system using two different non-coaxial views through the same lens is phase detection autofocus, I think.

I am not claiming that the parallax achievable through something like the 410 has any practical significance (that is one of the things I was trying to ask in the question) but I don't think it is theoretically zero, either.

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Re: Why are binocular eye tubes angled inward slightly?

#6 Post by MichaelG. » Thu Aug 13, 2020 6:36 am

wabutter wrote:
Wed Aug 12, 2020 9:45 pm
[…]
The philosophy from the designers was that the image being viewed through the microscope is virtual. A representation of an object at about 250mm in front of the viewers face. Not unlike reading a book, the eyes normally converge slightly at that distance.
[…]

As an added point, it provided American Optical a unique selling proposition when offering it competitive bids.
.

I believe this ^^^ is the essence of the design choice ... although the validity of the logic is debatable.

250mm is the reference length for calculation of visual magnification because it is the “typical” closest focusing distance of the unaided eye [*] ... but that is only really relevant if the viewer is repeatedly switching attention between microscope those papers.

There is another camp which suggests that when using the microscope for long durations, it is better to have the eyes relaxed [focused at the user’s ‘infinity’ and therefore parallel].

MichaelG.
.
[*]
Anyone who has studied Ergonomics will immediately realise that designing around a reference of “typical” is very poor practise.
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Re: Why are binocular eye tubes angled inward slightly?

#7 Post by apochronaut » Thu Aug 13, 2020 12:23 pm

Spencer adopted the practice a long way back, after the first world war, with the Jentsch heads. There is a physiological explanation in the old catalogues( 1920's) similar to what Wayne describes. The fact that they continued the practice when they switched to Seidentopf heads is a bit strange. I suppose it shows that they had a commitment to the principal . I think Wayne' s comment that the transference from microscope view to note taking would be less fatigueing with a parallax set into the system, bears some credence. I can also vouch for the fact that a slight mal-adjustment of the interpupillary distance when using the converging tubes results in a quite high degree of 3-D, both positive and negative, depending on whether the adjustment is over or under.

Is it more restful to be focusing at a point 250mm in front of you or gazing at infinity? Well, probably being focused and attentive to something irregardless of where it is, is less restfull whether it is 250mm away or a zillion miles ( George Dubbelya : How much is a Brazilian?) but if I ever find myself fixated, gazing blissfully down a pair of microscope eyetubes as though I am imbecilically staring into the vacuum of infinity, I don't think whether the tubes are parallel or pointed at the apex of Margaret Hamilton's hat, much matters.

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Re: Why are binocular eye tubes angled inward slightly?

#8 Post by Zuul » Thu Aug 13, 2020 11:54 pm

hans wrote:
Thu Aug 13, 2020 5:32 am
Zuul wrote:
Thu Aug 13, 2020 3:39 am
The exact same image is being split 2 ways. No amount of angle in the tubes will change the image.
This was the mental model I had, without having thought about it too carefully, until earlier today. But now I think there is no "exact same image" and that is why I brought up the possibility of parallax. I think there would only be an "exact same image" if there were something like a frosted glass plate in a conjugate of the specimen plane prior to the splitter (ignoring that no such plane exists in the 410) which preserved only the amplitude information present in the light field.
Perhaps "exact" is a poor choice of words. The beam splitter projects images to each optical path that are from the same perspective, and presumably you are are viewing with each eye centered in it's respective projection. I don't see how that could result in any significant parallax.
hans wrote:
Thu Aug 13, 2020 5:32 am
Zuul wrote:
Thu Aug 13, 2020 3:39 am
Stereo scopes use 2 distinct optical paths to achieve that.
What about common main objective designs I mentioned in the first post?
The projection is coming through the common lens from different angles. It does not negate the discrete optical paths. The compound microscope is taking one image and diverting half of the photons from a single image in a different direction.

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Re: Why are binocular eye tubes angled inward slightly?

#9 Post by hans » Fri Aug 14, 2020 3:41 am

Zuul, I think the splitter is causing confusion. If we ignore the splitter and mirrors and consider only the "unfolded" positions/angles of the eyepieces relative to the optical axis of the objective and telan lens, do you agree that should be equivalent for the purpose of discussing parallax?

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Re: Why are binocular eye tubes angled inward slightly?

#10 Post by Zuul » Fri Aug 14, 2020 4:34 am

hans wrote:
Fri Aug 14, 2020 3:41 am
Zuul, I think the splitter is causing confusion. If we ignore the splitter and mirrors and consider only the "unfolded" positions/angles of the eyepieces relative to the optical axis of the objective and telan lens, do you agree that should be equivalent for the purpose of discussing parallax?
No, I don’t. The splitter is everything. The direction your eye is pointing is only relevant if it reflects the perspective observed. In a compound microscope there is a single image, from a single perspective. It just happens to be duplicated. Parallax, by definition, is the result of viewing from 2 different perspectives.

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Re: Why are binocular eye tubes angled inward slightly?

#11 Post by MichaelG. » Fri Aug 14, 2020 6:30 am

apochronaut wrote:
Thu Aug 13, 2020 12:23 pm

Is it more restful to be focusing at a point 250mm in front of you or gazing at infinity? Well, probably being focused and attentive to something irregardless of where it is, is less restfull whether it is 250mm away or a zillion miles ( George Dubbelya : How much is a Brazilian?) but if I ever find myself fixated, gazing blissfully down a pair of microscope eyetubes as though I am imbecilically staring into the vacuum of infinity, I don't think whether the tubes are parallel or pointed at the apex of Margaret Hamilton's hat, much matters.
On the assumption [please correct me if that’s wrong] that you are responding to my post

My comment was based on simple human biology, not some romantic analysis of the observer’s mental state:
... There is another camp which suggests that when using the microscope for long durations, it is better to have the eyes relaxed [focused at the user’s ‘infinity’ and therefore parallel].
.
It is a simple fact that the muscles which control i) shape of the eye-lens, and ii) convergence of the eyes are all in a relaxed state when viewing at the user’s ‘infinity’

MichaelG.

.

P.S. ___ please also note that the optimum eye-tube divergence angle [user’s convergence angle] probably differs for the two scenarios mentioned, and therefore the ‘ergonomic’ solution would be to have it adjustable.
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Re: Why are binocular eye tubes angled inward slightly?

#12 Post by hans » Fri Aug 14, 2020 6:37 pm

Zuul wrote:
Fri Aug 14, 2020 4:34 am
No, I don’t. The splitter is everything. The direction your eye is pointing is only relevant if it reflects the perspective observed. In a compound microscope there is a single image, from a single perspective. It just happens to be duplicated. Parallax, by definition, is the result of viewing from 2 different perspectives.
I don't see how the splitter changes anything fundamentally. If the splitter was removed from the microscope you could look straight through it at an object and, without moving the object or splitter, obtain slightly different perspectives of the object by moving your head side-to-side? And a second observer could simultaneously do the same thing while looking through the right angle path of the same splitter? So in what sense, exactly, does the splitter "duplicate" a perspective? Any why does the presence of the splitter necessarily require both eye paths to be showing "a single image, from a single perspective"?

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Re: Why are binocular eye tubes angled inward slightly?

#13 Post by Zuul » Fri Aug 14, 2020 7:52 pm

hans wrote:
Fri Aug 14, 2020 6:37 pm
I don't see how the splitter changes anything fundamentally. If the splitter was removed from the microscope you could look straight through it at an object and, without moving the object or splitter, obtain slightly different perspectives of the object by moving your head side-to-side? And a second observer could simultaneously do the same thing while looking through the right angle path of the same splitter? So in what sense, exactly, does the splitter "duplicate" a perspective? Any why does the presence of the splitter necessarily require both eye paths to be showing "a single image, from a single perspective"?
That is not true. If you removed the splitter, only one eye could look straight through. The splitter replicates the image for the second eye. There is no second optical path. In a stereo, at no point do the two optical paths coincide. Even in shared objective designs, the angle viewed through the objective is different for each eye.
compound vs stereo.jpg
compound vs stereo.jpg (16.77 KiB) Viewed 2495 times
For compound microscopes, each eye is looking down the center of the objective. The angles built into the binocular head don't change that. A and B are fundamentally different.

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Re: Why are binocular eye tubes angled inward slightly?

#14 Post by hans » Sat Aug 15, 2020 12:15 am

Zuul wrote:
Fri Aug 14, 2020 7:52 pm
The splitter replicates the image for the second eye. There is no second optical path. In a stereo, at no point do the two optical paths coincide. Even in shared objective designs, the angle viewed through the objective is different for each eye. For compound microscopes, each eye is looking down the center of the objective. The angles built into the binocular head don't change that. A and B are fundamentally different.
I rearranged your "A" sketch a bit and added "C".

"A" is how the splitter is drawn in that patent I have been looking at, what Wayne said the behavior is in practice, and I think what you have in mind with statements like "replicates the image for the second eye"?

What I was wondering about is the possibility of a small amount of parallax as illustrated in "C" and I don't see how the presence of the splitter rules that out. As I said earlier, I am not claiming the angular separation possible in something like the 410 is large enough to have any noticeable effect. (Although if I understand correctly, apochronaut has actually seen some 3-D effect while adjusting IPD?)

To give a bit of background for why I was wondering about this: I was thinking that the difference in thickness between the right/left color correcting doublets from the other thread may have nothing to do with the color correcting function and could simply be a way to equalize a difference in the axial positions of the intermediate images resulting from the inward angle of the eye tubes. In that context I think it could matter whether the optics are more like "A" or "C" even if the angle is too small to give any perception of depth.
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Re: Why are binocular eye tubes angled inward slightly?

#15 Post by Zuul » Sat Aug 15, 2020 2:31 am

Both oculars are at an angle, so why would one be “off axis” more than the other? I think you are inventing problems to solve your mystery. What you are calling C could only exist at exceedingly small angles. Even a degree or 2 off-axis would be easily detected by eye. The angle of the eye tubes is not small, so why draw a correlation?

Have you swapped the right and left eye tubes to see if there is a visible difference?

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Re: Why are binocular eye tubes angled inward slightly?

#16 Post by hans » Sat Aug 15, 2020 3:21 am

Zuul wrote:
Sat Aug 15, 2020 2:31 am
Both oculars are at an angle, so why would one be “off axis” more than the other?
In the drawing with the remaining optics omitted one path been reflected, one has not. Ultimately both paths undergo the same number reflections and the situation ends up like "B".
Zuul wrote:
Sat Aug 15, 2020 2:31 am
What you are calling C could only exist at exceedingly small angles. Even a degree or 2 off-axis would be easily detected by eye. The angle of the eye tubes is not small, so why draw a correlation?
I agree, basically, but it is not obvious to me how several degrees on the eyepiece side translates into the specimen plane, in terms of depth perception being noticeable, especially given the narrow depth of field.
Zuul wrote:
Sat Aug 15, 2020 2:31 am
Have you swapped the right and left eye tubes to see if there is a visible difference?
I have done some part swapping and not seen any obvious difference in transverse CA which I why I was thinking maybe just path length difference. I also tried estimating axial position of the intermediate image directly using the adjustable eye tubes but got wildly varying results trying to judge it by eye, presumably because I am not that good at telling when my eye is focused at infinity. I think an accurate measurement would require using my DLSR manually focused at infinity in live view mode through the eyepiece, but that is kind of a pain to set up and execute. I may still try it at some point.

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Re: Why are binocular eye tubes angled inward slightly?

#17 Post by hans » Sat Aug 15, 2020 4:25 am

Regarding the intended equivalent viewing distance, I think 250 mm may be a number mixup. Using the 63 mm average IPD that would be almost 15 degrees. I have not tried to measure accurately but on the 410, holding up a protractor, I estimate more like 2-4 degrees.

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Re: Why are binocular eye tubes angled inward slightly?

#18 Post by MichaelG. » Sat Aug 15, 2020 7:07 am

hans wrote:
Sat Aug 15, 2020 4:25 am
Regarding the intended equivalent viewing distance, I think 250 mm may be a number mixup. Using the 63 mm average IPD that would be almost 15 degrees. I have not tried to measure accurately but on the 410, holding up a protractor, I estimate more like 2-4 degrees.
That would seem a much more reasonable angle to choose ... but it would help if we could we start from a basis of fact.
Obviously the design drawings would be ideal, but [in the real world] ; as I do not have access to a 410, I would be grateful if someone could provide a decent illustration of the binocular, from which I can take a measurement.

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Re: Why are binocular eye tubes angled inward slightly?

#19 Post by MichaelG. » Sat Aug 15, 2020 11:51 am

This may be of interest: https://worldwide.espacenet.com/patent/ ... US3309161A

Fig.1 is a mighty fine illustration : But the dreaded phrase ‘Do Not Scale’ comes to mind ... and it is obvious from the text commencing Column 3 line 51, that no novelty is claimed for the convergent viewing option.

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Re: Why are binocular eye tubes angled inward slightly?

#20 Post by hans » Sat Aug 15, 2020 5:16 pm

Nice find, Michael. Most interesting to me is figure 6 showing elements 18 and 20 with similar thickness difference as I have been wondering about in the 410 head. The description of them is vague and confusing and I am interpreting it something like: "For the purposes of this patent 18 and 20 can be considered to represent the field lenses of the eyepieces." I am guessing in an actual implementation the eyepieces would be interchangeable and the thickness difference would be in the glass window at the base of the eye tube? (So like the 410 but the windows are actually just plain glass windows of differing thickness?)

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Re: Why are binocular eye tubes angled inward slightly?

#21 Post by MichaelG. » Sun Aug 16, 2020 7:06 am

MichaelG. wrote:
Sat Aug 15, 2020 11:51 am

... the dreaded phrase ‘Do Not Scale’ comes to mind ...
But I couldn’t resist trying !

Measured to the best of my ability:
On Fig.1, each tube is angled at 1.5° to the main axis
... making the included eye-convergence angle 3°

MichaelG.

.

Edit: I also had a quick look around, to see if this general topic was discussed elsewhere
... google found this for me: viewtopic.php?t=7404
:lol:
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Re: Why are binocular eye tubes angled inward slightly?

#22 Post by hans » Sun Aug 16, 2020 6:33 pm

The 410 Siedentopf design is confusing because the inward angle varies with IPD. I measured 3.5 deg total with 90 deg bend in the Siedentopf mechanism which is toward the narrow end of the IPD range. The angle in the patent drawing of the optics is 5.4 deg with the Siedentopf mechanism flattened (not actually possible mechanically) but I think that is roughly consistent: (5.4 deg / 2) * sqrt(2) = 3.8 deg

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Re: Why are binocular eye tubes angled inward slightly?

#23 Post by wabutter » Tue Aug 18, 2020 5:25 am

Hans, I am not sure what happened to my earlier post on this thread, but I think you are WAY over thinking this.

[i how the did you do this? " I measured 3.5 deg total with 90 deg bend in the Siedentopf mechanism which is toward the narrow end of the IPD range."
[/i]
The telelens moves to compensate for the IPD change, so Sorry, this is not an issue. The inclusive angle of the converging eye tubes is about 5 degrees. Your post is even more confusing as it conflates IPD with convergences angles and doesn't take into consideration measurement error. Your measurements on the drawings from the patent are not relevant to the 410 tube design. The patent tube drawings are not Siedentopf.
The bottom line is no matter what the angles are, the systems works.

I don't want to be confrontational in this thread, although I guess am, it makes little sense to speculate on the design specifications of system that existed for more than 50 year and dominated a market segment for more than two decades. It worked.

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Re: Why are binocular eye tubes angled inward slightly?

#24 Post by hans » Tue Aug 18, 2020 7:37 am

Sorry for the confusion. Almost all of my comments in this thread were relating to the 410 head and patent US 4,715,697 which I originally posted about in the thread about the color correcting doublets in the eye tubes. The patent Michael linked, US 3,309,161 with the moving telelens, is a series 10 head I think? I am not familiar with series 10 and regarding that patent I only commented that it showed a similar difference in thickness of the right/left eye tube windows as is present in the 410 heads.

I still do not understand the right/left thickness difference and the comment about it possibly being related to path length was just a theory I had, which probably made more sense in the context of the 410, but may be completely incorrect.

On the 410 heads there is a slight wedge (~2-3 deg relative to the axis of rotation) in the castings where the eye tubes mount. If it were possible to fully "flatten" the binocular then the wedges would give a total around 5 deg between the two eye tubes. As the binocular is "folded" to decrease IPD the angle of each eye tube relative to the axis of rotation remains the same but the angle between the eye tubes decreases. With the IPD set corresponding to a 90 deg bend in the binocular the angle between the eye tubes should be (with small-angle approximation) 1/sqrt(2) times the "flattened" angle unless I am making a geometry error. That is the calculation I was showing to demonstrate rough agreement between the patent and measurement. The way I actually measured 3.5 deg was, with both eye tubes removed and the binocular folded to 90 deg, laying a protractor across between the two flat mounting faces to measure how much they deviate from being coplanar. There is some measurement error, of course, but these angles are not really that small and I am fairly confident the errors are less than 1 deg.

I am not questioning the wisdom of the design. The opposite, actually. I get the impression that it is a clever and efficient design with many subtle details carefully thought through. That is why I find it interesting to try to understand the reasoning behind it.

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Re: Why are binocular eye tubes angled inward slightly?

#25 Post by MichaelG. » Tue Aug 18, 2020 7:49 am

[/attachment]
wabutter wrote:
Tue Aug 18, 2020 5:25 am
[…] The inclusive angle of the converging eye tubes is about 5 degrees. Your post is even more confusing as it conflates IPD with convergences angles and doesn't take into consideration measurement error. Your measurements on the drawings from the patent are not relevant to the 410 tube design. The patent tube drawings are not Siedentopf.
The bottom line is no matter what the angles are, the systems works.

I don't want to be confrontational in this thread, although I guess am, it makes little sense to speculate on the design specifications of system that existed for more than 50 year and dominated a market segment for more than two decades. It worked.
.

May I just mention that the measurement of Fig.1 was done by me, as a reference to an historic design, simply to inform the wider discussion about Ergonomics.
.
N.B. This is NOT the 410 head ... it is for Ergonomics reference only
N.B. This is NOT the 410 head ... it is for Ergonomics reference only
5E1EE45B-92BE-48BF-94F0-90236A9962CD.jpeg (129.65 KiB) Viewed 2313 times
.

I am aware that patent 330961 is
not relevant to the 410 tube design. The patent tube drawings are not Siedentopf.
... but I do apologise if I caused any confusion by not emphasising that point.

MichaelG.
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Re: Why are binocular eye tubes angled inward slightly?

#26 Post by hans » Tue Aug 18, 2020 8:22 am

This is the "flattened" optical drawing of the 400-series binocular head I was referring to. The angle between the axes labelled 18 and 20 is 5.4 degrees in the drawing. (I did not see that angle given numerically in the patent.)
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Re: Why are binocular eye tubes angled inward slightly?

#27 Post by hans » Wed Aug 19, 2020 7:36 pm

This older Spencer Lens Co. patent has reasoning like Wayne and apochronaut recalled:
In a binocular microscope it is desirable to have the eyepieces converge away from the eyes at substantially the normal angle of convergence of one's eyes for reading or close work, that is about 8 degrees, so that a person using the microscope can remove his gaze from the instrument for reading, making notes, or doing other close eye-work, and return to the microscope without requiring readjustments or refocusing of the eyes with each change of use thereof.
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Re: Why are binocular eye tubes angled inward slightly?

#28 Post by MichaelG. » Wed Aug 19, 2020 8:11 pm

Thanks for sharing that, Hans

I have reasons to doubt the validity of the argument; but it’s good to see the claim in writing.

... Very much appreciated.

MichaelG.
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