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Old Monday 4th December 2017, 10:33   #1
Synaps
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Binocular length

I am trying to figure out what the advantages of longer binoculars are as I noticed that many top bins in 8x42 are quite long / tall and cheapo bins tend to be shorter. Can someone tell me the optical advantages the added length brings?
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Old Monday 4th December 2017, 12:11   #2
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I am happy to be shot down by more expert members but my understanding is that a longer length allows the design of lenses less prone to delivering chromatic aberration (colour fringing). I suspect this is one reason why Zeiss SF is a bit longer than most and is because for reasons of improved balance and handling, the objective lens group was reduced from 3 to 2 (with the assistance of the focusing lens). You would normally expect 3 objective lenses to control chromatic aberration and I am guessing that SF's extra length helped in this regard.

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Old Monday 4th December 2017, 14:13   #3
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Unless the optical designs, prism sizes and glass types are identical I don't think much can be predicted about the optical quality of a binocular by its physical length. Look at the cutaways below. The SF on the bottom is substantially physically longer than the EL-Swarovision on top, but the objective and eyepiece designs are so different that we have no idea which objective has a longer effective optical focal length, let alone what kinds of glass types are used.

The SF has an objective design that resembles the original Swaro EL (fixed doublet followed by a positive focusing singlet). As Lee suggested that design may simply require a longer physical focal length just to equal the optical focal length and aberration levels of the Swarovision design (fixed triplet followed by a negative focusing singlet).
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Old Monday 4th December 2017, 14:35   #4
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I notice that the prisms are reversed. I wonder if that was done to move the center of balance of the SF back closer to the eye cups?

Bob
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Old Monday 4th December 2017, 14:37   #5
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Hi,

in general terms - if we compare two instruments of similar design but with one having a slower focal ratio and longer focal length, that example will be easier to correct for CA (and other aberrations introduced by the very steep light cone in the fast instrument too).

But I'm not sure if the fairly small differences in length seen in 42mm bins are that significant to cause significantly easier to correct designs - and as has been pointed out - other measures can be taken to mitigate the ill effects of a faster design like air-spacing objective lenses (and thus giving the designer another degree of freedom) or using a triplet objective - or both.

One factor which obviously lengthens some models are Abbe-Koenig prisms, but they're fairly rare.

Joachim
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Old Monday 4th December 2017, 16:07   #6
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Longer focal length objectives have less aberrations.
In addition, the eyepiece will be a longer focal length, giving more eye relief generally and a simpler design.
However, a longer focal length eyepiece may not have such a wide field.

The Takahashi 22x60 is, I think, an example, maybe f/6 instead of f/4.
But it is really a binocular telescope.
An f/10 binocular could give excellent images, but small field, bulky and heavy. No longer a hand held binocular.

One of the reasons astro telescopes can be so good, even though very simple, is that they may have very long focal ratios compared to spotting scopes.
In addition, they have fewer problems with temperature changes.
A planetary Newtonian might be f/9 instead of a more common f/5 or f/6 nowadays.
In my experience short focal length Newtonians are a problem and much more critical to collimate.
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Old Tuesday 5th December 2017, 08:38   #7
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Okay, so it seems that longer bins give more freedom for design/ different construction choices and are less prone to aberrations, mainly chromatic. Doing a basic calculation I found that the distance between the outer lenses of the Zeiss SF is 31mm larger than on my old 42mm bin. which translates to a 25% larger focal length. This also translates to a 25% smaller entry angle (for the outer light rays into the next lens 90-10 deg= 80 deg and otherwise 90-12,5=77,5 deg). I was thinking about it for a bit last night, and I can imagine that the better entry angle (closer to 90deg) results in less reflection issues.

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Old Tuesday 5th December 2017, 09:26   #8
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There only s increased drive for short Newtonian for portability and Richfield applications. Correctors and modern eyepieces eliminate any optical concerns and as long as the scope is well built it can simply be collimated and hold collimation. My old wide angle 7x35 are very squat and probably around f4, give lovely images.

Peter
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Old Tuesday 5th December 2017, 12:13   #9
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Quote:
Originally Posted by ceasar View Post
I notice that the prisms are reversed. I wonder if that was done to move the center of balance of the SF back closer to the eye cups?

Bob
Yep



Chosun
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Old Tuesday 5th December 2017, 13:43   #10
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Okay, so it seems that longer bins give more freedom for design/ different construction choices and are less prone to aberrations, mainly chromatic. Doing a basic calculation I found that the distance between the outer lenses of the Zeiss SF is 31mm larger than on my old 42mm bin. which translates to a 25% larger focal length. This also translates to a 25% smaller entry angle (for the outer light rays into the next lens 90-10 deg= 80 deg and otherwise 90-12,5=77,5 deg). I was thinking about it for a bit last night, and I can imagine that the better entry angle (closer to 90deg) results in less reflection issues.
Thinking along these lines only works for binoculars with simple achromatic doublets for objectives. I don't think you can make any assumptions about aberrations, reflections or even the actual focal length of a modern complex binocular based on physical length.

In the cutaways I posted notice that the last element in the SF objective (focusing element) is a positive lens that acts as a focal reducer while the last element of the Swaro objective is negative and acts something like a Barlow lens to increase focal length. By the time the objective light cones reach the eyepiece fieldstops (after further massaging by the negative eyepiece field groups) the Swaro might be the one with the longer effective focal length and lower axial aberrations in spite of its shorter physical length. In fact, it seems probable that the Swaro's effective focal length is longer since its eyepiece fieldstop is about 50% larger than the SF's in spite of having a smaller real field.

Also, it's baffling, not "entry angle", that determines freedom from internal reflections.

Last edited by henry link : Tuesday 5th December 2017 at 14:04.
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Old Tuesday 5th December 2017, 14:08   #11
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Thinking along these lines only works for binoculars with simple achromatic doublets for objectives. ..... By the time the objective light cones reach the eyepiece fieldstops (after further massaging by the negative eyepiece field groups) the Swaro might be the one with the longer effective focal length and lower axial aberrations in spite of its shorter physical length.

Also, it's baffling, not "entry angle", that determines freedom from internal reflections.
I see what you are getting at, Henry, and I agree my reasoning about reflection on lens surfaces due to a different entry angle does not hold up in all situations, as your image clearly shows, there will be differences due to distances between lenses and how curved the lenses are. But the flat prism surface would always benefit, i think...

Until now, when I spoke of reflection, I have meant the reflection off the surface of lenses only, not internal reflections due to light bouncing off the tube walls which are insufficiently blackened or ridged. But.. I can imagine a longer tube has advantages here too, with the prism being further away from the objective lens.

I certainly do not think image quality of a bin can be judged solely by binocular length, but there must be some advantages to longer tubes and I like to know which benefits these are

Last edited by Synaps : Tuesday 5th December 2017 at 14:26.
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Old Tuesday 5th December 2017, 14:29   #12
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Hi Peter,
Post 8.

Try collimating a 20.5 inch f/3.9 Newtonian single handedly, even though it has a 1/20th wave mirror.

An ancient 6 inch f/8 secondhand AE Newtonian was amazingly good.
Unfortunately Jim Hysom, master optician (AE Optics) passed away recently.
He made most of the better U.K. larger amateur scopes with his brother.
He bought a very old Ross ships telescope from me for what I paid. I told him the optics were no good and despite his best efforts it was still no good.

Jim Hysom obtained the batch of specially made Russian 20x60 binoculars. George Alcock had one as I did.
Unfortunately he also had a batch of Japanese Orthoscopic eyepieces that had radioactive glass. It surprised me, and embarrassed him, when I monitored them. There were similar Swift eyepieces also.

The 8.5 inch f/6 C.Frank Newtonian was very good, but faster telescopes are just more difficult to keep aligned.

Faster refracting optics usually have steeper curves.
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Old Tuesday 5th December 2017, 14:58   #13
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I certainly do not think image quality of a bin can be judged solely by binocular length, but there must be some advantages to longer tubes and I like to know which benefits these are
Well, there's an advantage to a higher focal ratio as long as designs and glass types remain identical. You can see that in a star-test when you compare a binocular at full aperture to itself at a stopped down aperture. That's one reason I prefer binoculars with large exit pupils in daylight. The reduction in effective aperture and increase in focal ratio imposed by the eye will nearly always lead to lower aberrations.
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Old Tuesday 5th December 2017, 17:02   #14
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Well, there's an advantage to a higher focal ratio as long as designs and glass types remain identical. You can see that in a star-test when you compare a binocular at full aperture to itself at a stopped down aperture. That's one reason I prefer binoculars with large exit pupils in daylight. The reduction in effective aperture and increase in focal ratio imposed by the eye will nearly always lead to lower aberrations.
Hi, Henry:

Binocular owners are now learning what telescope makers have known for almost a hundred years. But, better late than never.

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Old Tuesday 5th December 2017, 18:51   #15
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400 years maybe.
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Old Tuesday 5th December 2017, 21:48   #16
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400 years maybe.
Not quite. But I was being kind by only invoking AMATEUR telescope makers. And their work began to take off in the late 1920s.

Bill
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Old Wednesday 6th December 2017, 13:40   #17
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So really longer length in a binocular does not necessarily mean better optical performance. It is dependent on the optical design. As Henry says because of optical design the SV probably has a longer focal length than the SF. Ergonomically, I prefer a shorter binocular.

Last edited by denco@comcast.n : Wednesday 6th December 2017 at 15:40.
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Old Wednesday 6th December 2017, 14:47   #18
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Dennis, I think most people do, which is in part what triggered my question.

Henry, Thanks for your answer. I am still trying to fully comprehend what you are saying, not so easy for me as I need to get familiar with the jargon a bit more, but I do have a question: in photography a stopped down diaphragm gives more depth of field. Would/ could that effect also take place in (stopped down) binoculars?

A rather controversial question I suppose, but hey... I am fairly new here, so I can ask stupid and controversial questions.
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Old Wednesday 6th December 2017, 16:20   #19
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Just read that long focal length telescopes have more depth of field! Guess stopping down to increase dof may also work for binoculars... Not sure how much effect the stopping down or increasing focal length by 25% has though, because in telescopes the increases in length are much larger....
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Old Wednesday 6th December 2017, 16:32   #20
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... but I do have a question: in photography a stopped down diaphragm gives more depth of field. Would/ could that effect also take place in (stopped down) binoculars?

A rather controversial question I suppose, but hey... I am fairly new here, so I can ask stupid and controversial questions.
That question has been thrashed around a few times here. My answer is no and yes; it depends on whether the exit pupil of the binocular is larger or smaller than the entrance pupil of the eye. Binoculars are afocal instruments, which means they don't actually bring light to a focus like a camera lens or the optics of the eye. When you look through a binocular it's the lens of your eye that brings the light to focus on your retina, not the binocular.

So, as long as the binocular exit pupil is larger than the eye's entrance pupil the only significant factor for DOF is magnification and the answer is no. However, if the binocular exit pupil is smaller than the entrance pupil of the eye that will increase the DOF by stopping down the eye's lens and thereby changing the focal ratio of the eye's optics just like the diaphragm in a camera lens and the answer is yes.

Henry
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Old Wednesday 6th December 2017, 18:30   #21
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Just read that long focal length telescopes have more depth of field! Guess stopping down to increase dof may also work for binoculars... Not sure how much effect the stopping down or increasing focal length by 25% has though, because in telescopes the increases in length are much larger....
Just noticed this post. My answer above applies to telescopes as well. If the telescope's exit pupil is larger than the eye's pupil magnification is the one and only significant factor for DOF. However, if the telescope objective is stopped down so that the exit pupil is smaller than the eye's pupil the DOF will increase, not because the focal ratio of the telescope is higher, but because the focal ratio of the the eye is now higher.

Henry
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Old Wednesday 6th December 2017, 19:23   #22
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So really longer length in a binocular does not necessarily mean better optical performance. It is dependent on the optical design. As Henry says because of optical design the SV probably has a longer focal length than the SF. Ergonomically, I prefer a shorter binocular.
“Necessarily?” NO. Traditionally? YES.

Shorter focal ratio systems CAN be equal to longer ones. However, it usually requires MORE EXOTIC glass types (read: $$), at least one ASPHERIC ELEMENT (read: $$), MORE ELEMENTS (read: extra weight).

Those unfamiliar with optical engineering have a proclivity to think that you can correct for optical aberrations onesie-twosie. ‘Tain’t so. You might correct one aberration to a desired level at the expense of increasing another a little and driving two others into the realm of the unreasonable. An engineer tries to design using his “degrees of freedom” to come up with a design that will meet the requirements of the MAJORITY of potential buyers.

This is also true of conceptual matters. I designed a Houghton telescope in which all photo-visual wavelengths would fit into the Airy Disc like a pea in the center of the platter. The contrast-destroying central obstruction was about 40%. By simply changing the designated stop from the primary mirror to the secondary mirror, I reduced the central obstruction to 30%. By so doing, the contrast was MUCH better. However, spots and OPDs looked like they had been concocted by a third grader.

Physics can be a harsh taskmaster.

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Old Wednesday 6th December 2017, 20:02   #23
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Don't know about DOF but I was able to measure distances of a mile, maybe more, with the 317mm Dall Kirkham, 4650mm focal length, just by the focus position of the eyepiece compared to the stars.
So I don't think the DOF was that great, although it could be.
I don't think the 317mm aperture was stopped down at high magnification.

I suppose it was similar to a 317mm base rangefinder with high magnification, although I haven't done calculations to compare.

I also had an approximately 6ft rangefinder possibly Barr and Stroud, which measured miles I think. I can't remember the maximum accurate range or the accuracy.

P.S.
Apparently the Americans used stereoscopic rangefinders and the British Barr and Stroud were coincidence rangefinders.
The stereoscopic and coincidence acuities were about equal in tests.
4.5m American and 3m British instruments.
Ranges 4,000 yards to 12,000 yards approx.

Laser rangefinders are used nowadays but can give away the observer's position.

I suppose the DOF in binoculars and telescopes depends on the observer's accommodation and acuity?

Last edited by Binastro : Wednesday 6th December 2017 at 20:28.
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