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Binocular length (1 Viewer)
- Thread starter Synaps
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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.
Lee
Lee
henry link
Well-known member
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).
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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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
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
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.
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.
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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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
Peter
Chosun Juan
Given to Fly
henry link
Well-known member
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.
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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 |:d|
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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.
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.
henry link
Well-known member
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.
WJC
Well-known member
Hi, Henry:
Binocular owners are now learning what telescope makers have known for almost a hundred years. But, better late than never. :cat:
Bill
WJC
Well-known member
Not quite. But I was being kind by only invoking AMATEUR telescope makers. And their work began to take off in the late 1920s. :cat:
Bill
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.
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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.
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.
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....:smoke:
henry link
Well-known member
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