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Weirdest binoculars on the market? (1 Viewer)

John Cantelo

Well-known member
Pentax have produced what must be one of the oddest binocular design ever produced: the Pentax VD 4x20 binoculars with a detachable barrel so that it converts into a 16x20 scope. I'm sure it's technically very clever but I struggle to see what market it's aimed at since 4x magnification is far too low for most uses. As I recall, most low power small OG instruments are opera glasses so presumably these are only of interest for people who want a close look when the fat lady sings ...
(see vd4x20-hires.jpg (700×700) (d1kmrw706gcgzi.cloudfront.net)
 

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John A Roberts

Well-known member
Australia
While true weirdness may be in the eye of the beholder, an historical overview of unusual optics can be found in Gijs’ 2017 presentation
’Multi-functional binoculars and telescopes in the past four centuries’

The 111 page, 19.4 MB document can be found at: https://www.houseofoutdoor.com/verrekijkers/verrekijkers-testen-en-vergelijken/

While appearing to be binocular, the Nicour combined telescope and 50 exposure dry glass plate camera of 1866, seems to be a good early attempt!


John
 

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pbjosh

missing the neotropics
Switzerland
The detachable aspect doesn't interest me. A decent 4x or 5x in the 20-25mm range could be really interesting, but $270 for a 40 degree AFOV just doesn't quite get me excited :)
 

Manolito

Active member
I find it very interesting. I find them very useful for what I do and to leave them in the car 24/7 just in case :giggle:
 

dries1

Member
Dennis when can we see you on camera like that other guy doing binocular reviews? I am a bit curious about these as well, just curious mind you. Look forward to see what you think of them. Hope fully you get a good sample.
By the way, I take it that they are IF - no mention of that on the video.

Andy W.
 
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pianoman

duck and diver, bobolink and weaver
I'm still looking for the holy grail of quality binoculars with the ability to capture the binocular view as a high res record digital image at the touch of a convenient button... Maybe that will never be possible
 

fazalmajid

Well-known member
I'm still looking for the holy grail of quality binoculars with the ability to capture the binocular view as a high res record digital image at the touch of a convenient button... Maybe that will never be possible
It seems surprisingly hard to put a large enough sensor to get decent results (so 1" or larger), with optics that have the right image circle, in a casing the size of normal binocular tubes. The human eye is a remarkable instrument.
 

John Cantelo

Well-known member
I confess that I'm surprised anyone should consider a 4x binoculars worth considering as useful tool for birding. Surely there are plenty of 7x20/21/25 central focus binoculars out there of a similar weight & quality that would serve much better.
 

jring

Well-known member
The detachable aspect doesn't interest me. A decent 4x or 5x in the 20-25mm range could be really interesting, but $270 for a 40 degree AFOV just doesn't quite get me excited :)

Hi,

you will probably not find any pair with a larger afov... because physics says no...

We have the following formula for the true field of view in degrees:

tfov in deg = 57.3 * field stop diameter in mm / objective focal length in mm

If we want to get 60 degrees of afov (and thus borderline wide angle) in a 4x instrument, we get 15 degrees of tfov. If we put this into the formula along with a 110mm guessed objective focal length (a bit more than the actual length due to some path inside the prisms) and solve for the field stop diameter, we get the following:

field stop diameter in mm = 15 * 110 / 57.3 = 29 mm

Which is a bit more than the objective aperture of the instrument... and make for huge prisms and eyepieces...

Joachim
 

Tringa45

Well-known member
Europe
If we want to get 60 degrees of afov (and thus borderline wide angle) in a 4x instrument, we get 15 degrees of tfov. If we put this into the formula along with a 110mm guessed objective focal length (a bit more than the actual length due to some path inside the prisms) and solve for the field stop diameter, we get the following:

field stop diameter in mm = 15 * 110 / 57.3 = 29 mm
Joachim,

I was unaware of the field stop calculation, but 110 mm focal length for a 20 mm objective? I would expect less than 80 mm and the glass path in the prisms would contribute by a factor of their respective refractive indices. Agree though that 60° AFoV is impractical..

John
 

pbjosh

missing the neotropics
Switzerland
Thank you for this post Joachim, I have a good interest in optics but sadly no formal training and little detail to my understanding. I think/thought I understood that in binoculars, the field stop is generally between the prism and the ocular lens in one way or another, and that in cheaper / simpler instruments it might inherently be the prism or it might be another essential lens, but in better instruments it will generally be a purposefully placed stop that helps control stray light further? Apologies if my question is more confusing than any possible answer. But that one way or another, the field stop is there, either explicitly or implicitly.

Among many things I don't fully understand is the effect of focal lengths in binoculars... there is some variability there in the design as I understand it, and you trade off TFOV for control of chromatic aberration and certainly other things that are beyond my understanding thus far! IE - it seems that, say, the 42mm SF models have an empirically longer focal length than compact 42mm bins like the Monarch HG or the UV HD. This should imply that it is harder to get a large FOV but easier to control CA, and should result in less depth of field. But there are also those who state as a maxim that depth of field/focus depends only on magnification. As I thought I understood it, this cannot be the case - it must be dependent on focal length as well?

In any case, it seems that a "magic" number looking at the market, for 6x bins is 30-32mm, several with very good AFOV, but a good FOV could be achieved down to smaller objective sizes. There are 5x25 bins with very good AFOV as well. I guess the formula you have presented would suggest that 5x25 might be the limit of low magnification and small objective diameter that can still, more or less, achieve a wide field. Smaller than that it becomes a trade off and a low magnification + wide AFOV implies either a larger instrument or, as you said, outsized prisms and eyepieces that do not make sense for the objective size.
 

jring

Well-known member
Joachim,

I was unaware of the field stop calculation, but 110 mm focal length for a 20 mm objective? I would expect less than 80 mm and the glass path in the prisms would contribute by a factor of their respective refractive indices. Agree though that 60° AFoV is impractical..

John
Hi,

I might of course be wrong, but 80mm is too low I think with an instrument length of 95mm... As for the prisms - I was not so much referring to the fact that we have glass path here but that the light path is folded a bit in there... which adds to the focal length...

The truth is probably somewhere between 80 and 110mm - but anyways reaching 60 deg afov is going to be difficult in a 4x instrument of that size.

Joachim
 

jring

Well-known member
Hi pbjosh,

I don't have formal training in optics either... just some reading. You are right that we often don't have a real field stop in any kind of eyepiece, but the field of view is rarely physically limited by the prisms - that would mean you get a non-circular field of view which sometimes can be seen in very cheap binoculars where the circle is cut off a bit by a too small prism.

Usually the lens mount for the front element of the eyepiece serves as a field stop, if there is no separate part for that function.

In general, a longer focal length and thus slower focal ratio (when assuming a fixed aperture) will make an optical design easier to perform well. It makes CA control easier, it will allow more simple eyepiece designs to work well and show all non design inherent aberrations introduced by the manufacturing process less prominently.

Unfortunately it makes for a longer instrument which most people don't really appreciate.

Joachim
 
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dries1

Member
Didn't Dennis say he was getting the Pentax/bino/monocular for a review? I guess the post was removed?

Andy W.
 

NDhunter

Experienced observer
United States
I am thinking it is more like a toy. I would be surprised if the optics are very good.

Jerry
 

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