Help on Understanding Optics.

[StuartJPP]

Not sure if this is the right place to put this but I'm sure someone here will know way more about it than me.

What I am trying to do is reduce the minimum focusing distance of my binoculars (8x42) so that I can focus on objects closer than say 2 meters. I'm not bothered that I will lose infinity focus at this point as it would only be temporary should the opportunity call for it.

In the photography world I can attach a close-up filter to my camera lens and it will allow me to focus the lens closer and closer depending on what diopter filter I attach.

I have tried one or two different ones that I already own (+1 or +2) but it shifts the minimum focusing distance from about 5 meters to about 20cm, way too close.

I have read somewhere that I'd need a +0.25 diopter or even +0.125 or less to perform this function.....is this correct?

Where on earth can I get such a filter in the size of the objective of the bins?

Probably in the days gone by I could have had an optician grind one but not even sure this is something plausible these days.

It seems like every genre of optics, photography, astronomy, microscopy etc. has its own vernacular despite the underlying physics being the same thing.

 

[vlaiv]

You don't need any additional optics to make binoculars focus closer - just increase separation between objective lenses and eyepieces somehow.

If your objective lenses can be unscrewed from the front of binoculars - it can be as simple as unscrewing them and adding appropriate distancing ring on that thread before screwing them back on.

[Peter Drew]

Just about to type the same reply.  Be aware of parallax issues.    🙂

[StuartJPP]

Thanks chaps unfortunately they are not removable, well not without any major surgery so that's why I was contemplating some form of front filter(s) that could be added and removed quickly.

I think I'd basically be adding a pair of glasses to the front of the bins to correct for hyperopia and need a prescription of the correct diopter.

I think the issue comes in that fractional positive dioptre filters are not very common.

I can't believe that there isn't a set available for exactly this type of application though it's obvious I don't really know what I'm talking about 😂

[Peter Drew]

Does this device have to be a binocular?  A finder could easily be modified for a close view.    🙂

[vlaiv]

2 hours ago, Peter Drew said:

Does this device have to be a binocular?  A finder could easily be modified for a close view.    🙂

Indeed.

In fact - regular finders when adopted to RACI will have issue focusing at infinity due to length of tube. Close focus will come "naturally" in that case.

[PeterW]

The Pentax papillon binoculars focus to 50cm and are  great for bugs and the like. Hopefully you’ll find a cheaper way to get such close in views.

 

Peter

[DaveL59]

47 minutes ago, PeterW said:

The Pentax papillon binoculars focus to 50cm and are  great for bugs and the like. Hopefully you’ll find a cheaper way to get such close in views.

 

Peter

They are indeed, I bought a pair for my daughter & grandaughter recently, new but damaged box so £99 vs the more usual £120 ish, link in the "What binos do you own" thread. They do come up on the bay-of-e so its possible to get a used pair for less of course.

[Astrobits]

The "filters" for close-up photography are simple low power lenses. For a diopter/dioptre ( D) of 0.25 you will need a lens of focal length 4 meters! For 0.125D the focal length is 8 meters! None of the scientific lens suppliers that I have looked at offer anything near your requirements. Try the online opticians for something with that low a D and choose the largest frames if they won't do the lenses by themselves.

I have, some time ago now, tried to get one of the cheap online specs makers to sell me the lenses without the frames but they refused ( don't understand the logic of that).  Have not pursued with any others yet but there are plenty more out there.

Nigel

[Peter Drew]

3 hours ago, Astrobits said:

The "filters" for close-up photography are simple low power lenses. For a diopter/dioptre ( D) of 0.25 you will need a lens of focal length 4 meters! For 0.125D the focal length is 8 meters! None of the scientific lens suppliers that I have looked at offer anything near your requirements. Try the online opticians for something with that low a D and choose the largest frames if they won't do the lenses by themselves.

I have, some time ago now, tried to get one of the cheap online specs makers to sell me the lenses without the frames but they refused ( don't understand the logic of that).  Have not pursued with any others yet but there are plenty more out there.

Nigel

The money is in the frames for the "spec" makers.      🙂

[Astrobits]

22 hours ago, Peter Drew said:

The money is in the frames for the "spec" makers.      🙂

That maybe so but as I was prepared to pay their "basic specs price" for just the lenses they would have made more profit than if I purchased the specs. Also they would have saved on cutting the lens to fit the frames, a further cost saving/increase profit.

Nigel

 

[Peter Drew]

I agree, that's why it doesn't seem logical.  Despite this, I think that companies that deal in large scale numbers of items simply can't be bothered with dealing with a request outside of their norm.    ☹️

[munirocks]

The problem is parallax. The closer the focus, the less overlap there is between the two fields of vision, and that overlap is what gives you the 3D effect. In most  close-focussing binoculars the near end of focus has only a thin sliver of overlap left between the fields so you lose the full 3D field of view for both eyes. The remarkable Pentax Papilio binoculars, however, redirect the two fields of view inward toward each when you focus on close objects, and the result is spectacular. If you want close-focussing binoculars, Papilio is the only choice. I recommend the 6x model over the 8x model, for steadiness, as you will end up happily chasing insects all over your garden without a tripod or monopod. The Papilio aperture is limited, however, so not so useful for astronomy.

[WJC]

Please keep in mind that Parallax and Collimation are NOT the same. 

Bill Cook, Chief Opticalman US Navy Ret.