Wandering diopter - what is happening inside / technical explanation? (1 Viewer)

[pbjosh]

First when I say wandering diopter, I mean a diopter that is clearly not functioning correctly - not a sneaky problem of having to adjust the diopter throughout the day a tiny amount depending on how tired you are, or having to adjust the diopter differently to achieve perfect near vs far focus, but rather a diopter that shifts over time. This had happened to me once about 10 years back. With perfectly corrected vision (ie, the diopter should have been in the center) I had to adjust it every day or two and after a month it was turned as far as it would go and still wouldn't achieve matching focus, yielding an essentially useless binocular.

I recently have this happening to another binocular (in this case a probably 10 year old but never used Minox BL I bought in a pinch after my primary bins developed a problem in the middle of a trip).

While I'm sure it likely (almost surely) varies due to internal design, I'm curious what is happening internally and if there is any chance that it is repairable, generally? I can imagine a few different designs that either shift a secondary focus lens entirely independently of the main focus mechanism or that would shift one of the focus lenses relative to the other in a perhaps lighter weight but slightly mechanically more complex design. The binocular in question is almost certainly not covered by a warranty (though I have sent in an inquiry) and is likely not worth the cost of a professional repair, but I am still curious what might be happening. I might well take it apart myself when/if it does become useless - less with the expectation of achieving a valid repair, more just for fun. I have plenty of experience with small parts and precision assembly (electronics and a few mechanical things) but no illusions that I will do anything more than play with it for a few hours, learn a bit, and then have a disassembled fishing weight instead of an intact fishing weight.

In any case, any comments / links to technical data / etc appreciated and if nothing else make for good discussion.

 

[pbjosh]

Here is the patient. I have essentially perfect distance vision (no diopter difference between eyes, no astigmatism) and invariably have the diopter perfectly centered for best results on all other bins. After about 20 days of field use, this is where the diopter is to achieve matching focus.

 

[dries1]

This occurred to me as well on a 8X42 Nikon, it was replaced. I would ascertain that parts will be needed, as something is likely worn out. Interesting to see what Minox now (Noblex?) say.

 

[pbjosh]

I really don't know enough about binocular internals to make much of an educated guess. With such little knowledge but just imagining what could be happening, my intuition is something like this: When using center focus, there is more force applied to the diopter mechanism in once direction than the other. Some fixed piece internally that should not be moving (a sleeve containing the entire diopter adjustment mechanism and one of the focusing lenses, or a stop to keep part of the diopter or focus mechanism in place) becomes loosened and moves in the direction where greater force is applied until reaching the end of it's travel or traveling beyond what the diopter setting can correct. However that is, as said, just a guess. Pondering this led to me asking the question.

Cheers

 

[Tringa45]

I suspect that the problem is not the helical dioptre adjustment itself but asynchronous movement of the focussing lenses in the left and right barrels.
Many complain of the uneven torque, clockwise and anti-clockwise, on the focussers of some Swarovski binoculars. This is due to the spring loading of the focussing lenses, which assures synchronous movement.

When you "focus" on an object you are placing the objective`'s image of that object in a certain position relative to the eyepieces. For the normally sighted that would be in the focal plane of the eyepieces so that the rays from that image emerge parallel from the eyepeces, i.e. as if the image were at infinity.
The short-sighted would place that image within the focal plane and the far sighted would place it outside the focal plane. The actual focus is achieved on the retina of your own eyes as on the fim or sensor of a camera.

Most central focussing Porro binoculars achieve "focus" by varying the distance between the eyepieces (on a bridge) and the objectives. The objectives would form an image of a near object further inside the objectives, so the eyepieces are extended and any differences in the observer's eyes are compensated by a helical adjustment of the right eyepiece.

The old Zeiss Dialyts had a focussing bridge but on most current roof prism binoculars the distance between objectives and eyepieces is fixed, so to reach focus on a near object one would have to reduce the focal length of the objectives. This is achieved by either moving diverging (-ve.) focussing lenses away from the other objective elements or shifting converging (+ve.) lenses towards them. On many roof prism binoculars the dioptre adjustment is accomplished by de-coupling the focussing lens in the left barrel.

John

PS:- On Birdforum we're all preoccupied with the optical qualities of our bins and scopes, but I think we should expect that the manufacturers devote similar attention to the mechanical aspects like reliability and longevity. Troubles like those of the OP seem to be all too common.

 

[Hermann]

Most central focussing Porro binoculars achieve "focus" by varying the distance between the eyepieces (on a bridge) and the objectives. The objectives would form an image of a near object further inside the objectives, so the eyepieces are extended and any differences in the observer's eyes are compensated by a helical adjustment of the right eyepiece.

And there's much to be said for such a system - it's simple and reliable. The more complex a system is, the more likely it is to go wrong.

On Birdforum we're all preoccupied with the optical qualities of our bins and scopes, but I think we should expect that the manufacturers devote similar attention to the mechanical aspects like reliability and longevity. Troubles like those of the OP seem to be all too common.

You can say that again.

Hermann

 

[qwerty5]

I also have the same problem on a Vortex Diamondback 10x42. It is still usable turned all the way, but it's definitely not right. I guess I'll be sending them to Vortex soon to try to fix it.

 

[ZeNiTh-PbArM]

I'm curious what is happening internally and if there is any chance that it is repairable, generally?

Hi

Under the focuser knob you'll find two brass screws with a groove each around the head. These are pushrods for the left and right focusing lenses.
If one of them screws into or unscrews from the focusing lens holder, you need to compensate using the diopter.
To repair this remove the focuser knob, set diopter to zero, turn the focuser all the way to infinity minus a quarter of a turn and screw/unscrew each pushrod until the corresponding image is in focus.

Cheers
zp*

 

[Alexis Powell]

I've seen wandering diopter problems caused in two ways, though I am sure there are many others. In one case, when a binocular has an external yoke for the oculars (as in the case of most porros and some roofs, such as the Zeiss 7x42 ClassiC), the yoke may be thermally unstable and shift in shape at different temperatures, shifting one or the other ocular up or down a bit, thus affecting the diopter setting. In the other case, which I've seen on numerous occasions with cheap roofs, the focusing system becomes temporarily decoupled when the threads of the gearing system slip on occasion on one side or the other, causing one focusing lens to stay still for moment while the other moves forward or back. The diopter then must be adjusted to compensate. This problem usually happens more when focusing one direction than the other, such that the oculars creep farther and farther apart and eventually the diopter reaches its limit for making up the difference.

--AP