Setting Diopter (2 Viewers)

[TheBlackGrouse]

The question was about distance and binoculars, and there's no obvious point in talking about eyes or surgery or contacts when the OP didn't mention having any such issues. Beyond this immediate issue of relevance, if odd interactions you refer to can sometimes arise for others in such circumstances, they don't seem to be very well documented, and you're providing no references.

Ask yourself, why does diopter adjustment exist? And look around in the world how many people need glasses/contact lenses. In the birding community this percentage is even higher so it's relevant. This is not a private conservation, this is a forum, other people read these threads too.

 

[Pinewood]

Are we talking about something akin to the concept of hyperfocal distance in photography?

Hello Ignatius,

Perhaps the Leica recommendation of 8m was considered a good average position on the focussing scale, between 2m and infinity. A quick look outside reveals that setting at 8m does not provide a sharp image at infinity. However, I doubt that Leica would make a recommendation for setting the diopter without a reason

Happy bird watching,
Arthur

 

[TheBlackGrouse]

Hello Ignatius,

Perhaps the Leica recommendation of 8m was considered a good average position on the focussing scale, between 2m and infinity. A quick look outside reveals that setting at 8m does not provide a sharp image at infinity. However, I doubt that Leica would make a recommendation for setting the diopter without a reason

Happy bird watching,
Arthur

The 8 meter is close to the distance that's used by opticians for measurements (it's a mirror). Tests are standardised for these distances. That may be a coincidence though, possibly Leica just picked a number.

Theoretically, distance doesn't matter, practice may show otherwise. According to some, it's better to avoid short distances because of accomodation, the working of eye-muscles in general and the less precise diopter-adjustment. And I'm just adding that eye-problems can make things worse. That's relevant in my opinion looking at the average age of the birding community.

Why take the risk, everyone can find a target outside at 50 - 100 meters.

Don't forget that diopter adjustment is redundant these days. Glasses/lenses give us good eyesight and no correction is needed, in principle. Again, practice shows otherwise, many birders take off their glasses when using binos and there goes the correction for astigmatism for instance.

 

[Tringa45]

Perhaps the Leica recommendation of 8m was considered a good average position on the focussing scale, between 2m and infinity. A quick look outside reveals that setting at 8m does not provide a sharp image at infinity. However, I doubt that Leica would make a recommendation for setting the diopter without a reason

Hello Arthur,
I think this is just a case of the sales and marketing people lacking any understanding of optics. There are numerous examples of this.
Have you read the link in post #4? If you have any doubts, I suggest you perform the experiment yourself with a tripod-mounted binocular near and far.

Regards,
John

 

[tenex]

Ask yourself, why does diopter adjustment exist?

Ask myself? Do you imagine I don't know? It's because people's two eyes tend to differ slightly, and the magnification of a binocular places much greater demands upon their accommodation than unaided vision. (Of course that's also why there's a focuser in the first place.) So even those who don't need to wear glasses/contacts can benefit from fine adjustment of the diopter, particularly when no longer young. If this isn't making sense yet, research how an afocal instrument like a telescope or binocular differs from, say, a camera.

Don't forget that diopter adjustment is redundant these days. Glasses/lenses give us good eyesight and no correction is needed, in principle. Again, practice shows otherwise, many birders take off their glasses when using binos and there goes the correction for astigmatism for instance.

Don't forget? Such confidence! Redundant? No, see above. Apparently, not understanding what a binocular diopter is for, you're confusing it with correction of vision. This explains a lot.

Perhaps the Leica recommendation of 8m was considered a good average position on the focussing scale, between 2m and infinity.

Yes, but why would such an average be their recommendation? It seems perfectly possible to adjust the diopter near close focus, though I doubt it occurs to anyone to do. I suppose atmospheric effects could degrade sharpness near infinity, so that's probably best avoided, as I should have thought to say initially. As to 8m, the precise distance to use would depend on what objects are present to focus on.

 

[Ignatius]

You have failed to understand the workings of a Keplerian telescope.
For any individual observer the image distances in left and right barrels set by focusser and dioptre are always the same regardless of object distance.
I have measured my image distance with glasses at 8 m, less than 1/20th of a dioptre away from my eye testing distance of 6 m.
A near-sighted observer needing an infinity correction of -3d and viewing without glasses would adjust every object to an image distance of 33 cm.

John

I do not understand this sentence. I mean semantically.

 

[Tringa45]

I do not understand this sentence. I mean semantically.

Perhaps poorly expressed. I meant that I had measured my personal virtual image distance when wearing glasses.
I focussed the binocular on an object and then set up a 35 mm single lens reflex camera with 85 mm lens on a second tripod behind the binocular eyepiece.
The 85 mm lens has a shallow DoF, so a wider distance scale and I regained focus (on the ground glass screen) at aroung 8 m distance setting.
There is no human factor involved in camera focus but it was a rather dim image at 7 mm aperture (8x56 SLC).
Eye testing distance is 20 ft or 6 m and prescriptions are usually in steps of 1/4 dioptre, so my preferred image distance only differed by 1/6 - 1/8, or about 0,04 dioptres.

John

 

[MiddleRiver]

BF cracks me up... here we are all arguing about setting diopter. Is it just me? I look through bins, focus, check both eyes. If one seems 'off', i try adjusting diopter till they look the same. I do it on whatever nearby object (not in my face and not a mile away) is handy. I don't worry about it unless I seem to be having trouble focusing.

I know... nothing wrong with knowing the right way to use a tool, and understanding optical devices, but sometimes it feels like we bog down in these conversations, to the point of it being laughable. No disrespect intended ;-)

 

[Trinovid]

BF cracks me up... here we are all arguing about setting diopter. Is it just me? I look through bins, focus, check both eyes. If one seems 'off', i try adjusting diopter till they look the same.

OMG, I was beginning to think I was the only one setting my diopter by just looking through the lenses!!

 

[Ignatius]

Sometimes, when I read through some of these threads, I think we could get a good deal on an omnibus order for those nice white jackets with the long sleeves and the buckles at the back.

 

[TheBlackGrouse]

Ask myself? Do you imagine I don't know? It's because people's two eyes tend to differ slightly, and the magnification of a binocular places much greater demands upon their accommodation than unaided vision. (Of course that's also why there's a focuser in the first place.) So even those who don't need to wear glasses/contacts can benefit from fine adjustment of the diopter, particularly when no longer young. If this isn't making sense yet, research how an afocal instrument like a telescope or binocular differs from, say, a camera.

Ok, so you know it.

But if ''the magnification of a binocular places much greater demands upon their accommodation than unaided vision.'' ... then it would be wise to avoid short distances (accomodation), in fact you say that distance does matter.

So, why all the excitement?

 

[Trinovid]

Sometimes, when I read through some of these threads, I think we could get a good deal on an omnibus order for those nice white jackets with the long sleeves and the buckles at the back.

 

[tenex]

Seriously?

This is from your post: "the magnification of a binocular places much greater demands upon their accommodation than unaided vision."

You are talking about accomodation and binoculars, I don't.

You criticize your own text.

You really are going to need to read in some detail about how a telescope works in conjunction with the eye, it may be too hard to interpret isolated remarks like these. Of course it's not just you, similar issues come up all the time with various questions (diopter, focusing, exit pupil, you name it) because people tend to be familiar with cameras and think in mistaken analogies. This would be high on a list of subjects we should have sticky FAQ posts about, but no one has done it. John might know a good source for you to read, and I'll look around a bit, if you don't get us kicked off the forum first.

 

[tenex]

You have failed to understand the workings of a Keplerian telescope.

It may not be surprising that few do. Google search results are of course useless, and this forum actually does little better, just tidbits here and there. I just checked Holger's Handbook which covers basic principles, virtual image, focusing, and accommodation quite separately, and in too concise and technical style for many. Astronomical sources have other priorities, and telescopes don't have diopters. Has anyone produced anything more accessible? Or is it just unnecessary because people who actually want to understand in detail will find all this and learn it, while the rest really don't want or need to, and will just accept a brief explanation such as I gave above for the focuser/diopter -- with the occasional exception not worth worrying about?

 

[tenex]

Here is a short nontechnical treatment. Any suggestions for improvement?


Accommodation refers to the ability of the eye's lens to vary its shape to focus on objects at various distances. When it's focused at some distance, light from objects closer or farther away converges slightly in front of or behind the retina rather than on it, making their image blurry to varying degrees. Ray diagrams of such situations can be found in photography tutorials on depth of field. As the lens becomes less flexible with age, the range of accommodation declines, leading to common problems like nearsightedness.

The magnification of an optical instrument places a greater strain on accommodation. Increasing the effective focal length of the optical (eye-binocular) system causes the axial distance between the in-focus image and the out-of-focus one to increase, which makes it even blurrier and gives the eye more trouble bringing it into focus instead: the range of accommodation shrinks as magnification increases. At some point around 6x, a growing number even of people with normal vision will be unable to simply look through a binocular and focus their eye on whatever they like, and the nearsighted will have difficulty even sooner.

Therefore a focusing mechanism is built in to help the eye accommodate objects at different distances, and along with it a diopter adjustment to correct for slight differences between a person's two eyes which have now begun to matter. Older users will have ever greater need for these adjustments. Focusing can be accomplished by moving the eyepieces or objectives, or by moving an internal focusing lens between them. Either way, the focuser does the heavy work of accommodating different distances, always placing the virtual image at a comfortable resting distance chosen by the individual user. The greater the magnification, the shallower the depth of field, and the greater the need for refocusing.