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Dioptre Setting: Fallacy and Fact (1 Viewer)

Tringa45

Well-known member
Europe
There are frequent misunderstandings and misconceptions on dioptre setting and many are of the opinion that the dioptre should be set on an object at or near infinity. Conversely, would they expect the dioptre setting to be wrong for objects at shorter distances?
On another thread I asserted that the distance of a target chosen for dioptre setting was irrelevant, and in expectation of contradiction decided to conduct an experiment.

Firstly though, it is important to establish that a telescope or binocular is an afocal instrument. Focus is achieved by the human eye on the retina, just like that of a camera lens on the film or sensor. The"focusser" of a binocular is there to place the image of the viewed object at an apparent distance that is comfortable to the observer, regardless of the real distance of the object.
For the normally sighted this would be at infinity, where the eyes are most relaxed and the rays from any point on the viewed object would emerge parallel from the eyepiece. For the near-sighted the rays would diverge and for the far-sighted they would converge.
The dioptre adjustment compensates for any differences in the observer's eyes, left and right. Observers wearing glasses would normally set the dioptre to zero.

For the experiment I used my Swarovski 10x42 EL SV. It has a dioptre scale of +/-5 but can actually be adjusted up to around +/-9 dioptres. Being of advanced age, the very limited accommodation of my own eyes was probably an aid to precision in the experiment.
I first set up the binocular on a tripod and focussed on a license plate at 75 m and confirmed the focus on both barrels as being identical by viewing through a 3x12 monocular with the same eye at the zero dioptre setting. This should be a matter of course with all binoculars but is not always the case.
I then retracted the focussing knob, which decouples the focussing lens in the left barrel, and placed a hand magnifier, whose focal length I had estimated at 18,9 cm (5,3 d) behind the right eyepiece. Seasoned Birdforum members will recognize here that I attended The Henry Link University :). A positive lens of 5,3 d simulates near-sightedness of that amount and it needs a negative lens of that value to correct it.
I regained focus with the dioptre knob and found that it ended up on the scale just a smidge beyond -5.
Repeating the test indoors at 3,6 m (arbitrarily chosen) on fine print the dioptre correction measured was about -5,5 d. Perhaps this very minor discrepancy of around 1/4 d could be attributed to experimental error or maybe it was caused by the shortening of objective focal length effected by the internal focussing lenses at shorter distances.
In any event, I think it can be ignored in view of the large difference in distances (3,6 m to 75 m in a 10x binocular represents about 26 dioptres) and a simulated 5,3 d difference between left and right eyes, and is confirmation that any distance of a high-contrast object is suitable for dioptre setting.

John

PS:- Sorry for the title, Bill. I just couldn't resist :).
 

Maljunulo

Well-known member
I find this discussion puzzling, because the diopter setting compensates for any difference between one’s two eyes.

That viewing objects at different distances would change this difference seems unlikely to me.

Things are more complicated when we don’t understand them.
 

Trinovid

mountain and glacier watcher
United States
I've not noticed any difficulty in adjusting my diopter setting and then leaving it alone afterwards and all of my binoculars have been simply left alone unless someone else needs to adjust them to accommodate their own eyes.
 

Troubador

Moderator
Staff member
Supporter
I would like to contribute to the Fallacy and Fact discussion by answering this question that has cropped up with regularity over the 10 years I have been a Birdforum member: "If I am wearing spectacles I don't need to adjust the dioptre do I? I can just leave it set to Zero can't I?"

I am a spectacle wearer myself and recommend setting the dioptre for the following 4 reasons:

Firstly, when your eyes are tested by an optometrist you are asked to choose which lens gives the sharpest image from a series of pairs of lenses. Normally the differences between the two lenses is quite easy to see but towards the end of the series it gets more difficult to be 100% certain which is the best. This means that it is possible that your final choice may be mistaken and this is not a disaster since this is of minor significance. Nevertheless it means a small discrepency may have crept in to your prescription.

Secondly, the lenses in my spectacles and in those of other spectacle wearers are made to tight tolerances but nevertheless there are tolerances involved so the lenses may differ from perfection by a tiny amount.

Thirdly, the dioptre mechanism on the binoculars should have be set to Zero during its manufacture so that when the dioptre wheel indicates Zero on the scale it means that the focus mechanisms in both optical tubes are set to the same position. However, binoculars are assembled by human hands and errors can occur, so it is wise not to assume that Zero means absolutely and perfectly Zero.

Finally, we spectacle wearers are accustomed to having to change our spectacles from time to time due to the changes taking place in our eyes as we age.

Setting the dioptre is a simple process so I believe spectacle wearers would be wise to take advantage of it.

Lee
 

Binastro

Well-known member
The tolerances for prescription glasses are rather poor both in dioptre and astigmatism angle.

With my last tests, which are some years ago because of Covid, I was still able to tell the difference repeatedly when the optician used his 1/8 dioptre lens. I.e. I could detect 1/16th dioptre differences.
Also 2.5 degrees of astigmatism. I.e 1.25 degree differences.

My tests took 90 minutes, normally one might have 20 minutes.

Prescription glasses are much worse than this, and the optician had to persuade the boss of the lab to make the glasses himself to the best of his ability for a crazy astronomer.

This is for 20/15 vision.

My eyes are now worse, but I haven't had a recent test.

For someone with 20/10 or 20/8 vision, normal prescription glasses are totally inadequate.

As to the zero marks on binoculars, it is only the best makes that are close to exact.
Lesser makes are often way off.

Also when moving locking systems on and off rotation occurs.

The twist system on one ocular is far more exact.

Eyes change during the day and from one day to the next.

Camera viewfinders are set at minus 1 dioptre.

With medium or low price Chinese binoculars there is stiction and the two barrels just are not in synch as one focuses.
Often dioptre settings are pretty useless.

Regards,
B.
 

Tringa45

Well-known member
Europe
I would like to contribute to the Fallacy and Fact discussion by answering this question that has cropped up with regularity over the 10 years I have been a Birdforum member: "If I am wearing spectacles I don't need to adjust the dioptre do I? I can just leave it set to Zero can't I?"

I am a spectacle wearer myself and recommend setting the dioptre for the following 4 reasons:

Firstly, when your eyes are tested by an optometrist you are asked to choose which lens gives the sharpest image from a series of pairs of lenses. Normally the differences between the two lenses is quite easy to see but towards the end of the series it gets more difficult to be 100% certain which is the best. This means that it is possible that your final choice may be mistaken and this is not a disaster since this is of minor significance. Nevertheless it means a small discrepency may have crept in to your prescription.

Secondly, the lenses in my spectacles and in those of other spectacle wearers are made to tight tolerances but nevertheless there are tolerances involved so the lenses may differ from perfection by a tiny amount.

Thirdly, the dioptre mechanism on the binoculars should have be set to Zero during its manufacture so that when the dioptre wheel indicates Zero on the scale it means that the focus mechanisms in both optical tubes are set to the same position. However, binoculars are assembled by human hands and errors can occur, so it is wise not to assume that Zero means absolutely and perfectly Zero.

Finally, we spectacle wearers are accustomed to having to change our spectacles from time to time due to the changes taking place in our eyes as we age.

Setting the dioptre is a simple process so I believe spectacle wearers would be wise to take advantage of it.

Lee
Lee,

I agree that glasses wearers should also check the dioptre setting as one cannot automatically assume that the focus on both barrels is identical at the zero dioptre setting. Other discussions here have centred on dioptre shift as a consequence of an imprecise internal focussing mechanism and there's no remedy for that other than a service by the manufacturer
As Binastro points out, one can demand much higher standards for spectacles than for binoculars.
I have just checked my 10x42 SV, which has 13 clicks of the dioptre setting for 5 dioptres. I assume that this is the same mechanism as for the 8,5x42 SV, which would translate to 0,5 d/click for that model.
My 8x56 SLC has 4 clicks per dioptre, but that probably stems from the greater sensitivity required of the 15x56 in the same family.

John
 
Last edited:

WJC

Well-known member
There are frequent misunderstandings and misconceptions on dioptre setting and many are of the opinion that the dioptre should be set on an object at or near infinity. Conversely, would they expect the dioptre setting to be wrong for objects at shorter distances?
On another thread I asserted that the distance of a target chosen for dioptre setting was irrelevant, and in expectation of contradiction decided to conduct an experiment.

Firstly though, it is important to establish that a telescope or binocular is an afocal instrument. Focus is achieved by the human eye on the retina, just like that of a camera lens on the film or sensor. The"focusser" of a binocular is there to place the image of the viewed object at an apparent distance that is comfortable to the observer, regardless of the real distance of the object.
For the normally sighted this would be at infinity, where the eyes are most relaxed and the rays from any point on the viewed object would emerge parallel from the eyepiece. For the near-sighted the rays would diverge and for the far-sighted they would converge.
The dioptre adjustment compensates for any differences in the observer's eyes, left and right. Observers wearing glasses would normally set the dioptre to zero.

For the experiment I used my Swarovski 10x42 EL SV. It has a dioptre scale of +/-5 but can actually be adjusted up to around +/-9 dioptres. Being of advanced age, the very limited accommodation of my own eyes was probably an aid to precision in the experiment.
I first set up the binocular on a tripod and focussed on a license plate at 75 m and confirmed the focus on both barrels as being identical by viewing through a 3x12 monocular with the same eye at the zero dioptre setting. This should be a matter of course with all binoculars but is not always the case.
I then retracted the focussing knob, which decouples the focussing lens in the left barrel, and placed a hand magnifier, whose focal length I had estimated at 18,9 cm (5,3 d) behind the right eyepiece. Seasoned Birdforum members will recognize here that I attended The Henry Link University :). A positive lens of 5,3 d simulates near-sightedness of that amount and it needs a negative lens of that value to correct it.
I regained focus with the dioptre knob and found that it ended up on the scale just a smidge beyond -5.
Repeating the test indoors at 3,6 m (arbitrarily chosen) on fine print the dioptre correction measured was about -5,5 d. Perhaps this very minor discrepancy of around 1/4 d could be attributed to experimental error or maybe it was caused by the shortening of objective focal length effected by the internal focussing lenses at shorter distances.
In any event, I think it can be ignored in view of the large difference in distances (3,6 m to 75 m in a 10x binocular represents about 26 dioptres) and a simulated 5,3 d difference between left and right eyes, and is confirmation that any distance of a high-contrast object is suitable for dioptre setting.

John

PS:- Sorry for the title, Bill. I just couldn't resist :).
Yeah, Yeah! Wait until you hear from my lawyer!!!

When I published that thing, I noticed how many other times that phrase had been used. I shrank with embarrassment. And, heck, I only 5, 4 to start with. Sometimes the bell can't be un-rung. Years ago, an editor said I should be someone more memorable than "Bill Cook." He said, "Bill Cook" sounded plain and uninspiring like a used car salesman or a stoker at Walmart. So, I became "Williman J. Cook." I thought it sounded a bit pompous and wanted to be just "William Cook." But when I saw how many of those there were on Amazon. I decided to keep the whole thing. Then, I was told for the sake of publishing continuity, I should keep it. So, I was trapped.
 

WJC

Well-known member
Nothing is so straightforward that it cannot be made complex by one who does not understand it.
That is why a fellow, who used to be here, is over on Cloudy Nights trying to make a name for himself producing charts and graphs far more complex than the Navy used to teach 3-Axis Collimation to Cory and me. When I was in optical “A” school, Master Chief Lou Corriveau told me that I needed to learn all that. But if he saw me going to such time-wasting lengths in the fleet, he would no longer consider me an Opticalman.

However, the last time I looked, this former member was getting all the adoration for his efforts he could use. ‘Takes all kinds!
 

WJC

Well-known member
I find this discussion puzzling, because the diopter setting compensates for any difference between one’s two eyes.

That viewing objects at different distances would change this difference seems unlikely to me.

Things are more complicated when we don’t understand them.

DIOPTRIC ACCOMMODATION and learning to stare

Perhaps the most important aspect of observing is found in dioptric accommodation, which is also a component of observing that can be controlled by the observer if they understand the most efficient focusing procedures. But what is there to know about simply focusing a binocular? Quite a bit, actually.

We have all seen bird watchers, amateur astronomers, or others repeatedly adjust the focus on their binocular when we know their target is not changing its proximity. Why, then, should an adjustment be necessary? The problem lies with a dioptric accommodation—a physiological correcting of a disparity of focus along the Z axis and the observer’s understanding of focusing procedures—novice or postdoc.

Your mother probably taught you, “It’s not nice to stare.” However, for the most pleasing, fiddle-free observing experience you might want to forget that advice. There are two parts to focusing a binocular. The first concerns the binocular’s focus mechanism—the actuation of the focus knob, diopter ring, or flip lever and it is the one seen almost exclusively by the observer. Over time, the second part can be even more important.

It works through the involuntary stretching or compressing of the eyelens by the eye’s ciliary muscles. Our impatient brains want to see things in focus quickly. This is a safety mechanism for mankind that has been with us since prehistory. But if given the chance, it will use those muscles to the detriment of the viewing experience.

Let us suppose you want to see a target at a certain distance and start turning the focus knob to achieve the best image. The target may become sharply focused for you at -3 diopters, although your at rest dioptric setting should be -1. Yes, the image is focused. But only because your brain was rushing ahead of your need and only achieved that focus at a strained setting. Then, as fatigue sets in, or your physiological focus starts to wander, you feel the need to start fiddling with the focus mechanism. Even so, without considering the importance of STARING, you will, more than likely, repeat the mistake and find that the problem remains.

Learning to stare and using staring in your focusing can preclude the eye’s involuntary input and limits focusing to that achieved through the binocular’s focus mechanism, which leaves the observer in control of the whole focusing operation, as opposed to having the two aspects of focusing fight each other while you try to attain a precise and comfortable focus for an object at a given distance. Not understanding the critical importance of learning to stare, observers have devised a few routines to circumvent the real problem.

Two of the most common involve trying to trick the brain. * Both can be beneficial to the operation and, although tantamount to placing a Band-Aid ®on a bullet hole, both have been endorsed in a number of articles. The first has the observer place a hand over one objective lens while focusing the other telescope. Then, the procedure is reversed for the other side. For some, even that is not good enough. In the second method, the observer must place the cap on one objective and reverse the process when a good focus appears to have been achieved. Some observers promote one or both of these methods and some satisfaction may be realized. Yet, how many times might a moving target disappear from sight while using the first method or fishing around in your pocket for a lens cap to perform the second? The person who has learned to stare need not bother with either of these stop-gap measures, too often used to pad the freelancer’s article.

RECOGNIZING FOR YOURSELF

PATIENTLY
focus your binocular as precisely as possible on a target a mile or more away. Observe your dioptric setting for that target. This is your natural, at rest, dioptric setting for a target at that distance.

Now, de-focus the setting by between 2 and 3 diopters. Then, when placing the binocular to your eyes, you may not initially see a disturbing difference in the focus. † Over time, however, you may note the image is not as crisp as when you started the experiment. Finally, as you continue to adjust the focus to attain a sharp image, you may see that the dioptric setting has returned to the one you had at the outset.

By learning to stare and bringing the image to its best—at rest—focus, you are allowing the mechanism to do its job and much of the unnecessary, and time wasting attempts at focusing can be avoided.

The center wheel adjusts your focus at different distances. The diopter ring is just used to set the different dioptric strength between your eyes and only 2-3% of observers have the same power in each eye. If you have reached this setting while staring, you’ll be fiddle-free AT ALL DISTANCES until the setting is inadvertently bumped or until the instrument is used by another observer.
 

Maljunulo

Well-known member
As far as I know Bill, I have zero ability to accommodate, since I have implants which have replaced my OEM lenses on both sides.

I do, however, fiddle with the focus mechanism in an attempt to squeeze that last bit of detail out of the image.

It’s probably more a nervous habit than anything which actually helps.
Richard
 

WJC

Well-known member
As far as I know Bill, I have zero ability to accommodate, since I have implants which have replaced my OEM lenses on both sides.

I do, however, fiddle with the focus mechanism in an attempt to squeeze that last bit of detail out of the image.

It’s probably more a nervous habit than anything which actually helps.
Richard
I happen to know the OEM. He bloody-well doesn't care about all this binocular stuff; he just worries about you.
 

WJC

Well-known member
Hi Bill,



Reading between the lines, it seems it's implied that "staring" means acommodating the eyes to a fixed, long distance?

Regards,

Henning
No. It means accommodating for the difference between where you stop focusing the binocular and where your NATURAL—at rest—focus (FOR WHATEVER DISTANCE is relevant at the time. I am getting so many questions on this issue that my novel is on hold the rest of the day while I gen up a complete ARTICLE on the subject.
 

Ted Y.

Well-known member
Canada
It seems the human eye is already in focus after 20m, therefore ciliary muscles are relaxed.
I cannot be sure, I just read about it.
 

WJC

Well-known member
It seems the human eye is already in focus after 20m, therefore ciliary muscles are relaxed.
I cannot be sure, I just read about it.
The farther away, the less accommodation. NOW superimpose a binocular and all bets are off.
 

Hauksen

Forum member
Antarctica
Hi Bill,

No. It means accommodating for the difference between where you stop focusing the binocular and where your NATURAL—at rest—focus (FOR WHATEVER DISTANCE is relevant at the time.

Is that is the definition of "staring"?

I would have thought that it would be advantageous to focus the binoculars so that the difference you describe is zero.

Regards,

Henning
 

WJC

Well-known member
Hi Bill,



Is that is the definition of "staring"?

I would have thought that it would be advantageous to focus the binoculars so that the difference you describe is zero.

Regards,

Henning
What your eyes see when they are at rest. 'Working on the article, now. Should be ready by the end of the day.
 

Maljunulo

Well-known member
I think the relaxed, unaccommodated (without arguing what that “means”) human eye will be focused on infinity.

A relaxed, unoccommodated eye is defined as an eye with the ciliary muscles which control the curvature of the OEM lens relaxed and in a state of zero contraction, so that the lens is at its minimum curvature.

Infinity is defined as 40 focal lengths by some who seem to know what they are talking about.

Offered without benefit of credentials, academic degrees, or industrial experience, so it may be totally wrong.
 

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