Left and Right Optical Inequality in Binoculars

[Robert / Seattle]

Hello all,

In subjecting my new 10x32 Ultravid HD binoculars to comparative tests against a few of my other high end binoculars, I've noticed for the first time that in some of them, the left and right scopes of a given unit do not neccesarilly offer the same optical performance. This new pair is absolutely fine, but I returned a previous purchase because the left scope did not achieve absolute focus, as determined when compared to the right scope. And in rigorous cross testing with other units offered by Leica, Swarovski and Zeiss I've noticed that some of them are inconsistent, left to right, with respect to brightness, contrast or resolving capability. All comparisons were done with tripod mounted units, using each eye on each scope respectively, and after diopter compensations were fully accommodated.

Has anyone else noticed this occurrence, and if so, what are your thoughts on the matter?

[henry link]

I've seen this many times. In fact, most binoculars I've tested show some quality differences between the barrels in a star test or resolution test with the magnification boosted to 40-80x. The usual defects revealed by a star test are astigmatism, pinched optics, coma (at the center of the field) and poorly made roof prisms. It's not uncommon to find one or more of those, at least to some degree, in even the most expensive binoculars. Most of the time the defects are minor enough so that there is no obvious problem at the intended low magnification, but as you see, sometimes the image is compromised.

[Surveyor]

Robert

As Henry says, of the binoculars that I have done more than a cursory examination, many, if not most, of them show some measurable difference between the barrels. It is not unusual to see one or two elements difference on a USAF resolution chart, a percent or two difference in magnification or a difference of collimation vector from either the other barrel or the axle. From observation, not measurements, I think there is even several percentage points difference in light transmission at times.

The current ISO 14133-1 standards call for metrics like power, entrance pupil and FOV etc. to be +/-5%. I may be wrong but I assume that means up to a 10% spread. They only two inter-barrel standards I am aware of are relative power, 2% and focus by central drive, 1D, which seems too large to me.

Although in normal use, I very seldom am able to see the differences.

Ron

[kabsetz]

Like Henry and Ron, I have seen plenty of examples of uneven performance between barrels. Whenever I test a pair, I always measure resolution for either barrel independently, both using my (slightly) better eye for both, as well as using a booster behind both. I have consequently learned to pretty much expect unequal performance as a given when it comes to boosted resolution or star-test patterns, but what I hope for is equal performance in the unaided eye tests. Alas, even this is not nearly as common as I would hope for. Fortunately, (or not, depending on your take) hand-holding introduces enough additional problems that much of the inconsistencies seen in a tripod-mounted binocular tend to go unseen in normal field use.

Kimmo

[ceasar]

Can these minor differences in the barrels be "corrected" by a diopter adjustment and minimized in most instances?
Bob

[Surveyor]

Can these minor differences in the barrels be "corrected" by a diopter adjustment and minimized in most instances?
Bob

Hello Bob;

I really doubt it. Most of the test measurements I make are conducted on one barrel at a time and focused to that barrel.

If I am being critical of a binocular I test the diopter control for setting accuracy by adjusting as normal on the right side and then inverting them and adjusting the diopter for the left eye after focusing for right eye (just reversing the normal process). If the diopter control is correct, the average of the two setting should be zero. I will check this at infinity and near focus. Since my correct diopter setting is +0.25, zero or ½ usually shows the same way for me all the way around. Recently I had a friend here on BF try this and he reported back that it worked well for him on several of his binos. I think he said he had a +2 setting. He reported that he read +2 right side and –2 left on several pairs. You might try this and let me know if it works for you. I do not have much feedback as to the results of others.

Have a good night
Ron

[Robert / Seattle]

Hello Bob;

I really doubt it. Most of the test measurements I make are conducted on one barrel at a time and focused to that barrel.

If I am being critical of a binocular I test the diopter control for setting accuracy by adjusting as normal on the right side and then inverting them and adjusting the diopter for the left eye after focusing for right eye (just reversing the normal process). If the diopter control is correct, the average of the two setting should be zero. I will check this at infinity and near focus. Since my correct diopter setting is +0.25, zero or ½ usually shows the same way for me all the way around. Recently I had a friend here on BF try this and he reported back that it worked well for him on several of his binos. I think he said he had a +2 setting. He reported that he read +2 right side and –2 left on several pairs. You might try this and let me know if it works for you. I do not have much feedback as to the results of others.

Have a good night
Ron

Hi Ron,

But the diopter itself can "float" from a theoretically ideal setting, can it not? I have perfect 20/20 in real sight, and most of my binocular settings agree with that. Yet a few of my bins come in anywhere from -1 to +1.5 diopter correction to match my eyes. It would be rather like an oven thermometer reading one setting when, in fact, the actual temperature is much different. It wouldn't matter what the reading indicates, so long as you knew it was skewed from absolute accuracy and understood that the necessary adjustments are matters of convenience as opposed to real optical measurements.

Eh?

Robert

[Surveyor]

Right Robert, the idea of doing it both ways is to determine how much "float' you have. For instance if your setting is normally +2 and you read +3 and -1 then the average would be +1 meaning that +1 on your bin is actually the 0 setting point.

[Robert / Seattle]

Right Robert, the idea of doing it both ways is to determine how much "float' you have. For instance if your setting is normally +2 and you read +3 and -1 then the average would be +1 meaning that +1 on your bin is actually the 0 setting point.

Ron,

I'd be delighted with that elegant formula if the "float" were consistent between units. But each binocular is different in it's empirically correct settings, therefore the resulting term "float" -- i.e., there is no normal setting as each or any given pair is different, up to a -1 to +1.5 spread.

Also, am I correct in assuming that this is a focus mechanism gearing phenomenon, and ultimately "correctable"?

Robert

[Surveyor]

Right, this only determines zero for the bin you are checking. Specs should be +/- 1D of zero. It is not something I check routinly, only if my normal setting is way off.

[Robert / Seattle]

Right, this only determines zero for the bin you are checking. Specs should be +/- 1D of nominal. It is not something I check routinly, only if my normal setting is way off.

That's good information Ron, and many thanks.

Even so, once "par focus" between barrels is achieved the question of unequal optical performance can remain. My resulting supposition is that the optical quality of the lens elements of the two barrels is (more or less) identical, and therefore that most of the discernible difference in brilliance and resolution may be attributed to the coatings, and perhaps the placement, of the prisms.

Thoughts?

Robert

[Surveyor]

Thoughts?


Hello Robert:
This is just thoughts, absolutely no idea of actual practice.

Just looked at a typical specs for achromats at Edmund Optics and for binocular sized lenses. The specs seem to be about +/- 2% for EFL, 0.2 for CT and axis 3 or 5 arc minutes depending on size.

Lets just imagine a scenario for the objective lens focal length only. As the lenses come to QC they may be graded with a number from 1 to 3, 1 being +2% and 3 being –2% with 2 being the nominal 0%. The lenses are checked and marked with the appropriate number.

In the next stage of assembly they try to match numbers and use all the 2’s first, then the 1’s and then the 3’s, etc. until out of stock of that run. I can see this for most of the critical elements of the optical chain.

I think the high dollar big 3 makers spend a lot more time and money on QC and grading and matching of components and the very low end binocular makers, very close to none since a +2% and a –2% in the same instrument would probably still fall in the +/-5% tolerance but the barrel to barrel differences would be greater.

My thoughts are that this process may well be applied to all components including coatings and castings since all components are going to have a tolerance.

Have a good day
Ron

[Robert / Seattle]

Thanks Ron,

Makes perfectly good sense; great insight. I can now imagine how the assembly process might produce the so-called "cherries" that are so coveted by more discerning consumers. Too bad, though, that it's the luck of the draw at point of purchase.

Happy New Year to you,

Robert

[henry link]

My resulting supposition is that the optical quality of the lens elements of the two barrels is (more or less) identical, and therefore that most of the discernible difference in brilliance and resolution may be attributed to the coatings, and perhaps the placement, of the prisms.

Thoughts?

Robert

Since the diopter adjustment only changes focus it will do nothing at all to improve defects in one barrel. Robert, I wouldn't assume that the optical quality of the lens elements (and prisms) is more or less identical in both barrels. Buying a binocular is like buying two separate telescopes. You wouldn't expect exactly the same optical defects in both. Defects like astigmatism, coma or a bad roof edge are usually worse in one barrel than the other and that may cause a visible loss of resolution or contrast in the more defective barrel. Those kinds of defects may originate in the glass or may result from assembly problems like, for instance, lens retaining rings that are screwed on too tightly. Coatings will have no significant effect on resolution or sharpness. I've read that batch variations in multi-coated lenses can cause transmission differences of as much as .25% per surface (4% for a complex binocular with 16 coated surfaces) which would be difficult to detect. A transmission difference of more than that between right and left would be hard to explain.

[kabsetz]

Henry,

Concerning the transmission difference, since it is highly unlikely that manufacturers would try to consciously select into one barrel only lens/prism elements with worse than average transmission, I don't think I need to do any statistical calculations to say that it would appear to be exceedingly unlikely that anything close to the maximum theoretical difference would ever occur in real life.

Ron and Robert,

For determining diopter setting accuracy, I again use my booster. If I have a tripod-mounted binocular focused precisely to a target using the booster and the non-adjustable eyepiece, I can switch the booster into the adjustable eyepiece and, without touching the focus wheel, focus with the diopter adjustment until I have perfect focus, and then check what the diopter scale says. In a perfect instrument, it should say 0. To be really sure, especially with roof-prism internal focusing binoculars, I check this both with reaching the perfect focus from near side and infinity side separately, since getting different diopter readings would tell of excess slop and consequent diopter adjustment difficulties with that particular binocular.

Kimmo

[Surveyor]

Thanks Kimmo
I agree, a boosted image is more accurate. I have used that method and use it when I want more accuracy and have the equipment set up but I also reverse now as a check even when using the double collimator as a booster (I will email you a photo for comments).

I should mention that what drove this change for me was the friend I mentioned before had two presccriptions that showed differences of diopter setting and was looking for a way to check which one was the more correct. That was when I modified my procedure since by inverting you get approximately twice your individual diopter setting so you can note it and use on other instruments if you have not had it measured before.

Have a good day
Ron

I also flip the double collimator. This not only checks focus but helps me detect/compensate for parallex at the higher magnifications.