What is the Best Distance for Initial Focus/Diopter Setting (1 Viewer)

[BruceH]

Let’s assume the following:

This post is about the common center focus binocular that focuses both barrels simultaneously and has a separate diopter adjustment to make plus or minus adjustments to the right barrel. It can be either a roof or porro binocular.

Focus procedure:
1) Block the view of the right objective, then focus the left barrel using the center focus knob.
2) Block the view of the left objective, then focus the right barrel using the diopter adjustment. Do not change the center focus from the previous step and sight on the same object used to focus the left barrel.

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Ever since I was very young, I have always initially set the focus/diopter setting using a distant object out at infinity. I have never given it much thought, I just did it that way.

I noticed that some instructions I have say choose a distant object, Zen-Ray says choose an object at least 75 feet away, and some instructions say choose an object with no mention of distance with the understanding I guess that it has to be at least past the close focus distance.

There are some issues from a practical stand point when using an object at infinity for initial setup. It can be difficult to find a target that provides good contrast and a sharp edge. Another issue is distortion from heat waves.

I was also thinking depth of field could possibly come into play. The far object has a large depth of field and the close object has a short depth of field.

What are the technical reasons for choosing either a far target or a close target?

What are practical reasons for choosing either a far target or a close target?

Does it make any difference?

I am looking forward to hearing what others are doing and why!

 

[John Russell]

I doubt that depth of field plays any role here. What is important is depth of focus, i.e. the amount of movement of the focussing lens/eyepiece in which a sharp image can be maintained. I don't think depth of focus would change much with different focussed distances, so IMHO the distance of the object you choose to set the diopter doesn't really matter.

Of course, the ability to accommodate plays a significant role and accommodation deteriorates with increased eye pupil diameter, but I can't say whether it would be advisable to set the diopter under low ambient lighting. Perhaps Ed (Elkcub) can elaborate on this.

John

 

[Alexis Powell]

You should use an object at infinity so that the muscles on your eye lenses can be completely relaxed at infinity, and as relaxed as possible at intermediate distances, when the diopter is adjusted properly.

My favorite object for diopter setting is the moon.

--AP

 

[stereotruckdriver]

I have experimented and find that 25ft to 2-300yds it didn't matter? I always use tripod and get the same result? I used to use the infinity method and found i generally had to make small adjustments. That said i use the 25ft method and then check again at 2-300yds and find i don't have to reset. YMMV, Bryce...

 

[John Russell]

You should use an object at infinity so that the muscles on your eye lenses can be completely relaxed at infinity

Alexis,

The image you see through the eyepiece is always at infinity, or rather your most comfortable viewing distance. That's what the focusser is there for.
The short-sighted need focus overtravel so that they can comfortably view objects at infinity. I am far-sighted so without glasses my comfortable image would be "beyond infinity", i.e. covergent rays emanating from the eyepiece.

John

 

[Alexis Powell]

Alexis,

The image you see through the eyepiece is always at infinity, or rather your most comfortable viewing distance. That's what the focusser is there for.
The short-sighted need focus overtravel so that they can comfortably view objects at infinity. I am far-sighted so without glasses my comfortable image would be "beyond infinity", i.e. covergent rays emanating from the eyepiece.

John

If you use a nearby object to set the diopter, you can achieve focus in three ways: (a) by using your eye's lens muscles, (b) by using the diopter, (c) by using the center focus. If the object is at infinity, and you use the center focus for the non-diopter side, you facilitate relaxation of your eyes.

--AP

 

[John Russell]

Alexis,

If you have set focus and diopter correctly then the VIRTUAL image will be at infinity (or your most comfortable viewing distance with the naked eye) REGARDLESS of the object distance.

John

 

[Alexis Powell]

Alexis,

If you have set focus and diopter correctly then the VIRTUAL image will be at infinity (or your most comfortable viewing distance with the naked eye) REGARDLESS of the object distance.

John

The question is how to efficiently get the focus/diopter properly set. I actually keep both eyes open, but for those who prefer to block one eye, then the other, etc in the course of set-up, I think it is difficult to get the setting right when using a nearby object because several different combinations of eye-focus + bino focus will yield a focused view, and there is no way to ensure that one's eye focus doesn't change from moment to moment, especially when looking through one eye, then the other. By using a distant object, one can focus the bino drive or diopter from near to far, allow the eye to relax, and maybe focus outward just a bit more to the point where the eye has reached its own distant focus limit = state of greatest lens muscle relaxation. That limit is quite consistent in most people, so it can be used to set the diopter/center focus one eye at a time fairly effectively.

--AP

 

[newfie ghost]

I look at a high contrast object such as a tree trunk (maple with lots of lines and detail) at about 40-50 yards with both eyes open (and no lenses blocked) and get the sharpest image possible. Then I adjust the diopter so the image is perfect again with both eyes open. This works much better for me than the one eye closed technique and or blocking a lens.

With some glass I am on 0, others a little positive, others a little negative. My vision is corrected so in theory i shouldn't need a diopter. But with most glass i have to adjust to get the best view. So much for consistency eh? Jupiter and it's moons is a good test as well. Just adjust until there is a perfect disk.

 

[absolut_beethoven]

I look at a high contrast object such as a tree trunk at about 40-50 yards with both eyes open and get the sharpest image possible. Then I adjust the diopter so the image is perfect again with both eyes open. This works much better for me than the one eye closed technique.

With some glass I am on 0, others a little positive, others a little negative. So much for consistency eh? Jupiter and it's moons is a good test as well. Just adjust until there is a perfect disk.

I agree about a well lit high contrast subject, but I personally wouldn't use a tree. Preferably a billboard with clear sharp solid writing on a light background so that sharp lines are well defined.

And I also agree with BruceH about blocking off the objectives one at a time, rather than tiring your eye muscles by closing them.

 

[elkcub]

Interesting discussion. Being an old guy I've attached pages 76-77 from Dr. Henry Paul's 1964 edition of "Binoculars and All-Purpose Telescopes, How to choose, test, and use them." At least for daytime uses, his recommendation always seemed fine to me and corresponds with the OP's description. He recommends a clear target at 500' or more, stabilized viewing, both eyes open while focusing, and several replications. It's hard to improve on that. That's what I used to do.

If I might contribute a personal thought, it dawned on me recently that the universal presence of a diopter control on modern binoculars is basically an anachronism. If one's eyesight were perfect there would be no need for a diopter control; if it were imperfect one would need spectacles. With spectacles (or contact lenses) there is no need for a diopter control, because they correct for differential magnification.

So why do we have a diopter control? Essentially, it's an industry copout. Many manufacturers have not yet addressed eye relief properly, hiding behind the claim that 15mm or 16mm is "adequate" or "sufficient," when it really is not. That induces many people who are pre-astigmatic to use their binoculars without eyeglasses, and revert to the handy-dandy, old-fashioned diopter control as a crutch. But, as the severity of astigmatism (and other problems) degrade their eyesight, the diopter control eventually proves to be inadequate. At that point, regardless of optical quality, those expensive binoculars become about as useful as a bubble-pack toy.

Just a thought. :king:

Ed

 

[ceasar]

Ed,

Your last paragraph, like some diopters, needs to be tweaked!:h?:

Bob

PS: Thanks, I think you edited that paragraph I referred to in your addendum.

Bob

 

[elkcub]

... thanks. :t:

 

[John Russell]

Ed,

I had hoped you might be able to elaborate on the interrelationship of dilated pupils, accommodation (and/or reduced DOF of the eye) and possible loss of accuity of the eye at reduced light levels.

I have checked the instructions of two different Swarovskis and a Meopta and no mention is made of preferred distances for diopter setting, so I think that, provided the distance can be maintained precisely during the procedure it is irrelevant.

The extreme close focus of many modern bins is of little use and the 1,5 m of my 10x42 SVs would require squinting equivalent to 15 cm with the naked eyes! However, I see no reason why one should not use 2 m to set the diopter. Here one is succesively setting two monoculars, the depth of FOCUS of the bins is essentially the same as at infinity, and the image as seen through the bins is in the comfort zone, i.e. at infinity for the normally sighted, somewhere closer for the short sighted and "beyond infinity" for the far sighted.

John

 

[mayoayo]

Adding a little wikipedia to the debate.....

Depth of focus vs depth of field

While the phrase depth of focus was historically used, and is sometimes still used, to mean depth of field, in modern times it is more often reserved for the image-side depth.
Depth of field is the range of distances in object space for which object points are imaged with acceptable sharpness with a fixed position of the image plane (the plane of the film or electronic sensor). Depth of focus can have two slightly different meanings. The first is the distance over which the image plane can be displaced while a single object plane remains in acceptably sharp focus;[1][2] the second is the image-side conjugate of depth of field.[2] With the first meaning, the depth of focus is symmetrical about the image plane; with the second, the depth of focus is greater on the far side of the image plane, though in most cases the distances are approximately equal.
Where depth of field often can be measured in macroscopic units such as meters and feet, depth of focus is typically measured in microscopic units such as fractions of a millimeter or thousandths of an inch.
The same factors that determine depth of field also determine depth of focus, but these factors can have different effects than they have in depth of field. Both depth of field and depth of focus increase with smaller apertures. For distant subjects (beyond macro range), depth of focus is relatively insensitive to focal length and subject distance, for a fixed f-number. In the macro region, depth of focus increases with longer focal length or closer subject distance, while depth of field decreases

 

[jaymoynihan]

My favorite object for diopter setting is the moon.

:t:

 

[John Russell]

Adding a little wikipedia to the debate.....

Thanks for adding that , Manuel.

For distant subjects (beyond macro range), depth of focus is relatively insensitive to focal length and subject distance, for a fixed f-number. In the macro region, depth of focus increases with longer focal length or closer subject distance, while depth of field decreases

Depth of focus in bins should be even less sensitive to subject distance than in most cameras because they are not used in the macro region, have long objective focal lengths and in many cases internal focussing.

John

 

[elkcub]

Hmmm. The issue at hand is to agree on a procedure for setting the diopter. Would everyone agree that the observer looks through the instrument? If so, the optical system is defined by the combination of the optics of the eyes and those of the binoculars.

When acting alone, the human eye is similar to a camera. Like a camera it has a measurable 'depth of field', which under fixed ambient light conditions is determined by the focal length (state of accommodation) and effective aperture (pupil diameter). Let's think of DOF as a range over which all objects are in acceptable focus. The hyperfocal distance is then defined as the point that retains all objects in focus from there to infinity.

When the eye is used in combination with a telescope, the depth of field of the optical system can be shown to be the depth of field of the eye reduced by the magnification of the instrument squared (1/M^2). (An afocal instrument itself has no definable DOF.) Now if we proceed to focus the left side of our binoculars on a very distant object from near to far, we are essentially setting the optical system, i.e., eye+telescope, at its hyperfocal distance. Focusing beyond that point will simply reduce the near points that are also in focus. If the diopter is then adjusted the same way (from near to far) on the right side, in theory we have assured that the in-focus range for both eyes is matched optimally. That's the rationale for why good instructions call for focusing on a distant object. (People who write instructions don't always understand what they're writing about.)

Now consider a similar procedure focusing on a very near object, recognizing that the eye is also accommodating in conjunction with the instrument. In theory, this introduces instrument focus variability from one observation to the next, depending on how much visual accommodation is used. In addition, due to fatigue the observer's accommodation ability is not constant, or necessarily matched between the two eyes. How much difference this makes in practice, however, is another matter. When I was making diopter settings I tended to use intermediate range targets, but that may have been more out of laziness than correctness.

Finally, it is generally mentioned that the diopter adjustment should be done in good light, which means the pupil of the eye will be between 2.0 and 2.5mm. Larger pupils introduce significant aberrations from the outer surface of the lens, which reduce acuity. Going through the procedure at nighttime introduces a number of other issues, not the least of which is the adaptive state of the retina and the fact that maximum sensitivity switches away from the fovea. These are complications better left to astronomers to figure out.

Ed

PS. I just became conscious of something I hadn't realized before. Notice in my earlier attachment that the woman in Fig. 7.3 is wearing glasses. Rather misleading. I never found it necessary to adjust the diopter control with my glasses on.

PPS. In the rationale stated above, depth of field refers to object distances, and depth of focus refers to retinal blur. Focusing is based on minimizing the latter.

 

[mayoayo]

Ok,,I think You have to find this for Yourself,only as a personal formula and perhaps even with a particular pair of bins at a time..Sometimes My eyes make me change the setting,from close range (a bit minus)to infinity( a bit plus),if ever so slightly...Even with glasses,because the vision can be corrected with glasses ,but SIMULTANEOUS focussing is muscular activity ,and stamina, nutrition or muscle development can be different for each eye,at a particular moment....ultimately,and with my current pair of binos,I use the diopter settings naturally as a part of the focus system,and let my eyes tell me when im there.....

 

[John Russell]

Ed,

I liked your concept of the binocular/eye as a complete optical system and the depth of field of the eye being reduced by the square of the magnification. If one regards the magnification as being the ratio of the angles subtended by the image and the object itself (or rather the tangents thereof) I can appreciate that the diameter of the circle of confusion will increase by a factor of the magnification and its area by the square.

We were using the term "depth of focus" in two different contexts. I meant the depth of focus of the image formed by the binocular objective. This image is what we are viewing so we focus on an image, not an object. The eyepiece enables us to place its virtual image at infinity, but in reality the objective's image is only separated from the eyepiece by the eyepiece focal length, a few millimeters.
Consequently, I still think that object distance is irrelevant to diopter compensation. If we take the microscope analogy, object distances are very short .

John

PS: I think we could agree on the desirability of moving from close focus in the direction of extended focus, but for diopter setting that would require knowledge of whether the right eye is positive or negative with respect to the left eye, and a bit of thought whether to preset the diopter at maximum positive or maximum negative!