Focussing: Just Do It! (2 Viewers)

[Tringa45]

John, I would love to hear if Miss C did in fact repeatably set the same focussing wheel position for the same distance, in case you checked that (which you'd probably have mentioned, but I thought I'd ask anyway .

Hi Henning,

Yes , that was my original intention but it's rather difficult to mark an exact focus setting. I then realized that dioptre setting is the correlation of two independent focus settings.
If the correlation remains constant, that's a good indication that there was consistency in the focus settings.

Regards,
John

 

[Hauksen]

Hi Bill,

Also, I used the term "stare" because dealing with the series of scientific/medical occurrences involved is beyond my capacity to explain and beyond* the capacity of many others to appreciate.

I admit that I always had difficulties understanding your use of the term, but now I believe I see what you're aiming at. "Working point" might be a term more appropriate to electronics than to physiology, but I'll keep that concept in mind in case I ever stumble upon something highlighting the background, which is beyond my capacity at the moment, too!

Regards,

Henning

 

[elkcub]

I’d like to see some literature citations on the business of the effects of accommodation on focusing of optics.

Has this been investigated systematically?

I know of none, so that is what I was trying to show.

Sorry being late to the party. There is considerable vision research related to this issue under the rubric "Instrument Myopia" dating back well over fifty years. A seminal article by my friend and colleague, Bob Hennessy is attached from 1975. A 1998 book entitled "Visual Instrumentation: optical design and engineering principles" by Pantazis Mouroulis also provides and overview of the subject in the context of instrument design, a small section of which is also attached below.

Like Triaga, I am not aware of research (or theory) that substantiates the notion that "staring behavior," whatever that means, can improve viewing quality or effect the normal course of events. So I remain very skeptical.

Ed

 

[Hauksen]

Hi Ed,

Like Triaga, I am not aware of research (or theory) that substantiates the notion that "staring behavior," whatever that means, can improve viewing quality or effect the normal course of events. So I remain very skeptical.

Thanks a lot, that's really interesting material! Couldn't Bill's "staring" be understood as a technique to either avoid instrument myopia, or to focus the instrument so that the eye's refractive state corresponds to Hennessy's "intermediate resting state"? I can't judge whether this is feasible, or whether this would actually generate any practical advantage, but I'm curious ...

Regards,

Henning

 

[Alexis Powell]

Hi Ed,



Thanks a lot, that's really interesting material! Couldn't Bill's "staring" be understood as a technique to either avoid instrument myopia, or to focus the instrument so that the eye's refractive state corresponds to Hennessy's "intermediate resting state"? I can't judge whether this is feasible, or whether this would actually generate any practical advantage, but I'm curious ...

Regards,

Henning

I wonder the same.

...Like Triaga, I am not aware of research (or theory) that substantiates the notion that "staring behavior," whatever that means, can improve viewing quality or effect the normal course of events. So I remain very skeptical...

In the distant past (e.g. What is the Best Distance for Initial Focus/Diopter Setting), you've argued that subject distance didn't matter but that focusing from near to far might matter for getting the best focus and diopter settings. Has your understanding changed since then?

--AP

 

[elkcub]

Hi Ed,



Thanks a lot, that's really interesting material! Couldn't Bill's "staring" be understood as a technique to either avoid instrument myopia, or to focus the instrument so that the eye's refractive state corresponds to Hennessy's "intermediate resting state"? I can't judge whether this is feasible, or whether this would actually generate any practical advantage, but I'm curious ...

Regards,

Henning

I believe Hennessy's intermediate resting state hypothesis has been well supported over the years. Personally, I don't see how "learning to stare" (whatever learning means in this context) would have much effect on a process that takes place all by itself anyway. Fighting it (for whatever reason) would clearly be uncomfortable, and facilitating it simply gilding the lily. To date I'm not aware of negative value judgments about it, just that it's a curious behavioral phenomenon that warrants scientific explanation. (Well, an exception might be made in the case of microscope usage, but I haven't researched that enough to comment.)

Ed

 

[elkcub]

I wonder the same.

In the distant past (e.g. What is the Best Distance for Initial Focus/Diopter Setting), you've argued that subject distance didn't matter but that focusing from near to far might matter for getting the best focus and diopter settings. Has your understanding changed since then?

--AP

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

Hello AP,

At the time (see below) I was making the point that, taking depth-of-field (DOF) into consideration, it would make sense to focus from near to far so that one could obtain the hyperfocal setting for the eye-instrument combination. Thank you for pointing it out, because it is consistent with the instrument myopia phenomenon. Indeed, Mouroulis (somewhere) points out that fixed focus instruments should be preadjusted to their hyperfocal distance. But, a binocular is afocal so it can't be set except in conjunction with the eye.

With regard to the object distance for setting the hyperfocal, I don't recall commenting on that but I do use objects much closer than the moon which are at optical infinity. I was trying to provide a rationale for focusing from near to far.

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.)

Ed

 

[elkcub]

Oops, I forgot to mention that the literature doesn't support the simple notion that the lens is ever completely relaxed, much less at optical infinity, ... well, except at death. Arguably while the organism is living the most relaxed it gets is probably at what Hennessy calls its intermediate resting state. Several invasive experiments have been done on monkeys in this area of inquiry.

Pondering it, it's not really that weird an idea. For example, the brain, heart, lungs, ... etc. are never really at rest, they just settle down to a tonic state. Why shouldn't that also be true for the eye lens?

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

Ed

 

[Maljunulo]

When the ophthalmologist dilates your eyes, the drops paralyze the muscles of the iris …… do those drops also paralyze the muscles which change the focal length of the (OEM) lens?

If so, at what distance is the eye then focused?

 

[elkcub]

Nothing like the Internet nowadays.

Ed

 

[Alexis Powell]

Hello AP,

At the time (see below) I was making the point that, taking the depth-of-field (DOF) into consideration, it would make sense to focus from near to far so that one could obtain the hyperfocal setting for the eye-instrument combination. Thank you for pointing it out, because it is consistent with the instrument myopia phenomenon. Indeed, Mouroulis (somewhere) points out that fixed focus instruments should be preadjusted to their hyperfocal distance. But, a binocular is afocal so it can't be set except in conjunction with the eye.

With regard to the subject distance for setting the hyperfocal, I don't recall commenting on that but I do use objects much closer than the moon which are at optical infinity. I was trying to provide a rationale for focusing from near to far.

Yes, I understand, I think, what you were trying to explain back then. If I might add, my own understanding has changed a fair bit since that thread of 10 years ago. John Russell cleared up the question of apparent distance of objects seen through binoculars in that thread, but it took me a while to fully absorb it. I still very much prefer to use distant objects for diopter setting, but my thought as to why they work best for me was strongly influenced by that thread plus a lot more thought about the complications that the slow to fully tension/rebound lenses of my middle-aged eyes impose on the process of finding final focus and best diopter settings.

--AP

 

[Tringa45]

With regard to the object distance for setting the hyperfocal, I don't recall commenting on that but I do use objects much closer than the moon which are at optical infinity. I was trying to provide a rationale for focusing from near to far.

Hi Ed,

Thanks for jumping in here.
Firstly, I think your valid method of establishing hyperfocal distance by focussing from near to far has been falsely interpreted by some as a universal focussing recommendation!
The concept of a binocular as an afocal instrument also still seems to cause problems for many. I recently received a quote from one of the participants here:
"I don't find the exit pupil of a binocular trained on the sun to be especially hot on my hand."
This is not only an extremely dangerous implication but an indication that he had then failed to understand afocality and probably continues to do so.

While I agree that depth of field has to be seen as a binocular/eye combination, I nevertheless think a definition for the binocular alone would be possible if one defined a limit for perceived sharpness.
One often reads in Birdforum of users' capability of significantly bettering manufacturers' close focus specifications. This can mostly be attributed to myopia or good accommodation. However, I suspect the manufacturer's definition would be the closest object distance in which the focal planes of objective and ocular could be brought to coincidence.
In Holger Merlitz' book, "Handferngläser" he quotes Albert König as stating that a circle of confusion of 3,4 arcminutes would be perceived by most as sharp, even though the resolution limit of the human eye is often considerably better. Consequently, it would be possible to define DoF for any object distance if anyone were interested.
However we can mostly be content in knowing that DoF diminishes with the inverse square of the magnification and this is probably a factor in in the preference of many of us presbiopic shaky old dogs for the lower magnifications.

John

PS: I vaguely recall an old discussion on Birdforum on "Instrument Myopia" which was based on the zero settings of an IF binocular. It was (falsely?) assumed that zero represented infinity, but would not the hyperfocal distance for the normally sighted make more sense, particularly for the marine applications for which these binoculars are often used?

 

[Binastro]

At what distance do our eyes focus when we are asleep?

Does this change when we are dreaming?

Regards,
B.

 

[Canip]

At what distance do our eyes focus when we are asleep?

Does this change when we are dreaming?

Regards,
B.

I will measure if you wake me up at the right moment.

 

[Maljunulo]

I will measure if you wake me up at the right moment.

How?

 

[tenex]

The previous discussion of "instrument myopia" referred to was conducted in an unusually erratic and frustrating manner, but as I recall the experimental evidence was that the eye's resting focus is not at infinity but in the 1-2m range (varying individually), which does make more sense in terms of overall effort from an evolutionary point of view; and therefore the suggestion was made that it would be natural or best to focus an instrument on a target so the virtual image distance matches that. But the whole issue of precise focus is complicated by accommodation and depth of field to a degree that most people manage diopter and focus settings quite easily without any special tricks.

 

[Maljunulo]

All of the hand-waving and speculation aside, is there any objective measurement of the distance at which a “relaxed” human eye is focused? It strains my credulity that it is 1-2 meters, and I would like to see a citation from the peer-reviewed literature In support of that figure. As for when we are sleeping, I cannot conceive of a way to test or measure that.

Back in the days when I still had control of my OEM focusing apparatus, I recall that I had the impression that when I “relaxed” my eyes and allowed them to “go out of focus” that they were not in focus at any distance.

I have no clue what that “means” (if anything) in terms of the present discussion.

 

[Binastro]

Camera viewfinders were I think traditionally set at minus one dioptre.

Specialist MRI scans could possibly measure the eye when a person is asleep, and deduce the rest position of the lens.
Some universities have very sensitive MRI equipment.

B.

 

[tenex]

All of the hand-waving and speculation aside, is there any objective measurement of the distance at which a “relaxed” human eye is focused? It strains my credibility that it is 1-2 meters, and I would like to see a citation from the peer-reviewed literature In support of that figure.

I think you mean credulity but science often challenges that. I'm no expert on this subject, but the widespread assumption(!) of relaxed focus at infinity is generally attributed to Helmholtz, who perhaps was just thinking of the perceptible effort of focusing very close. (And not thinking of how to explain myopia?)
These two papers on resting/dark focus were referenced in the discussion here (see Experiment III in the first):

https://www.birdforum.net/attachments/hennessey_1975_instrument_myopea-pdf.741467/

https://www.birdforum.net/attachments/owens_1984_resting_state_of_the_eye-pdf.741627/

 

[Maljunulo]

I think you mean credulity but science often challenges that. I'm no expert on this subject, but the widespread assumption(!) of relaxed focus at infinity is generally attributed to Helmholtz, who perhaps was just thinking of the perceptible effort of focusing very close. (And not thinking of how to explain myopia?)
These two papers on resting/dark focus were referenced in the discussion here (see Experiment III in the first):

https://www.birdforum.net/attachments/hennessey_1975_instrument_myopea-pdf.741467/

https://www.birdforum.net/attachments/owens_1984_resting_state_of_the_eye-pdf.741627/

Alas, you are right. Off to edit. (I could blame auto-correct, but that would be bad form)

Thanks for the references.

ETA: Fascinating. I got pretty well lost in the first one, which was way over my head, and the second one was easier for me to follow. I like the experiment at the end, which even I could understand.

Live & learn.