Focussing: Just Do It! (1 Viewer)

[Tringa45]

Regarding the Home and Pool study, you don't have to believe them.

Omid,

I am reasonably convinced that the concept of "Instrument Myopia" is humbug.
It seems that you have just edited out your suggestion of testing with a camera, but I have nevertheless performed the test.
I set up my 8x56 Swarovski SLC on a tripod and focussed on a license plate at 75 m. I repeated the test on a closer object, but I think we are at least in agreement that the object distance is immaterial and that the virtual image distance for any one user will be consistent.
On a second tripod I placed the f/1,8 85 mm objective of a single lens reflex camera behind the binocular's eyepiece. The 85 mm lens, having somewhat reduced depth of field has an extended distance scale marked at infinity, 20 m, 10 m, 7 m, 5m and so on down to 0,85 m.
Wearing glasses for both binocular and camera focus I achieved camera focus at 8 m, i.e. my personal virtual image distance, which is fairly consistent with my glasses prescription, which was established at 6 m (20 ft).

John

PS: "Instrument myopia" of -1 dioptre, i.e. a focus setting of 1 m on the camera lens would have required 105 mm of rotation from infinity. The 8 m setting required a mere 13 mm!

 

[Omid]

Hi John,

I edited out my suggested experiment since I figured it would cause you undue trouble and likely lead to erratic results. Now, since you already took the trouble to do an experiment, I'll write some additional notes:

Regarding your experiment: What is your near point of vision when wearing your glasses? 6m? If so, it goes without saying that you will not focus your binoculars to show an image at 1m!

The proper test would be to invite your young lady friend (who I presume can comfortably focus her eyes from 25cm to "far") and ask her to focus your 8X56 binoculars at same target. Then, use your SLR camera with a close-focus lens and split-image viewfinder to precisely determine the distance of the virtual image she saw through the eyepiece.

Instrument myopia is a misleading term. It shouldn't be called "myopia", a more proper name for this phenomenon is "instrument accommodation": our eyes prefer to accommodate when looking through an optical instrument.

My own experiments: I used Nikon 105mm f/2.8 AF-S Micro lens in my early tests of this phenomenon. This particular lens can focus as close as 31 cm and has a very precise focusing scale on its manual focus ring. I performed most of my tests on riflescopes which are the primary optical devices that I am interested in. Because riflescopes have a very long eye relief (80mm to 120mm in some special models) and because their exit pupil is much larger (up to 13mm at lower magnifications), I could get reliable focus readings using the above lens attached my Nikon D3 SLR camera body.

I tested 5 or 6 riflescopes which I routinely use on my hunting rifles and found that their focus (diopter setting) was set such that the "reticle" appeared to be at distance from eye between 60cm to 1.2m (if I recall correctly). This indicates that my visual system "prefers" to see the target images somewhere between 0.8D to 1.5D. It is interesting that I had not set the reticle to same dioptric distance in all the tested scopes.

Earlier this year, Dr. Mike Mross from Vermont Photonics was kind enough to lend me a Moller-Wedel diopter scope for two weeks. I used it to test virtual image position in several of my riflescopes and binoculars. The results were more or less consistent with my primitive "SLR camera" experiments. The images were always focused at finite distances, not at infinity.


Cheers,
Omid

 

[Tringa45]

What is your near point of vision when wearing your glasses? 6m? If so, it goes without saying that you will not focus your binoculars to show an image at 1m!

Then, use your SLR camera with a close-focus lens and split-image viewfinder to precisely determine the distance of the virtual image

Omid,

Much ado about nothing.
I do still have some accommodation, so if there were such a thing as instrument myopia, I would have achieved a value under 6 m. Instead I hit my relaxed viewing distance at +0,08 dioptres, i.e. within my prescription tolerance.
Why would I need a close-focus lens if the one I used already had a -1,2 dioptre capacity?
And why would I need a split-image viewfinder, which blacks out at f/12 (85/7) or f/15 in your case? A matte focussing screen can be precise enough and I used one for many years on a Nikon F2.
Perhaps my experimental method using two tripods with both instruments aligned and stationary was just a little more reliable.

John

 

[Omid]

Omid,

Much ado about nothing.
I do still have some accommodation, so if there were such a thing as instrument myopia, I would have achieved a value under 6 m. Instead I hit my relaxed viewing distance at +0,08 dioptres, i.e. within my prescription tolerance.
Why would I need a close-focus lens if the one I used already had a -1,2 dioptre capacity?
And why would I need a split-image viewfinder, which blacks out at f/12 (85/7) or f/15 in your case? A matte focussing screen can be precise enough and I used one for many years on a Nikon F2.
Perhaps my experimental method using two tripods with both instruments aligned and stationary was just a little more reliable.

John

Hi again John,

The reason for suggesting a split-image view finder was to enhance focusing precision. The reason for suggesting a close-focus lens was to be able to measure images as close as possible in case you asked a young adult person to volunteer for this experiment.

Thanks again for taking the trouble and conducting your own experiment.

-Omid

 

[Omid]

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

2- Does this change when we are dreaming?

Regards,
B.

Answer 1: Tests on monkeys with full range of accommodation has shown that their eyes accommodate between 2.5D to 3D during light sleep. Ref.: Gerald Westheimer, Sidney M. Blair, Accommodation of the eye during sleep and anesthesia, Vision Research, Volume 13, Issue 6, 1973

Answer 2: Yes! If you dream of a sunny Hawaiian beach, your pupils will constrict, and your accommodation state will likely change to match the new tone of your autonomic nervous system.

 

[Omid]

My ability to accommodate is exactly zero, since my lenses are both implants.

You are underestimating yourself! The human eye is not a camera. You eye likely has about 1D residual accommodation!

French vision scientist Yves Le Grand associates this capability to the retina/brain adaptation to the really high chromatic aberration of the eye's natural lens (which is more than 2.5D between 400nm and 700nm wavelengths). When describing the challenge of explaining eye accommodation in the face of such drastic chromatic aberration, Le Grand says (Form and Space Vision, 1967, Page 46):

One notes that the physical criterion of the "best image" does not play any part, and the eye adjust itself so that it spares its accommodation.

He then goes on to say that this explains the residual accommodation observed among aphakics (persons who have lost their eye lens).

Note to elkub: Le Grand's statement I quoted above is truly remarkable! His observation that "best retinal image" does not play any part in accommodation is in line with my statement in Post 55 where I said "image blur" cannot be the stimulus for accommodation.

Note to John: This thread belongs to you and its original theme was focusing binoculars. I apologize for jumping in. I will go back to my atelier and focus on my achromatic imaginations now

-Omid

 

[Hauksen]

Hi John,

Users will always adjust the focusser to place the virtual image at their comfortable viewing distance. For a near-sighted user with a -5d prescription this would be 20 cm and rays from any point on the focussed object would diverge from the eyepiece.

As an interested layman, I find it increasingly difficult to follow the highly interesting discussion whenever additional elements get added to the experiment, or when measurements are mentioned, as I'm not always sure of the precise context (which the pros obviously are).

In other words, a sketch of the relevant elements and dimensions of interest would really help me a lot

Regards,

Henning

 

[Tringa45]

In other words, a sketch of the relevant elements and dimensions of interest would really help me a lot

Hi Henning,

Unfortunately, as a digital dyslexic I couldn't even upload a straight line!
However, here is a sketch from German Wikipedia, Fernrohr – Wikipedia, which is somewhat clearer than the one on the English site.

Otherwise, Holger Merlitz' book, "Handferngläser" or "Telescope Optics" by Rutten & van Venrooij go into great depth.

Although Omid and I are not in complete agreement, his suggestion of using a camera to establish one's personal virtual image distance was very helpful.
Perhaps when Miss C returns from holiday I will get her to repeat the test, although I am fairly confident it will not show any confirmation of "Instrument Myopia".

Regards,
John

 

[Hauksen]

Hi John,

Otherwise, Holger Merlitz' book, "Handferngläser" or "Telescope Optics" by Rutten & van Venrooij go into great depth.

Thanks for the recommendation, I just ordered a copy of Holger's book!

Regards,

Henning

 

[tenex]

although I am fairly confident it will not show any confirmation of "Instrument Myopia"

Confirmation of what exactly? This unfortunate term was coined under the false assumption of a resting focus at infinity, which made accommodation at modest distances when using optics (or in dim light etc) look like some sort of "myopia", a problem to be explained, whereas resting focus at modest distance is merely a feature of human vision. (Or are you disputing that?) I think confusion around this can be purely linguistic.

 

[Tringa45]

Confirmation of what exactly? This unfortunate term was coined under the false assumption of a resting focus at infinity, which made accommodation at modest distances when using optics (or in dim light etc) look like some sort of "myopia", a problem to be explained, whereas resting focus at modest distance is merely a feature of human vision. (Or are you disputing that?) I think confusion around this can be purely linguistic.

Eric,

What do you mean by modest distance? A distance of 5 m is only -0,2 dioptres but there are assertions here in the thread and in some "scientific" studies that the normally sighted, when focussing through an optical device would place the virtual image at 1 m, i.e. -1 dioptre, e.g. Home & Poole.
I dispute this and in the test above my preferred virtual image distance was 8 m with corrected vision. This coincides fairly closely with the 6 m or 20 ft testing distance, which eye doctors regard as infinity for the human eye.

John

 

[Maljunulo]

I seem to recall reading many years ago, that infinity was defined as fifty (50) focal lengths.

Perhaps I remember incorrectly.

Unfortunately, about the time these discussions get really interesting, they go beyond my ability to understand and follow.

 

[tenex]

What do you mean by modest distance? A distance of 5 m is only -0,2 dioptres but there are assertions here in the thread and in some "scientific" studies that the normally sighted, when focussing through an optical device would place the virtual image at 1 m, i.e. -1 dioptre, e.g. Home & Poole.
I dispute this and in the test above my preferred virtual image distance was 8 m with corrected vision. This coincides fairly closely with the 6 m or 20 ft testing distance, which eye doctors regard as infinity for the human eye.

Well, from papers like Hennessey 1974 and Owens 1984 (linked above), typical values are -1 to -2D, a meter or less. When I used Omid's SLR camera method myself, I got about 2m, -0.5D, which is on the long end of the distribution, so I'm not sure how to explain your result of 8m; it would be interesting to have more people here try this and build our own data set. Anyway to a non-expert like me, it seems that intermediate resting focus is now widely accepted, and if you wanted to dispute it you would have to find some flaw in the laser-optometer approach that's been used to measure focus without stimulating accommodation, and a different explanation for the apparent consistency of its results. The fact that similar results are also found under conditions that frustrate accommodation (accounting for twilight and empty-field "myopias") also seems very persuasive to me, so I'd like to hear more about your doubts.

 

[Tringa45]

Well, from papers like Hennessey 1974 and Owens 1984 (linked above), typical values are -1 to -2D, a meter or less. When I used Omid's SLR camera method myself, I got about 2m, -0.5D, which is on the long end of the distribution, so I'm not sure how to explain your result of 8m; it would be interesting to have more people here try this and build our own data set. Anyway to a non-expert like me, it seems that intermediate resting focus is now widely accepted, and if you wanted to dispute it you would have to find some flaw in the laser-optometer approach that's been used to measure focus without stimulating accommodation, and a different explanation for the apparent consistency of its results. The fact that similar results are also found under conditions that frustrate accommodation (accounting for twilight and empty-field "myopias") also seems very persuasive to me, so I'd like to hear more about your doubts.

The problem here seems to be that the physiologists have a very limited understanding of optics and that those of us with a basic understanding of optics have a very limited understanding of physiology.
As far as I can tell there was no attempt made in all these studies to test the eyesight of the individual test persons, so lacking any basis for comparison, how could one assert that they experienced "instrument myopia"?
When conducting an eye test one views through a stack of spherical and cylindrical corrective lenses, so why would the eye suddenly switch to a myopic condition when the test apparatus were replaced by a microscope or binocular? This assertion seems to me to border on the absurd.
These verbose treatises tax the patience of any reader, so I think few would make any serious attempt to understand them, let alone verify them experimentally, and in the end the author gets a PhD!
I'm reminded of "The Emporer's New Clothes".

John

 

[tenex]

As far as I can tell there was no attempt made in all these studies to test the eyesight of the individual test persons, so lacking any basis for comparison, how could one assert that they experienced "instrument myopia"?

As I understand it, this is not a verdict on anyone's eyesight, it's just a question of the distance at which the resting eye is focused, which is being measured in a clever way (laser interferometer etc) that doesn't itself affect the eye's focusing. And investigators who expected that distance to be infinity (or rough equivalent) were surprised to find it coming out around 1m, and called this result "instrument" (or "twilight" or "empty-field") "myopia", as if people were accommodating closer than they should... and somehow the term continues to be used. It's as if chemists today were still talking about the negative weight of phlogiston.

When conducting an eye test one views through a stack of spherical and cylindrical corrective lenses, so why would the eye suddenly switch to a myopic condition when the test apparatus were replaced by a microscope or binocular?

As just described there is no actual "myopic condition" here.

These verbose treatises tax the patience of any reader, so I think few would make any serious attempt to understand them, let alone verify them experimentally

Well, I've tried to understand some of them, and found the Owens paper particularly readable. And this had all better be right, because fighter jet displays and so on are being designed on the basis of it!

 

[Hauksen]

Hi Tenex,

Well, I've tried to understand some of them, and found the Owens paper particularly readable. And this had all better be right, because fighter jet displays and so on are being designed on the basis of it!

Are you referring to Head-Up Displays? I would think these only make sense if its displayed image is in the same focal plane as the objects the pilot is looking at in flight, which would be "near infinity".

Regards,

Henning

 

[tenex]

Are you referring to Head-Up Displays? I would think these only make sense if its displayed image is in the same focal plane as the objects the pilot is looking at in flight, which would be "near infinity".

I can't easily find what I read about this, as it's not my field, but think the general context was customizing for the resting focus of an individual pilot's eye under stress or disorientation (much like the discussion of night driving in the Owens paper). Perhaps someone else can step in.

 

[elkcub]

All,

The attached 2016 article might be of particular interest to those speculating about the visual capability of subjects participating in published studies. In the fairly recent 2016 study by Liua, et al:

"All subjects were required to have a best-corrected monocular visual acuity of 20/25 or better, uncorrected (or residual) refractive error of ≤ 1.25 D hyperopia, ≤ 0.50 D myopia, ≤ 1.00 D uncorrected astigmatism or anisometropia. If correction was necessary for the subjects during the study, they were required to have habitual contact lens correction that had been prescribed and worn for a least one month. The requirement for contact lens correction during objective AR recording is necessitated by our study protocol of prolonged (10 minutes) continuous AR recording during reading task. The spectacle reflection resulted in excessive loss of data during auto-refraction recording. The residual uncorrected refractive error was determined by making three static auto-refraction recordings while participants viewed the 20/25 row of a Snellen chart at 6 m from the right-eye. Recordings were made under subjects’ habitual viewing condition, either uncorrected, or wearing their habitual contact lens correction.
Major exclusion criteria for this study included history of treatment for binocular disorder, corneal refractive surgery, epilepsy or head trauma, multiple sclerosis, Graves’s thyroid disease, myasthenia gravis, diabetes or Parkinson’s disease. Participants were also ineligible if they were currently taking non-SSRI anti-anxiety drugs, anti-arrhythmic agents, anticholinergic or tri-cyclic antidepressants. Additionally, participants that were deaf or stuttering were excluded."

Hennessy's 1975 article (again attached below), contained three experiments:

Study #1 "Sixteen emmetropes, age 18 to 32, served as subjects. ... "
Study #2 "Twelve emmetropic subjects, ages 18 to 32, served as subjects."
Study #3 "Fifteen emmetropes, ages 18 to 25, served as sub- jects."

It's of particular importance to stress that subjects are always considered to be independent from the experimenters, and usually not told the purpose of the experiment. With rare exceptions, such as research notes, etc., the experimenter's personal observations are not included in scientific publications for what should be obvious reasons of bias. Equally important, scientific studies (with rare exception) use representative samples, with N > 12 in most cases to meet statistical requirements. Otherwise, it would not possible to assess group differences, e.g., age, sex, ..., or measurement error.

Ed

PS. I also thought this article might be of interest to Omid, wrt the accommodation stimulus. Nah, you've probably read it.

 

[Hauksen]

Hi Ed,

PS. I also thought this article might be of interest to Omid, wrt the accommodation stimulus.

That is indeed fascinating, thanks a lot!

Longitudinal chromatic aberration does seem to check the box for providing the direction for accommodation, too.

However, if that's the primary mechanism, there are secondary mechanisms as well? For example, it's possible to focus under monochromatic lighting where chromatic aberration is absent.

I just found this article ... Accommodation with and without short-wavelength-sensitive cones and chromatic aberration

... stating:

The mean lag of accommodation for 550 nm light increased from 0.18 D without LCA to 0.50 D with LCA. In the presence of LCA blue light (420 nm) was focused in front of the retina (−0.69 D) while green light (550 nm) was focused behind the retina (0.50 D).

Is "lag of accommodation" another way of referring to "instrument myopia" (or maybe a corresponding but more general term, if the effect occurs "instrument-less")?

Another statement I found interesting is:

In fact the wide variation in accommodative response [among subjects] is typical of all types of accommodation, and is a hallmark of the accommodation response.

As the question of individual vs. random variations was mentioned above, that sounds to me as if individuals might in fact be able to produce well-repeatable results, even if a larger variation is measured over an entire test subject population.

However, I can't claim to have read or understood the article in full, I'm just looking for interesting keywords which I believe fit into this thread!

Regards,

Henning

 

[Omid]

Is "lag of accommodation" another way of referring to "instrument myopia" (or maybe a corresponding but more general term, if the effect occurs "instrument-less")?
Henning

No. Lag of accommodation refers to the fact that human eye doesn't accommodate enough on objects which are too close and accommodates too much on objects which are far. At some intermediate distance (called the resting point of accommodation, about 80cm to 1.5m), the eye accommodates accurately:



Two more facts:

a) Eye lens has so much chromatic aberration (and substantial spherical abberation) that focusing to produce a "sharp image" is literally meaningless (exactly as LeGrand said).


b) The human fovea doesn't have its RGB pixels (cone receptors) distributed uniformly. Even if the lens could focus polychromatic light sharply into a spot, there would be no red, green and blue receptors waiting on that spot to "register" that pixel's intensity and color correctly. The human eye has many more red receptors than green and there are very few blue ones. Also, the green receptors are often lumped together as shown in the pseudo-color micrograph on the left:




So, the eye lens is very bad, the focusing mechanism is poor, the retina doesn't have enough receptors to register RGB colors at every pixel.

How do we see the world so nicely and vividly then??! I don't know of a convincing explanation. Several prominent scholars including James Gibson, Ulrich Neisser and Margaret Livingstone (Cornell, Harvard, Harvard) have completely given up the idea of the eye taking "pictures" of the world. I have started to believe in this very hard-to-digest notion too.

-Omid