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Old Saturday 15th February 2020, 18:16   #126
Binastro
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I would query whether one always needs to focus a binocular.

It depends what one is observing and the binocular used.

I have very little accommodation, yet I hardly ever need to focus my 6x18 waterproof low priced IF binocular.
It has a curved field. In fact I don't focus it at all.
It is set for maybe 30ft, which is O.K. for longer distances.
When I want to see closer objects I just raise the binocular a little or a lot and use the lower half of the field.
Things are sharp at fairly close distances if need be.

I use a similar technique with larger binoculars with curved fields. Most of my binoculars have curved fields, although the Canon IS are mostly flat field, but these are generally used at a distance.
I did not refocus the Canon 18x50 IS for about ten years. I only changed the focus when my eyes changed enough to warrant a change.

My PST telescope has never had a change of focus in about 15 years. It is on a distant target, but the H Alpha image needs changing as one moves across the Sun. I achieve this by using the curved Kelner eyepiece field. I just move across the field as much as needed for things to be sharp.

As to why flat field binoculars need to refocus. The image is magnified for one reason.
Focus is mainly needed for close observing. At medium to far distance it may be that focus can stay the same.

Incidentally, the sharpest binocular I have for daylight use at medium distances in the Minolta 8x23 AF binocular.
I can never focus a standard binocular that accurately on a moving target.

Autofocus in cameras is now so precise, I think that autofocus binoculars should be looked at again.

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Old Saturday 15th February 2020, 20:17   #127
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Autofocus in cameras is now so precise, I think that autofocus binoculars should be looked at again.

B.
Spot on!
The auto focus on cameras is at the point where the exact on eye focus is standard for bird photographs, achieved in a fraction of a second. It makes it appear that the binocular community is caught in an early 20th century time warp.
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Old Sunday 16th February 2020, 06:55   #128
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Why isn't the eye's amazing accommodative power sufficient to bring objects seen through binoculars into sharp focus?
One reason is because there is no such thing as 'the eye', as in one eye that everyone has two of and which all perform in an identical way.

The power of accommodation varies from person to person and is certainly age-dependent.

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Old Sunday 16th February 2020, 08:23   #129
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Spot on!
The auto focus on cameras is at the point where the exact on eye focus is standard for bird photographs, achieved in a fraction of a second. It makes it appear that the binocular community is caught in an early 20th century time warp.
Autofocus is a huge boon to photography and I couldn't have taken the photo below without it. But photos like this and photos of birds are deceptive because they don't reveal how many times the autofocus focused on vegetation in front of the bird or insect, vegetation that in your concentration on your subject you didn't notice. Chasing warblers in trees and scrub or reedbeds would be a frustrating task with manual focusing being necessary on many occasions.

And of course autofocus needs electronics, a sensor, switches and batteries.

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Old Sunday 16th February 2020, 13:42   #130
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Hi Jack,

Thank you for your comments. Regarding sculpture vs painting, I sated my opinion but its a subject we can leave to art experts to discuss. Here's one expert's opinion:

http://www.italianrenaissanceresourc...sus-sculpture/

Regarding 3D cinema -once sufficiently refined- being preferable to 2D, I don't agree for the reasons I stated before. The issue is not technical quality of the images. It is far more fundamental. I came up with the notion of 3D film and cinema (or 3D art in general) adding little to enhance visual aesthetics on my own. Since then, I did some research and noticed that I am not alone in noticing this. Here is an excellent article on the subject:

https://www.dr-lex.be/info-stuff/3dfail.html

This author's view is in agreement with the "utilitarian nature" of stereoscopic vision I mentioned in my above post. He explains some further aspects too. For example, he argues that watching a film is a passive experience, and that is not a bad thing. People go to a cinema or sit behind their TV to relax and do absolutely nothing during 90 to 120 minutes. While looking at things in a 3D world is not a passive process, it is very active.

The notion of stereoscopic vision being important when we need to actively interact with the environment is a very deep concept. Depth perception is needed if you are a hunter (like me) or predator (like a lion). If you are a software engineer working for Google or an antelope running away from a lion, you don't need it


Sincerely,
-Omid
Hi Omid,
I guess we'll just have to check back in twenty years and see!
Best regards,
Jack
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Old Sunday 16th February 2020, 20:24   #131
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One reason is because there is no such thing as 'the eye', as in one eye that everyone has two of and which all perform in an identical way. The power of accommodation varies from person to person and is certainly age-dependent.

Lee
Of course there is a need for some "initial focusing means" to help each individual focus the binoculars for his eyes. Something like the diopter setup knob.

But once the initial adjustment is done, why can't our eyes focus on various objects when using binoculars? A binocular produces a virtual image of various objects in its field view. This virtual image for each object is located "somewhere" in front of the eye. The eye should be able to focus on the virtual image provided by the eyepiece as long as it is not too close (i.e. closer than 20 cm). But this doesn't seem to be the case ??

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Old Sunday 16th February 2020, 20:49   #132
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Perhaps the virtual image is not actually in focus.

B.
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Old Monday 17th February 2020, 16:41   #133
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Of course there is a need for some "initial focusing means" to help each individual focus the binoculars for his eyes. Something like the diopter setup knob.

But once the initial adjustment is done, why can't our eyes focus on various objects when using binoculars? A binocular produces a virtual image of various objects in its field view. This virtual image for each object is located "somewhere" in front of the eye. The eye should be able to focus on the virtual image provided by the eyepiece as long as it is not too close (i.e. closer than 20 cm). But this doesn't seem to be the case ??
Whatever the cause, 2 eyes are not needed. Anyone with a telephoto lens with shallow depth of field, or a spotting scope will encounter the same effect. It seems the optics generate/present a shallow depth of field that the eye can't compensate for. The focusing lens on a binocular physically moves a virtual focal plane back and forth in space while the eye lens can only deform itself to change focus. So, maybe what Binastro says: the out of focus elements contained in the focal plane are 'unfocusable'...

-Bill

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Old Tuesday 18th February 2020, 20:27   #134
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Here's another question:
Can eye relief be made a user adjustable function of a binocular system, and if so, at what cost optically?
My reasoning is that if one had to sacrifice no more than .25 of fov to pick up 3-4 mm of eye relief, it would be worth it for the viewer.

-Bill

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Old Tuesday 18th February 2020, 22:13   #135
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Whatever the cause, 2 eyes are not needed. Anyone with a telephoto lens with shallow depth of field, or a spotting scope will encounter the same effect. It seems the optics generate/present a shallow depth of field that the eye can't compensate for.
-Bill
Hi Bill,

I too was considering if stereoscopic vision has anything to do with it. It might but, as you explained, it can not be the only cause of shallow depth of field. Spotting scopes show same limitation and they are much worse than binoculars (have extremely shallow depth of field).

I am thinking the main cause must be the eyepiece. A well-corrected objective lens forms real images of all objects in its field of view. The image of far objects is formed closer to the objective focal plane. Images of near objects are formed slightly behind the focal plane. But a focused image of every object is formed somewhere near the focal plane of the objective.

In photography, one can simply shift the position of the film to focus any object on it. The famous CONTAX AX camera used this method for auto-focus. It used manual-focus Zeiss lenses but could auto-focus by shifting the film plane.

Why the eye positioned behind an eyepiece can not bring any arbitrary real image point formed by the objective into sharp focus? What does the eyepiece do that impedes the eye's amazing accommodation ability?

-Omid

PS. Regarding variable (adjustable) eye-relief, I have some solutions and I am investigating them for use in riflescopes. It is possible to do.
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Old Wednesday 19th February 2020, 00:00   #136
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[David Williamson:] "The mystery remains of how the eye was designed and made that way so long ago".
Just catching up on this thread... a curious remark. Of course there is no such mystery, as the eye was neither designed nor made. It certainly is interesting though.

As to why focusing is necessary with binoculars, I'm not sure of the answer. I thought it a straightforward question that someone knowledgable would have answered by now, but since that doesn't seem to be happening, I'll offer an observation: a camera lens forms a roughly planar image which we then view on a screen or print. A binocular or scope with an ocular doesn't form an image that the eye then views, but helps the eye form one itself, on its retina which is curved.

Part of the problem may lie in how the question is phrased, as to what's "impeding" the eye's focusing ability. Perhaps we just can't expect the eye to work the same way with a bino as it does without one, because it's no longer functioning as only "the eye", but a compound system of eye plus bino that apparently requires adjustments to the spacing of glass elements to accomplish focus. If the magnification is sufficiently small, the eye actually does seem to be able to accommodate; even 6x or 7x binos don't require much refocusing, which some people like. For some reason yet to be explained, there's ever more of an issue as the magnification increases from there. Does this dependence on magnification suggest the rest of an explanation of what's going on, why mechanical focusing is necessary?

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Old Wednesday 19th February 2020, 00:44   #137
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What emerges from the eyepiece is not a focused image. It's afocal light just like what struck the front of the the objective lens. When it strikes the lens of the eye it's brought to focus on the retina just like a camera lens focuses the afocal light from the scene in front of it onto a sensor.

The reason DOF appears shallow in binoculars and scopes is because, like everything else in the image that forms on the retina of an observer using binoculars, the circles of confusion in front and behind best focus are magnified compared to way they appear to the eye alone. An unfocused circle of confusion near best focus may be so small as to appear sharp to the eye alone but quite unfocused when it's 8 times larger in a 8x binocular. That's why DOF shrinks with increasing magnification. The circles of confusion just keep getting bigger and nothing can be done about it, except to reduce the magnification or call upon whatever accommodation you have to shift the focus point closer.

I played around with the idea of setting an old Kern 8x30 binocular to it's hyperfocal distance and then checking how close my pitiful old man accommodation of about 1 diopter would allow me to reach focus. It was about 40 m. I picked the Kern because it has accurate 1 diopter increments marked on it's rotating eyepieces all the way to +/-10 diopters. The hyperfocal setting is just about 0 D for my right eye.

An average person about 42 years old would have about 5 diopters of accommodation, so I rotated the eyepiece to +5 D to simulate that much accommodation. Now I could focus as close as about 10 m. A 25 year old person has about 10 D accommodation. so I moved the eyepiece to +10 D and could focus to about 6m. A 10 year old can have accommodation of about 15 D. To simulate that I set the binocular at +9 D and positioned a +6 D magnifier between my eye and the eyepiece. Now I could focus down to about 3 m, a decent close focus distance. These are all pretty sloppy ball park figures, but close enough for demonstration purposes.

So, if you're 10 years old you probably could successfully use any 8x binocular without changing the focuser once it's set to the hyperfocal distance, but life is cruel so gradually you will lose accommodation as you eye's lens stiffens and by the time you're 30 or 40 you will definitely want a focusing mechanism on your binoculars.

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Old Wednesday 19th February 2020, 00:59   #138
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Omid and Tenex set up the question, and Henry hits it out of the park.

Well done, and thank you.

-Bill
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Old Wednesday 19th February 2020, 13:09   #139
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Bill,
Re.post 134.

The sacrifice in FOV is only needed in practice by those wearing glasses.
I certainly prefer less eye relief and bigger FOV up to a point.

Old 20x50 Porros can be a problem, but not much for me. I find them very useful as night as much fainter stars are visible than in a 10x50.
The 30x80 is a problem as the eye relief is so little I have to push the eye lenses almost touching my eyes.
In addition the magnification is too high for stability even pointed up in the sky.

The eye relief is dependent on the eyepiece.

One solution for varying eye relief is to have a mild zoom binocular. The eye relief typically changes with magnification.

For me, something like the 5x25 VisionKing has a 15 degree field. The field stops are easily in view and I could use wider field eyepieces.
By using my reading glasses I increase the magnification to 5.5x25 still with a 15 degree field, with the field stops just visible.

By using curved field eyepieces I get greater focus ranges.
In addition, by using the extreme bottom of the field I get even closer objects in focus, but this is by vignetting.
In daylight I just tried it and the view is still bright enough even when looking at much closer distances than the central focus position.

I am far sighted and if I have no glasses with me I make a tiny pinhole with my fist, and can easily read things and focus pretty well on near objects. Not perfect, but very functional.

Modern binoculars seem to cater for glasses wearers and consequently have smaller fields than I want.
Some are pretty good.
The Conquest HD 10x42 has a field of 6.65 degrees. I find the performance just as good as the Nikon 10x42 SE up to 6.0 degree field, and then I have the extra field thrown in.

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B.
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Old Wednesday 19th February 2020, 19:08   #140
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Omid and Tenex set up the question, and Henry hits it out of the park.
OK, I've actually had this question for a long time... Where does one learn this stuff? Seriously, here's Omid experimenting with novel ocular designs, and he didn't know. I studied optics in physics years ago but it didn't cover this. I have heard of "circles of confusion" in photography, but didn't see the connection here because a binocular doesn't form an image. Rays look very simple and neat in diagrams, but even now I have only the vaguest idea of the whole mess of them converging (sort of) from all the points (at different distances) in a scene. Where is practical, applied optics taught? Where do companies get their optical designers?
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Old Wednesday 19th February 2020, 19:18   #141
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Omid and Tenex set up the question, and Henry hits it out of the park.

Well done, and thank you.

-Bill
I hope Omid will be paying tuition fee$ to the member$ here !




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Old Wednesday 19th February 2020, 19:38   #142
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....but life is cruel so gradually you will lose accommodation as you eye's lens stiffens and by the time you're 30 or 40 you will definitely want a focusing mechanism on your binoculars.
Perhaps binoculars should come in different models, just like pet food for different aged pets. I think I qualify for the senior citizen model....

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Old Thursday 20th February 2020, 00:54   #143
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The reason DOF appears shallow in binoculars and scopes is because ...
Hi Henry,

Thank you for your explanation. It is an interesting perspective and might be the correct reason why binoculars provide a shallow depth of field. However, we have not fully solved the mystery yet. What I like to understand is that why a healthy human eye can not focus on individual objects located at various distances while looking through binoculars? I am not saying why those objects don't all appear sharp. I am asking why it is not possible to look at individual image points and see them in sharp focus?

To use a camera analogy, the COTAX AX camera with a 50mm f/1.4 lens mentioned above can focus on any point in a scene (say from 1m to infinity). If the aperture is at f/1.4, other points slightly before or after the object on which the camera has focused will appear blurry. But it is possible to focus on any desired distance.

Why can't human eye -when positioned behind an eyepiece- do the same? Why can't we focus on various objects in front of our binoculars and see a shallow-depth-of-field view of only the object we are concentrating on?

-Omid

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Old Thursday 20th February 2020, 03:08   #144
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Old 20x50 Porros can be a problem, but not much for me. I find them very useful as night as much fainter stars are visible than in a 10x50.

B.
Since we are dealing with a "point source", magnification has no effect on the size of the image formed on the retina.

Post #77 enplanes why a faint star could be more visible in a 20X50 binoculars (exit light beam diameter 2.5mm) compared to 10X50 (exit light beam diameter 5mm). Same amount of input light is concentrated in a narrower pencil of light when using 20X50 leading to increased perceived brightness.
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Old Thursday 20th February 2020, 08:34   #145
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Hi Henry,

Thank you for your explanation. It is an interesting perspective and might be the correct reason why binoculars provide a shallow depth of field. However, we have not fully solved the mystery yet. What I like to understand is that why a healthy human eye can not focus on individual objects located at various distances while looking through binoculars? I am not saying why those objects don't all appear sharp. I am asking why it is not possible to look at individual image points and see them in sharp focus?

To use a camera analogy, the COTAX AX camera with a 50mm f/1.4 lens mentioned above can focus on any point in a scene (say from 1m to infinity). If the aperture is at f/1.4, other points slightly before or after the object on which the camera has focused will appear blurry. But it is possible to focus on any desired distance.

Why can't human eye -when positioned behind an eyepiece- do the same? Why can't we focus on various objects in front of our binoculars and see a shallow-depth-of-field view of only the object we are concentrating on?

-Omid
Doesn't Henry's description already explain this? My understanding is the binocular presents to the eye a single distance at a time with its associated depth of field and the eye can only accomodate and examine this image. To examine other objects at different distances the binocular has to have its point of focus adjusted so that that eye is presented with another selection or slice of the view which it can then work with.

The example of the Contax with its 50mm lens only works because its magnification isn't high enough to be comparable to 8x or 10x binos.

I am happy to be proved wrong but at the moment I feel that the culprit is the magnification and this is what Henry described.

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Old Thursday 20th February 2020, 13:03   #146
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Yes, Henry did explain this, and very well too. His "sloppy, ball park only" tests are also highly illuminating.

In Omid's example of the Contax camera, focusing the lens does what the human eye pupil will do if you have young eyes and no extra magnification in front of it.
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Old Thursday 20th February 2020, 14:35   #147
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I also think Henry got it right. While the source of light from the binocular going to the eye can be Omid's point source, no matter what the point source is, 6x, 8x, 10x, it is still a point source many times more powerful than the 1x of our DNA modelled optical system that is dealing with the magnified point source. As Henry pointed out, we are left with our natural accommodation. When the limits of that accommodation are reached, focus on the binocular seems to become necessary. That seems further obvious that the closer the magnification gets to 1 x, the increasing depth of focus makes focusing less necessary.
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Old Thursday 20th February 2020, 15:20   #148
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Sub millimetre pupils and focus.

This morning in dull conditions I tried to find out the pinhole size that I use with an almost closed fist to bring distant objects into focus.

I used an accurate 1.0mm pinhole in stainless steel as my test pinhole. This was the smallest I had nearby although I may have smaller ones.

I estimate that a 0.7mm pinhole is what I was using with my fist.
This brought chimney pots at 120m and 124m into quite good focus.
Comparable to that using simple 2.25 dioptre glasses. I am far sighted and things are blurred with my normal vision without glasses.
These 2.25 dioptre glasses don't give the fine resolution I get from my prescription glasses, but the image is pretty good.

It is raining today, so I don't have bright sunshine.
But in bright light I think I use 0.5mm or 0.4mm pinholes for best images.

People here might complain about diffraction, too dull images etc. But this is what I actually use.

In addition, in bright light the best magnification for detail in my 90mm Maksutov is 125x or 0.72mm (0.7mm) exit pupil.

There are other cameras besides the Contax that use a movable film plane to focus.

As to how optical designers learn.
They used to go to the optical imaging department of Universities, but now they usually learn on the job. They have Mathematics degrees as a basis.

In the past people like Gauss, Fraunhofer, Newton, Schmidt, Baker, Vaisala or Dall just worked it out themselves.

Quite off topic, the new large liquid mirror telescope in northern India situated near the Himalayas at 8,500 ft uses thin Mylar just above the surface to stop air currents disturbing the rotating Mercury. So it seems this does not impact the image.

The proposed 20m to 100m Liquid mirror telescope at the Moon's north pole cannot used Mercury, as it would immediately evaporate there. Other liquids are contemplated.

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Old Thursday 20th February 2020, 17:16   #149
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What I like to understand is that why a healthy human eye can not focus on individual objects located at various distances while looking through binoculars? I am not saying why those objects don't all appear sharp. I am asking why it is not possible to look at individual image points and see them in sharp focus?
Yes, Henry's post answered this question. The eye has only a limited range of accommodation, which is more than adequate for the entire range of distance with unaided vision (which after all it evolved for!) but cannot handle the proportionally greater amount of defocus that comes from typical magnifications of 8x or more. And this only gets worse with age.

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Old Friday 21st February 2020, 16:33   #150
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Yes, Henry's post answered this question. The eye has only a limited range of accommodation, which is more than adequate for the entire range of distance with unaided vision (which after all it evolved for!) but cannot handle the proportionally greater amount of defocus that comes from typical magnifications of 8x or more. And this only gets worse with age.
I am not convinced by the arguments presented so far. A binocular is an "afocal" teleconverter attached to the eye. If a healthy eye can focus from 20cm to infinity; the same eye with a 10X binocular in front of it should be able to focus from 2m to infinity. No focusing inside the binoculars should be necessary. The focused image may have a shallow depth of field due the combined f-number of the entire optical system but that's not my concern. The very ability to focus (or lack thereof) is what I am trying to understand.

In any case, let's not spend any more time on this question for now. If I find an explanation, I'll post back later on. It is Friday and nice weather here in westside of Los Angeles. Life in this part of LA is all about the "image" you project and the shallower your "depth", the better:

https://youtu.be/-q-4XKTHJGs


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