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Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Zone?

coati

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Visit this article by Bruce L. Harding to learn more:

Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Zone?

Garry Holloway was the first to suggest a meaningful explanation for this: he suggested that, because there is a small difference between pavilion main & half facet slopes (less than 2º in a typical round brilliant), this may cause one to be dark when the other is bright – producing contrast in the gem’s image. Studies of human optical response say that this is attractive to viewers; it may be why the people Tolkowsky polled chose the proportions they did.

Thanks to Bruce L. Harding for his contribution to the PS Journal.
 

ariel144

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Can you please explain the question? ... "best" cuts on the edge of the table?
"bezel zone"?

A new language to me...Sorry, over my head since I don't study all the angles etc on MRB's. thanks
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

ariel144|1390353706|3598098 said:
Can you please explain the question? ... "best" cuts on the edge of the table?
"bezel zone"?

A new language to me...Sorry, over my head since I don't study all the angles etc on MRB's. thanks
Hi Ariel,
Bruce wrote this article in 1975 http://www.gemology.ru/cut/english/faceting/ that was published in the GIA journal Gems & Gemology.
Unfortunately it passed by the heads of the industry unnoticed (that was a very clever pun).

The basic idea is that the observers head cause obstruction of light sources that is always there (sound of one hand clapping stuff). Based on that and simple rules developed from where light can come from and where it can go to (physics and different gem materials) Bruce developed rules and sweet spot proportions for each gem material. The debate we had in about 2004 was that some head obstruction is good because it causes contrast - and that play of dark / bright has a positive effect in our heads.

Bruce is currently one of many people reading a just released 40 page article published by the Australian Gemmologist written by Sergey Sivovolenko (AKA Serg), Yuri Shelementiev (AKA Yuri), Garry Holloway (me), Janak Mistry, Roman Serov, Stepan Zhulin and Kristina Zipa.
We hope to receive permission from the Aust Gem editor to publish both a summary article (which I have nearly completed) and the full journal article here online on Pricescope.
The title is:
How diamond performance attributes: Brilliance, Scintillation and Fire depend on human vision features.

Bruce Harding's AKA is Beryl https://www.pricescope.com/users/beryl and you can find posts from Bruce and comments on other topics. You will notice that he is succinct; he has a beautiful mind.

Keep the questions coming.
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Mains not driving light return in a modern RB.
https://www.pricescope.com/journal/do_pavilion_mains_drive_light_return_modern_round_brilliant

Article about contrast patterns and symmetry.
https://www.pricescope.com/journal/hearts_arrows_diamonds_its_not_all_about_light_return

Of you want to read about how contrast patterns are formed see here:
https://www.pricescope.com/journal/virtual-facets-and-patterns-discussion-about-step-cuts
They are formed the same way for all cuts.
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

The over emphasis on head shadow has lead to the badly broken contrast metric in the AGS grading system.
ASET and IS before it do not show a real world representation of contrast a diamond will ever see.
I am looking forward to reading the full article to see if this flawed thinking was addressed.
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

The original tolk was an oec design with short lgf%
Somewhere around 65 degrees,
http://www.folds.net/diamond_design/index.html#brilliant
The comparison to a 80% lgf% stones is an issue.

Also that being the case the mains did drive light return in a Tolkowsky to a much greater degree than they do in a modern cut as my article points out in comparison to Morse/Wade.
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Here is an interesting question: if Tolkowsky/Morse/Wade were working with modern lighting would they have come up with the same numbers?
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl_K|1390446519|3599163 said:
The over emphasis on head shadow has lead to the badly broken contrast metric in the AGS grading system.
ASET and IS before it do not show a real world representation of contrast a diamond will ever see.
I am looking forward to reading the full article to see if this flawed thinking was addressed.

Hi Karl,
That is what I meant that Bruce's work was ignored any many aspects.
I hope we do address that issue to your satisfaction - two extracts:
Harding (1975) discussed that a real observer has two eyes and that the minimal obscuration angle is less than for a cyclops viewer with the same head size. The non-symmetrical obscuration of light by an average observer’s head size in relation to a diamond from a viewing distance of 30 cm is approximately 8 degrees for each eye (Harding mentions 10 degrees) in one direction, and 20 degrees in another (see Fig. 3.11) which has a significantly different effect on the appearance and brilliancy of a diamond when compared to a cyclops modelled obscuration angle of 14 degrees in each direction (see Fig. 3.12).
And
A practical example of the difference in cyclops and stereo observer's views can be illustrated by the ASET™ device which predicts obscuration which will be represented as blue. However the non-symmetrical eye position of a real observer reduces the obscuration angle by a factor of about two times, i.e. total head obscuration is around 28 degrees (a cyclops view has about 14 degrees from each side of the imaginary head) whereas a human has around 8 degrees obscuration from one eye and 20 degrees from the other. Thus many facets in a diamond considered unable to return light from surrounding sources by the ASET™ device may actually capture light from sources and return it to one or the other observer’s eye.

Another example illustrating the importance of human stereo vision is areas that the eye sees as dark because of light leakage through the diamond pavilion. Tools for viewing leakage using structured illumination, such as the Firescope™, Ideal-Scope™ and ASET™ assess leakage from a single monoscopic view in the face-up position. However often when observing the same area through two eyes, one eye may see the dark leakage zone, but the other eye may see a bright area and the brain will perceive this area as bright.
 

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl_K|1390447060|3599172 said:
The original tolk was an oec design with short lgf%
Somewhere around 65 degrees,
http://www.folds.net/diamond_design/index.html#brilliant
The comparison to a 80% lgf% stones is an issue.

Also that being the case the mains did drive light return in a Tolkowsky to a much greater degree than they do in a modern cut as my article points out in comparison to Morse/Wade.
I figured I would explain some more.
When you change the lgf% from 65% to 75% to 80% and leave the other angles the same you change the very nature of the diamond performance and which facets are doing the heavy hitting.
At 65% the mains have the greatest influence on brightness but by 75% the lower halves have taken over and at 80% even more so.
The angle of the lower halves change also. Which changes where they are looking.
The virtual facet pattern and number of virtual facets of the diamond changes dramatically from 65% to 75%, which accounts for the radical change in the look of the diamond. The step out to 80% is a much smaller change in the number and pattern of virtual facets.
The change from 65 to 75 is larger because of the change in drivers from the mains to the halves as they extend out under the table.
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl_K|1390447060|3599172 said:
The original tolk was an oec design with short lgf%
Somewhere around 65 degrees,
http://www.folds.net/diamond_design/index.html#brilliant
The comparison to a 80% lgf% stones is an issue.

Also that being the case the mains did drive light return in a Tolkowsky to a much greater degree than they do in a modern cut as my article points out in comparison to Morse/Wade.

One of the primary discoveries in the Aust Gem article Karl is that a main cause of the perception of brilliance is binocular rivalry - this happens when one eye sees a darkness and the other sees a bright area on exactly the same (virtual) facet.
This rivalry in our brain is a prime cause of the perception of brilliance and diamond may be one of the best devices ever to produce it.
I can not prove it, but I think the main facets seen inside the table may well be better at providing the chance for this to happen.
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Garry H (Cut Nut)|1390451566|3599230 said:
One of the primary discoveries in the Aust Gem article Karl is that a main cause of the perception of brilliance is binocular rivalry - this happens when one eye sees a darkness and the other sees a bright area on exactly the same (virtual) facet.
This rivalry in our brain is a prime cause of the perception of brilliance and diamond may be one of the best devices ever to produce it.
That is an interesting concept but I am not sure I buy it at this point. Will have to see what I think after reading the article.
 

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Garry H (Cut Nut)|1390451566|3599230 said:
I can not prove it, but I think the main facets seen inside the table may well be better at providing the chance for this to happen.
The steeper angle of the halves under the table would be more directional in a modern RB on the other hand the mains form a larger virtual facet.
This if it happens is going to be very distance dependent and they may swap depending on distance.
Mains more likely at some distances halves at others.
That's my thoughts.
But like I said I'm not sure I buy the concept yet.
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl_K|1390452383|3599239 said:
Garry H (Cut Nut)|1390451566|3599230 said:
I can not prove it, but I think the main facets seen inside the table may well be better at providing the chance for this to happen.
The steeper angle of the halves under the table would be more directional in a modern RB on the other hand the mains form a larger virtual facet.
This if it happens is going to be very distance dependent and they may swap depending on distance.
Mains more likely at some distances halves at others.
That's my thoughts.
But like I said I'm not sure I buy the concept yet.
I am with you on what you wrote Karl.
The article is not prescriptive. We make an analogy that diamond cut knowledge is about at the point where chronometers were developed for mapping and navigation purposes. To create effective metrics (one aim) one must understand what the brain does with the two feeds coming from each eye.
There is a myriad of optical illusions where no one is even close to a mathematical model that explains what we "see".
Scientists struggle to explain WHAT, which is required usually before we can quantify a visual experience.

Here is one of the simpler illusions which is comparatively well understood (after more than 100 years of study).

I think this illusion helps explain something of the contrast effects that Bruce is discussing and why slightly shallower diamonds than his 1975 work predicted are actually more attractive.

_13975.jpg
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl_K|1390446161|3599157 said:
Article about contrast patterns and symmetry.
https://www.pricescope.com/journal/hearts_arrows_diamonds_its_not_all_about_light_return

Quote: Some people will say that it doesn't matter because people aren't cyclops with one eye in the center of their forehead.
I disagree I can see the arrow pattern under a variety of lighting and have read testimonials from several others who can also.
Those that buy the more chaotic looking diamonds may scoff at it as being not needed but it does have an appeal to many people.
If nothing else it brings another look to the diamond under common conditions that some get enjoyment from.
GH comment - if one eye sees the top star returning a light source, and the other sees a dark place, e.g. ones own head, then you may see that star as being brighter. The top star usually seems brighter.
The stars near the lower portion are usually darker and more persistent (because they are often getting (or not) light from our body. The arrows on the sides tend to both come and go with the slightest rocking. But most obviously, if you close one eye the opposite side of the stone has half the stars and vice a versa
.
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Garry,
After some observations I am scepticle.
I had to work really hard to get into a position and lighting condition where a main was bright to one eye and dark to another and had to use a large cz to do so. I could not do it with my wifeys .42.
I don't think that condition exists enough to be a huge factor.
To me it looked brighter when the main was returning light to both eyes.
That said my eyesight is far from normal and that could be a factor.
 

ariel144

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Ok, thanks Garry and Karl...you all are so techie but I do comprehend head obstruction and maybe should somehow be addressed when grading diamond performance (that is if I'm comprehending the issue here)...thanks
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

ariel144|1390595977|3600527 said:
Ok, thanks Garry and Karl...you all are so techie but I do comprehend head obstruction and maybe should somehow be addressed when grading diamond performance (that is if I'm comprehending the issue here)...thanks
What we are debating is how, when and how much obstruction(head shadow is a part of obstruction but not the only source) be factored into diamond performance as it relates to human vision.
My problem is my eyesight is very far from normal so it makes it hard for me to confirm it for myself.
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Ariel144:
. I apologize for not explaining 'table-table' and 'table-bezel' zones here; the readers knew them then. I hope this clarifies.
. There are three types of ray path into and out of the crown of a faceted gem: 1) those which enter the table and exit the table. 2) those which enter the table and exit the bezel (or vice-versa), and 3) those which enter the bezel and leave the bezel (this type never occurs in a diamond of typical proportions).
. 'Faceting Limits' (G&G, Fall 1975) showed that when the angle between entry and exit paths of a ray are less than 10 degrees the viewer's head blocks entry rays which should reach the viewer's eye (see illustrated head & 10-degree ray on charts in this article). The dark upper zone of the chart is for table-table rays which diverge less than 10 degrees. The dark lower zone is for table-bezel rays which do likewise. So if the crown/pavilion slope combination is within these dark zones, the viewer can see no light. Gary guessed that the Tolkowsky diamond is on the edge of a zone because a facet changes from light to dark with very little motion of stone, light, or viewer.
. Note that I show the ray coming to the eye from the SAME side of the head; this is the best condition. The angular obstruction is much greater from the OTHER side of the head, but if one eye sees light, the brain sees light. This is discussed in the new GAA article.
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

[quote="Bruce wrote this article in 1975 ... that was published in the GIA journal Gems & Gemology.
Unfortunately it passed by the heads of the industry unnoticed ..."

This needs correction.
. GIA had an 'advisory board' of experts, mostly professors, who reviewed the manuscript and had objections. Richard Liddicoat, then President of GIA, passed their comments on to me*. I countered each objection and advised Mr. Liddicoat that he was 'in the middle' and had to decide. He published it, for which I am grateful and so should the industry now be - what was 'heresy' is now 'gospel'. I think Mr. Liddicoat got in a lot of trouble for this; our frequent correspondence stopped and he rejected my article 'Tolkowsky Revisited' (now at Russian website) because it might upset faith in the GIA cut-grading system (which was later discontinued). The promised sequels were never published. For some odd reason, GIA rarely references this article and their early computer studies ignored it (however, it does not affect their final conclusions).
* One professor said that the viewer's head had no place in a scientific study of diamond optics. Now we agree that it (and the viewer's body) cause the contrast we see.
. Credit also goes to Michael Cowing, who brought it to the attention of a 'Diamond Talk' forum in late 2000, where Gary and others were discussing the crown/pavilion slope relationship. Anton Vasiliev had one of few copies of this article in Russia; he was on that early team at MSU in late 1900's.
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Here is an item I sent to a selected few 2 years ago; it will give you insight to the stereo part of new GAA article Gary mentions here.
Original text and illustrations were presented in 'Diamond Talk' late 2003. I think Gary left 'DT' about that time.
Oops - sorry - I don't know how to copy it here.
 

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

beryl|1390710642|3601328 said:
[quote="Bruce wrote this article in 1975 ... that was published in the GIA journal Gems & Gemology.
Unfortunately it passed by the heads of the industry unnoticed ..."

This needs correction.
. GIA had an 'advisory board' of experts, mostly professors, who reviewed the manuscript and had objections. Richard Liddicoat, then President of GIA, passed their comments on to me*. I countered each objection and advised Mr. Liddicoat that he was 'in the middle' and had to decide. He published it, for which I am grateful and so should the industry now be - what was 'heresy' is now 'gospel'. I think Mr. Liddicoat got in a lot of trouble for this; our frequent correspondence stopped and he rejected my article 'Tolkowsky Revisited' (now at Russian website) because it might upset faith in the GIA cut-grading system (which was later discontinued). The promised sequels were never published. For some odd reason, GIA rarely references this article and their early computer studies ignored it (however, it does not affect their final conclusions).
* One professor said that the viewer's head had no place in a scientific study of diamond optics. Now we agree that it (and the viewer's body) cause the contrast we see.
. Credit also goes to Michael Cowing, who brought it to the attention of a 'Diamond Talk' forum in late 2000, where Gary and others were discussing the crown/pavilion slope relationship. Anton Vasiliev had one of few copies of this article in Russia; he was on that early team at MSU in late 1900's.


re:for which I am grateful and so should the industry now be - what was 'heresy' is now 'gospel'.

Not , yet. It is 'heresy' for many( may be for most in same Labs) 'advisory board' of experts until now. :(
 

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Serg|1390718890|3601364 said:
re:for which I am grateful and so should the industry now be - what was 'heresy' is now 'gospel'.

Not , yet. It is 'heresy' for many( may be for most in same Labs) 'advisory board' of experts until now. :(
I don't feel its heresy nor gospel but I do not feel the proper model for accounting for obstruction has been found.
It is certainly broken in the reflector technology based grading systems and is a serious issue in observation studies.
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl:
. Please explain your latest comment re 'reflector technology based grading systems', etc. I have been out of the loop for a long time and do not know the latest activity.
. Perhaps 'issues in observation studies' are due to stereo effects, whereby obstruction may occur to one eye and not the other at the same time, This is explained in the new article by Serg, Gary, etal in GAA Journal and the pic I tried to send,
. Most studies ignore the viewer's body, which is an obstruction to rays from the side or below that are sometimes sent to the viewer's eye. Vasiliev's software 'Facet Designer' includes this option; perhaps it has been added to Octonus 'DiamCalc' as well. 'Facet Designer' is available free from me (including my tutorial) if you are interested.
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl (and anyone else):
. I can send you my thing on stereo by e-mail; it is a 'Word' document - only one page, including text and illustrations.
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

beryl|1390710642|3601328 said:
[quote="Bruce wrote this article in 1975 ... that was published in the GIA journal Gems & Gemology.
Unfortunately it passed by the heads of the industry unnoticed ..."

This needs correction.
. GIA had an 'advisory board' of experts, mostly professors, who reviewed the manuscript and had objections. Richard Liddicoat, then President of GIA, passed their comments on to me*. I countered each objection and advised Mr. Liddicoat that he was 'in the middle' and had to decide. He published it, for which I am grateful and so should the industry now be - what was 'heresy' is now 'gospel'.

A further clarification for newbie readers. R. Liddicoat was one of the founding father's of GIA. He developed the D-Z grading system and many other parts of it.
http://en.wikipedia.org/wiki/Richard_T._Liddicoat
 

Garry H (Cut Nut)

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

beryl|1390770078|3601619 said:
Karl (and anyone else):
. I can send you my thing on stereo by e-mail; it is a 'Word' document - only one page, including text and illustrations.
Hi Bruce,
email addresses may not be published here.
Send to me (I probably have it already?) and I will send it on to anyone who wants it - or we publish it here also :)
 

Karl_K

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

beryl|1390766958|3601590 said:
Karl:
. Please explain your latest comment re 'reflector technology based grading systems', etc. I have been out of the loop for a long time and do not know the latest activity.
. Perhaps 'issues in observation studies' are due to stereo effects, whereby obstruction may occur to one eye and not the other at the same time, This is explained in the new article by Serg, Gary, etal in GAA Journal and the pic I tried to send,
. Most studies ignore the viewer's body, which is an obstruction to rays from the side or below that are sometimes sent to the viewer's eye. Vasiliev's software 'Facet Designer' includes this option; perhaps it has been added to Octonus 'DiamCalc' as well. 'Facet Designer' is available free from me (including my tutorial) if you are interested.
I am interested my email is in the link in my signature.

Reflector based systems are ASET/AGS and a lesser extent IS because IS is billed as a rejection system rather than a grading system. But it gets used as a grading system on PS all the time so to be fair I have to include it.

Observation studies is mainly GIA, and the biggest problem with obstruction in their study is lack of real world conditions and distances.
Unless people are looking at the diamond like they would in the real world it will not show accurate results.

There is a problem with any system that says at this distance and looking at it this way.
How many people are going to look at a diamond at that exact difference and relationship to the eyes?
I am worried that cut research/grading is being herded down a path rather than letting the real world lead.
We have part of the picture but a huge part of it is missing and some of what we think we know may be wrong.

Which brings me to my above question:
"Here is an interesting question: if Tolkowsky/Morse/Wade were working with modern lighting would they have come up with the same numbers?"
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Serg:
. I am impressed by the extensive work by you and your team. You have done much since we met 10 years ago!
. At that time I was impressed by your discussion of angular size of light sources and pupil reception angles.
. I am pleased to see you use the term' slope' instead of 'angle' for facet inclination from the girdle plane; there are so many different angles to consider in a faceted gem that it is nice to have a specific term for this feature. A mathematician would use 'slope'.
. I can't understand that some still cannot accept the viewer's obstruction of light as a cause of some dark areas. It is obvious.
 

beryl

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl:
. I think you will appreciate GAA article more than most readers; it addresses some of the issues you bring up. Get it.
. My previous computer is temporarily dead and much memory is in it only. I don't know anyone I trust to fix it; now using laptop previously used only for work. I hope to find a copy of 'Facet Designer' in my flash drives. I understand that Sergey did something similar more recently but am not familiar with it. I wrote the tutorial to Anton's work in 2004 and so I know it well.
. Not only do mono-vision devices, and experts with loupes, see gems differently than consumers, but different consumers see them differently. For example the man with a bald head sees it differently than his lady with a big hairdo or broad-brimmed hat. And it's all changed when the diamond gets dirty shortly after putting it on! Then there is the difference in lighting, as you mention.

Garry:
. You already have the things I described. Send Karl my e-mail address = less confusion. Sorry I misspelled your name lately.
 

diagem

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Re: Why are the ‘Best’ Cuts on the Edge of the Table-Bezel Z

Karl_K|1390531445|3599960 said:
Garry,
After some observations I am scepticle.
I had to work really hard to get into a position and lighting condition where a main was bright to one eye and dark to another and had to use a large cz to do so. I could not do it with my wifeys .42.
I don't think that condition exists enough to be a huge factor.
To me it looked brighter when the main was returning light to both eyes.
That said my eyesight is far from normal and that could be a factor.
Not skeptical and always ready to learn...
Probably not a huge factor but again its all relative. I can tell you one thing. At the highest precision..., every nuance becomes relevant.
I just ordered my copy, it sounds very interesting!

ETA..., nice to see you on the boards Bruce...:)
 
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