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How does lower girdle length affect face up performance?

Discussion in 'RockyTalky' started by agc, Jun 8, 2008.

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  1. agc
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    by agc » Jun 8, 2008
    I know there has been several threads in the past but there seems to be some contradictory info in them. In a well cut RB diamond how does a lgf of 80+ affect the face up performance? Some have said it only increases brilliance in direct/spot lighting and suffers in low/diffuse lighting while others suggest it is more brilliant in diffuse lighting and more pin fire flash in direct lighting. I understand that as the lower girdles get longer the pavilions get narrower thus the arrows narrow in the face up position and contrast changes. I realize the hearts with develop clefts but how does all this affect what we see in the face up postion with different lighting conditions? While I''m at it I would love to hear what affect longer stars(say around 58%) have on fire/brilliance etc. As always everyones help is and has been greatly appreciated.

    Andy
     
    


    


  2. Rhino
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    by Rhino » Jun 8, 2008
    Longer LGF''s suffer in diffuse lighting? Not by a long shot. Equally as beautiful as shorter. Yes it does affect the shape of the Hearts pattern but is in no way inferior. Rounds with longer LGF''s can be cut equally if not more precise than shorter LGF''s and their optics are not compromised one iota in any lighting environment. Just a different flavored Hearts pattern. Which a person deems better is a matter of personal preference and some enjoy both appearances equally. Like me. [​IMG] I have one here with 65% LGF% too ... you should see what that looks like! [​IMG]
     
  3. oldminer
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    by oldminer » Jun 8, 2008
    Isolating this one facet length and adjusting it in various ways does alter how light is handled. In moderation, slight changes only cause slight overall effects. In gross changes, the effects can be quite visible. Most cutters use good judgment in not going to extremes on these facets since they want beauty as the outcome.

    There are so many other variables at work in the facet sizes and angles of a total diamond, that the question of how long a certain facet will change the looks is somewhat the same as asking if a longer or shorter big toe on a champion runner will alter their performance capability. We can imagine that such a measure of toe length might have some effect, especially at extremes, but condition, muscle type, and natural abillity must play a far more important role in the overall performance of successful athletes.

    Your question shows you are hungry for the knowledge and are attempting to digest a great deal of the information at play here. Nothing wong with that effort. It will serve you well. There has been much discussion of very fine points which are nearly always overcome by buying diamonds cut to near top specs.
     
  4. John Pollard
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    by John Pollard » Jun 8, 2008
    Like many proportions, the lower half % isn’t a stand-alone number. X % will look different in a small-tabled stone (50%) than it will in larger tables (>60%). The size of the diamond itself also matters.

    In top near-Tolkowsky makes there are trade-offs in character at the low versus high ends but these are not necessarily trade-offs in performance. It’s like equally lovely Chardonnays; one end has more oak, the other has more butter. People may prefer one over the other, love them equally or not even notice a difference. The most common range for EX diamonds coming out of GIA is 73-82%. In my experience finely-cut diamonds in that range, or slightly above and below, can be absolutely spectacular (so I wouldn’t say ‘suffer’). What you will like is a matter of taste.

    In general, there are more medium-to-large scintillation events as lower halves get shorter and more medium-to-small events as they get longer.

    You can estimate lower half % by viewing the ‘arrows’ in these simulations. Those are the pavilion mains. The images below are near-Tolkowsky makes, modeled at 56/40.8/34.7. They assume perfect optical symmetry (great cut precision enhances the impact of lower halves).

    1-LowerHalves65-90JS.jpg
     
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  5. John Pollard
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    by John Pollard » Jun 8, 2008
    It wasn’t until 2006 that GIA graded cut and gave a range for EX - see above. Their central number for lower halves is 77.5%. AGS uses 78.5% length on their cut guides (for my fellow nerds, height is cited on the guides).

    Diamond cutters are pretty smart people. In real-world terms a difference of several percent makes little practical difference so it’s no surprise that top superideal brands targeted at-and-around those central numbers for many years before the labs reinforced them.

    You can see the size of the pavilion mains well in magnified reflector photos like IS and ASET.

    2-LowerHalves65-90IS-ASET.jpg
     
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  6. John Pollard
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    by John Pollard » Jun 8, 2008
    Turning the diamond over, here is what the pavilion image looks like (called 'hearts' in some cases) - assuming perfect optical symmetry.

    3-LowerHalves65-90Hearts.jpg
     
  7. John Pollard
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    by John Pollard » Jun 8, 2008
    While these screen shots don’t give the depth, detail and contrast of real life they can illustrate how lower half length impacts the character of scintillation.

    Tilt a diamond and the arrows and lower halves swap brightness…tilt it more & they swap again…and so forth and so on. Do it rapidly and you have scintillation.

    With shorter lower halves the large flashes are created by the wider pavilion mains (called arrows by some) flashing. Narrower mains create smaller flashes. The size of the virtual facets is also dependent on this to a degree. Diamonds in the optimal range have the same visible performance impact, depending on lighting, it's just distributed differently.

    4-LowerHalvesThroughTilt.jpg
     
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  8. agc
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    by agc » Jun 8, 2008
    Rhino and David I appreciate the info. It has been suggested in several threads that middle of the row lgf lengths were the best all around performer. I have seen numerous H&A diamonds(most with average/middle lgf length) but just a couple with long lgf''s. The longs looked stunning (not that the others weren''t beautiful) and the only thing I could see that was different than the others was the fact that the stars and lgf''s were longer. All had complementary CA/PA combos, superb optical symetry, AGS 0 or GIA EX with good idealscope/aset etc and I am trying to make some sense out of it. Any thoughts? What factor does the longer stars (around 58) have?
     
  9. agc
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    by agc » Jun 8, 2008
    John, great info and graphics. I continue to learn and appreciate everyones time and expertise.
     
  10. John Pollard
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    by John Pollard » Jun 8, 2008
    I disagree when it comes to commercial markets Rhino.

    Diamonds with long lower halves lacking optical symmetry/cut consistency or at borderline angular combos can look nice in showroom spotlights but they go dark outside the store, especially around 85%+. There is more "wiggle room" to perform well nearer the center of the range when cut precision is sloppy or the angles aren't charry.

    I know you wouldn't sell such diamonds (nor would I promote them) but there are far more average-and-mediocre diamonds in the world than fine-make and in that context the highlighted seems off to me.
     
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  11. agc
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    by agc » Jun 8, 2008
    John, just noticed something on your reflector images. Starting at 80 there is a dark area forming at the bases of the arrows and it gets more pronounced as lgf increases. What is this?
     
  12. John Pollard
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    by John Pollard » Jun 8, 2008
    Andy, you''re welcome of course. You''re in a good place to learn and it sounds like you''re seeing some great diamonds.

    As you might have learned, it’s rare that you will ever hear much discussion of this subject in markets or stores away from PS. With that in mind you may agree that, like many things we discuss here, the “idea” of a few % difference can seem more dire in your mind than it will ever be once you have that firecracker on your finger.
     
  13. John Pollard
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    by John Pollard » Jun 8, 2008
    Oldminer was right...you are hungry. You realize the silent PS choppers and guys in black neoprene suits may come and get you tonight. [​IMG] It's how we recruit.

    You're seeing table reflection; the reflection of the table seen in the pavilion of the diamond. It is more obvious in diffuse lighting than in direct or filtered.

    More info and graphics on table reflection: https://www.pricescope.com/community/threads/cut-question-for-non-aca-diamond.86196/
     
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  14. Wink
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    by Wink » Jun 8, 2008
    Well, I was going to chime in that I think that when the lower girdles are towards the long end of the Exellent to Ideal range (GIA and AGS) that the flashes get smaller and often the diamond will show less dispersion, ESPECIALLY IN LOW LIGHT.

    That is because dispersion''s flashes must be broader than the pupil of the eye at the point at which that light reaches the eye. The longer lower girdle facets create smaller flashes (shorter) flashes to begin with so more of the dispersion events will not be perceived as dispersion, but as white light since when all of the light enters the eye at the same time it is perceived as white by our eyes. Only when the entire flash of dispersion is NOT received completely in the iris of the eye do we perceive those wonderful flashes of color that some of us love so intensely.

    Add to the already shorter width of the dispersive flashes in the longer lower girdle facets the widening of the iris in dim light and you begin to understand that if you are a dispersion fan you will probably want diamonds more to the middle or lower range of the lower girdle facet lengths if you wish to see the maximum dispersion in your diamond in lower light situations.

    Then I was going to add that this is completely a matter of taste and that provided both stones are well cut that they will both be beautiful, but with slightly different flavors.

    Then I saw the epic work provided by John Pollard and I decided not to expose the lack of time and effort that it would take me to respond what I was thinking when he has delved so deeply into the subject. It would just be too embarrassing...

    Wink
     
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  15. agc
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    by agc » Jun 8, 2008
    Well that clears that up and I won''t be up all night trying to figure it out.
     
    


    


  16. agc
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    by agc » Jun 8, 2008
    Hey Wink, now your going to have me up all night reading ophtalmology books. LOL
     
  17. John Pollard
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    by John Pollard » Jun 8, 2008
    Or just go by what your eyes have told you. It sounds like you''ve seen beauty spanning a comfortable range.

    Besides...you need to be listening for those choppers.
     
  18. strmrdr
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    by strmrdr » Jun 8, 2008
    While everything said so far is true.... it depends on the c/p combination and there are exceptions to everything said here.
    For example a high crown fic that makes GIA EX AGS0 will have greater dispersion with a 80% lgf in low light than a diamond on the other end of the range with 75% lgf%.
    The lgf% can not move the diamonds basic nature set by the c/p combo it can only modify it.
    There are lgf% mismatches where the modifiers don''t match the nature of the c/p combo and they become in conflict lessening the performance.
    In the midrange right around tolk the lgf% has far less effect than say in a shallow/steep or a steep/shallow as far as modifying the performance curve.
    In simple words some c/p combos are more sensitive to lgf% changes than others.

    The stars 50%-60% and in some combos out to 65% has little to no visible effect.
     
  19. strmrdr
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    by strmrdr » Jun 8, 2008
    In other words asking what does X lgf% does is like asking how fast does this blue car go?
    Without the rest of the information it either cant be answered or the answer may not apply.
     
  20. honey22
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    by honey22 » Jun 8, 2008
    Thanks for those pics John, they are very helpful! After reading a few threads on this topic I was still clueless about it, but now it makes sense! Some of us just need pretty pictures [​IMG]

    Maybe you could put these pics into the helpful threads section? I think it would be useful for others too. Thanks!
     
  21. agc
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    by agc » Jun 8, 2008
    Storm,

    I happened to be one of the odd folks that seems to like C/P combos like 34.2-34.4/40.9-41. What do you think of these combos with longer lgf''s?
     
  22. strmrdr
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    by strmrdr » Jun 8, 2008
    Keeping in mind my above post lets take a 34.5/40.8 with a 56% table combo and change the lgf% from 75-80 what is the effect:

    Direct light: slightly smaller flashes at 80, more white light is perceived at times at 80% and it may appear slightly brighter. 75% more fiery
    Diffused lighting with head shadow: the ratio of light and dark contrast is altered the 80% lgf% can appear brighter(smaller arrows)
    Candle light: the short lgf% have a slight advantage.
     
  23. strmrdr
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    by strmrdr » Jun 8, 2008
    personal preference here! personal preference here! personal preference here!
    love them :}
    I have a personal preference for them.
    I am sensitive to contrast in rounds and big arrows bug me.
    I like lots of white light return in rounds and the brighter the better.
    If I want a fireball give me a step cut like an asscher as no RB can do large flashes the way they can.
    The largest flashes will come from an oec in a round and I love them 2.
    So its kinda strange that my 2 favorite rounds are at the opposite ends of the performance spectrum and I''m not as thrilled with the ones in the middle.
    That does not mean the ones in the middle are bad diamonds and not beautiful because they can be they just don''t speak to me as well as these do.

    personal preference here! personal preference here! personal preference here!

    Sorry about the bold but I wanted to make that very clear so I don''t get yelled at for the next 2 days.
     
  24. agc
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    by agc » Jun 8, 2008
    Thanks Storm.
     
  25. lesco
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    by lesco » Jun 8, 2008
    Sorry for the slight threadjack agc but maybe my diamond could be a good example, for discussion purposes. I think my stone is one of those "borderline extreme" crown/pav angles and long lgf dudes we are discussing here:

    Stats:
    Table 57
    Crown % 34
    Pav % 41
    Star 50%
    lgf 80%

    Strmdr, I know you have been pretty busy lately, but If I can bother you for a couple minutes, Can you elaborate on my diamonds "personality"? If I read this whole thing right, I should expect less fire / more white light return and less fatty arrows?
     
  26. strmrdr
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    by strmrdr » Jun 8, 2008
    welcome
     
  27. strmrdr
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    by strmrdr » Jun 8, 2008
    basically yes depending on what you are comparing it too.
    but to clarify the less fire isn't the total amount of fire in optimal fire conditions but a difference in the range of lighting conditions it shows fire vs white light return.
     
  28. lesco
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    by lesco » Jun 8, 2008
    Thank you strmdr
     
  29. Wink
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    by Wink » Jun 8, 2008
    AHHHHH! Personal preference, a thing of beauty. Never will I yell at you for personal preference!

    I happen to enjoy a good OEC my own sweet self, LOVE LOVE LOVE those large flashes of color!

    Wink
     
  30. Rhino
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    by Rhino » Jun 8, 2008
    I agree John. I should have qualified my statement by prempting it with the fact that I really like longer lgf''s as much as others as long as the proportions combos fall in the Ex/Id zenith and with superior/premium optical symmetry.

    Of the average and mediocre types which dominate the market ... I don''t care what the lgf''s are. [​IMG] Great graphics too. We''re on the same page and good to cya round.

    All the best,
     
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