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 Post subject: 20x triplet
PostPosted: Thu Jan 17, 2008 4:19 pm 
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I was in Kassoy (NYC showroom) a couple days ago and bought a Kruss 20x triplet - not easy to use, but I wanted to have a higher power magnification for some situations when away from the microscope.

It is a 21mm triplet - but still has distortion around the edges (thinking it would allow more light - someone who uses the smaller diameter one recommended the larger for that reason) - is the distortion a function of the larger diameter?

In a very small depth of field and working area in the view free of distortion, I can see the surface features of three phase inclusions and other inclusions - which is what I was aiming for. Another thought is looking at all obliquely through the lens instead of exactly straight through, might be the source of the distortion - this is easy to do with the large diameter.


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 Post subject: Re: 20x triplet
PostPosted: Thu Jan 17, 2008 10:40 pm 
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Quote:
is the distortion a function of the larger diameter?


Yes, image distortion increases with radial distance away from the optic axis. But this is self-evident from the fact that you can view a small circular zone with "tolerable level of distortion " surrounded by a region of "larger distortion."

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Another thought is looking at all obliquely through the lens instead of exactly straight through, might be the source of the distortion


Oblique viewing definitely increases distortion, but that is not the source of the distortion.


Last edited by Brian on Thu Jan 17, 2008 11:17 pm, edited 1 time in total.

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 Post subject: Re: 20x triplet
PostPosted: Thu Jan 17, 2008 11:14 pm 
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Brian wrote:
I thought about writing a long explanation of the hows and whys of optical aberration, but I think the short answers suffice here.

No, wait, please go on.... I am so glad you chimed in here.

Would it have been better to have gotten the smaller diameter loupe or does the light factor in?

I didn't realize that there should be aberration in a triplet, but going back and examining lines with other 10x loupes B&L Hastings, GIA and Gem-A triplets - all small diameter, there is some distortion along the perimeter. I've never tried a loupe of this larger size so it took me by surprise and I started to wonder if it was a cheap knock-off in Kruss disguise.

I can't remember which part of a triplet corrects for color and then for spherical aberration - I am being lazy - but, is the correction only partial and so more pronounced in the larger diameter? I think you answered that, I just haven't digested it yet... more please.


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 Post subject: Re: 20x triplet
PostPosted: Thu Jan 17, 2008 11:30 pm 
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Brian wrote:
Yes, image distortion increases with radial distance away from the optic axis. But this is self-evident from the fact that you can view a small circular zone with "tolerable level of distortion " surrounded by a region of "larger distortion."

That is perfect (is that what you wrote the first time or after editing, b/c this makes sense - maybe I just didn't get it the first time).

I also find that with this loupe, I have to be careful using it and minimizing eye strain which seems to come on quicker than with a 10x (both eyes open).

Yesterday a master goldsmith, another collegue and I were all talking about this 20x subject, along with the advent of the need for drug store magnifying glasses... bemoaning the facts of aging eyes. The goldsmith wears one loupe in the eye, adds a 10x loupe in front occaisionally and then pulls down the drugstore glasses....it's hell to get old, but the alternative is not acceptable.


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PostPosted: Fri Jan 18, 2008 12:47 am 
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I think that 'circular region of "tolerable distortion" ' phrase stuck after the third or fourth edit.

I find looking through any magnifying optics causes major eyestrain. I hate looking through microscopes for any length of time. So I let cameras do the looking for me.

I don't even want to talk about age and vision, having just gotten my first bifocals less than a month ago. Of course, I demanded Zeiss lenses... can't let my work optics outclass my personal optics. On the other hand, my dad is quite happy with his drugstore reading glasses.

Now, let's see...

Distortion of the magnified image of some object is an aberration called lens distortion (of course, :roll: ) . The distortion occurs because parts of the object closer to the lens's optic axis are magnified less than parts that are further away. The image exhibits a sort of pincushion effect.

In a magnifying lens, lens distortion increases with distance from the optic axis of the lens. Increasing magnification tends to aggravate this aberration. Also, a lens can minimize lens distortion or any other aberration only in one view plane; trying to view the image from another plane tilted at some angle with respect to optimized plane increases aberration such as lens distortion.

There is an art to designing lens systems, but roughly speaking, reducing lens distortion requires an increasing number of lens elements. A singlet lens typically exhibits the worst lens distortion. The three elements of a triplet reduces lens distortion, but it doesn't eliminate it. To reduce lens distortion and produce a "flat field" of view, a microscope objective lens might use a dozen elements. So the triplet isn't anything close to the be-all, end-all lens design. But it is a nice, compact design.

You can use a a 2x singlet lens to magnify the letters in a newpaper. Lens distortion is tolerable, and you have to look at the edge of the lens to detect distortion. But no one uses a 10x singlet to study of gems. Increasing the singlet's magnification that much leads to lens distortion that overwhelms the benefits of magnification.

So you use a 10x triplet loupe for gem study. Lens distortion is tolerable across the field of view. You have to look closely at the edge to detect the distortion.

Now you increase triplet magnification to 20x. If a 20x loupe was as easy to use as a 10x, then why would anyone be using a 10x? Just as with the singlet, increasing magnification results in much more distortion.

You say you want to look at 50x magnification? Hmmm, better invest in a microscope.

As for moving from smaller diameter to larger diameter 20x loupe. I've been trying to figure out a reason why a larger loupe would be advantageous. Increasing aperture (light gathering capability) doesn't helps reduce lens distortion, although it does improve some aberrations that would produce a better-focused image. Even so, the increase in aperture is not very much.

... ok, um, I warned you it would be long.


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PostPosted: Fri Jan 18, 2008 12:21 pm 
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Brian wrote:
Now you increase triplet magnification to 20x. If a 20x loupe was as easy to use as a 10x, then why would anyone be using a 10x? Just as with the singlet, increasing magnification results in much more distortion.

You say you want to look at 50x magnification? Hmmm, better invest in a microscope.

Now you are famous... have printed it out and distributed it to all members of the loupe discussion panel.... maybe will include my dentist who was recently showing off his $2,000 duel eye loupes attached to his glasses-- he looked like Wile E. Coyote with one of his ACME purchaces (never try to talk optics with your dentist while he's trying to fit a crown).

I wouldn't go higher than 20x with a loupe and, as I said, only for certain situations where I was away from my scope and wanted to get at the nature of an inclusion - and then somewhere I read that John K. finds all his great inclusion specimens at Tuscon using his 10x loupe......would that I could be so skilled someday. But, I do think it is amazing to be able to see the surface features of a crystal inside a multi-phase inclusion before my eye starts to hurt.

All this was setoff as I was putting together a field pack by cannibalizing several other combinations of instruments - I have it all in a nifty padded 6"x8" zippered bag that I picked up in Bangkok which has all kinds of zippered compartments- not that I would take all of this into all situations but it is easily broken down into smaller portable units:

Obivious redundancy (I really only need my megaloupe and a spectroscope), but all in one place and if I go off to work with someone, teaching, consulting,etc, I have everything I need in one place and can leave out some things once I get to where I am going. I have one of Jeff Wildman's portapacs similar to this one but stripped down to just the cheaper kit comprised of a refractometer, polariscope and dichroscope (It is one of the nicest refractometers) - the case fits nicely in a slightly larger padded shoulder bag into which the small bag above fits also-- all about 10x12" - sort of like a miniature maxilab.... now if I could only shrink my microscope.....anyone ever see the Fantastic Voyage movie?


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PostPosted: Fri Jan 18, 2008 6:01 pm 
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http://www.amateurgeologist.com/categor ... agnifiers/

These folks carry the Belomo 20x quadrulet which I think I will have to buy. I had a Belomo 10x Triplet but it has vanished. I really liked it. It had characteristics of both the Hastings triplet and the Coddington. very clever and nicely made. At the time it was like $15. They are a bargain.
Zeiss has just purchased an interest in Belomo but they make much more than loupes. For example spy satellite cameras.


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PostPosted: Fri Jan 18, 2008 6:38 pm 
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I have the Belomo 10x and 20x... actually, I think Barbra does too. I am very satisfied with the 10x and the 20x is also good... although not quite as good as the 10x. Of course, you also get a closer-up view... so it's a trade-off. :)


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PostPosted: Fri Jan 18, 2008 6:49 pm 
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Gene,
If you figure out a way to order the 20X Belomo from your URL please let me know. They're out of stock and I can't find any way to put one on back order or contact them to reserve one.

Meanwhile, another Belomo reseller says the factory in Belarus has just raised prices about 45% so they're not planning to carry them again. I bought several 10X triplets -- two at the 'really old' price and a couple more at the 'new old price.' All were bargains. I will now guard them with my life!

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PostPosted: Fri Jan 18, 2008 11:49 pm 
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I got mine from a pal who ordered it from that website I think , several years ago. But I haven't been able to put my hands on it for a while.
I thought I saw something that said they were on the way to be available soon. But if that info about prices is good then it might be time to grab a couple of them.

I actually don't care much for loupes. I like having stereoscopic vision.
I think the Belomo is as good as any I have ever seen.

The post linked to the Gem Associations four page .pdf It appears they are going the way of GIA in not carrying any but the largest movers of gem instruments. I can remember (not all that long ago I THINK) when it was fun to look at the GIA catalog and leer at all the high priced instruments. (I know I should have gotten a life) But now the pickins have gotten pretty slim. There should be MORE goodies available not less.


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PostPosted: Fri Jan 18, 2008 11:56 pm 
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Sidebar: my personal choice: GemologyPro/Harald Schneider L1 10X Loupe
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You'll LOVE it.


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PostPosted: Sat Jan 19, 2008 12:05 am 
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Ha! Barbra beat me to it.. I LOVE my schneider loupe.. not only is it 3x as big as anything else, it is totally clear and distortion free right to the edge of the visual field. They use a lot of esoteric lenses fitted into that thing and got a patent on it.

You can find it at www.color-wright.com Kulakofsky sells them.

You'll never pick anything else up after that one.. ;)

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PostPosted: Sat Jan 19, 2008 12:05 am 
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For $300 it better be great. :)


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PostPosted: Sat Jan 19, 2008 12:21 am 
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And, it is!


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PostPosted: Sat Jan 19, 2008 12:50 am 
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jleb wrote:
not only is it 3x as big as anything else, it is totally clear and distortion free right to the edge of the visual field.

Would this also be true of a 20x, or going back to what Brian wrote, would there be distortion in the larger diameter, higher power loupe. I am quite happy with my B&L little 10x Hastings triplet - I pick it up to use probably more than any others. Maybe sometime I'll run into someone with a Schneider loupe and I'll change my mind though.

Purely as a novelty gift for a collegue, when I was in Kassoy I picked up a triplet with 2 little LED lights mounted in the casing . Today he called to say he used it to examine a stone he thought might be lead glass filled and the 3 people watching fell in love with the thing (not the star which was indeed filled) and now want to put in a mass order for the store. Turned out to be a fun gift (it throws a reflection that you have to work around, but I guess it has its uses).


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