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fld optical glass by sigma

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#1 sg80

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Posted 23 February 2012 - 03:17 PM

It is said to have performance equal to florite. but at a lower cost and lighter than reg optical glass.anybody heard of this and can it be used for refractors and ca correcters!!!!

#2 peleuba

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Posted 23 February 2012 - 03:45 PM

It is said to have performance equal to florite. but at a lower cost and lighter than reg optical glass.anybody heard of this and can it be used for refractors and ca correcters!!!!


Currently only used in camera lenses which do not require the precision, purity and homogeneousness of glass needed for a telescope.

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#3 DanaJ

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Posted 23 February 2012 - 03:56 PM

You mean it's only used in certain Sigma lenses, or that all camera lenses have lower requirements than all telescopes? I find the latter hard to believe.

#4 peleuba

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Posted 23 February 2012 - 04:10 PM

You mean it's only used in certain Sigma lenses, or that all camera lenses have lower requirements than all telescopes? I find the latter hard to believe.


Camera lenses have lower requirements then telescope objective lenses.

#5 sg80

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Posted 23 February 2012 - 05:14 PM

It did say it could be used in larger lenses.I can,t post the link but it,s from 2010 the glass is new so all of it,s uses may not be known yet.Sorry Paul posted link in his first post

#6 Jon Isaacs

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Posted 23 February 2012 - 05:25 PM

It did say it could be used in larger lenses.I can,t post the link but it,s from 2010 the glass is new so all of it,s uses may not be known yet.


Have you found the Abbe number for the FLD glass. It is one thing to put out a press release claiming it is the equivalent of Fluorite, it is another to actually show the test results.

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#7 sg80

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Posted 23 February 2012 - 06:11 PM

Have you found the Abbe number for the FLD glass. no but good question. I,ll try

#8 orlyandico

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Posted 23 February 2012 - 06:36 PM

1) isn't FPL-53 "equivalent" to fluorite?

2) some camera lenses can give telescopes a serious run for their money - http://www.samirkhar...evue_canon.html

#9 DanaJ

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Posted 23 February 2012 - 06:42 PM

Camera lenses have lower requirements then telescope objective lenses.

In some areas, but I find the idea that Canon, Nikon, Leica, Zeiss, etc. use less "precision" for the elements in a $10k+ lens than Synta does for a a sub-$100 doublet to be hard to swallow. Or perhaps I'm being too literal with your quote. All camera lenses have lower requirements in all areas than every telescope objective? You can't possibly mean that -- is Hasselblad going to tell Zeiss they're switching to Sears/Jason lenses?

Also tangential to whether this FLD is available with the qualities needed for telescopes. I recall looking at some catalogs (e.g. Schott pocket catalog) where one can see lots of quality levels possible for a wide array of properties. Some glasses are not going to be available with some tolerances (e.g. transmittance, bubble quantities, etc.), or might have unfavorable pricing. All it takes is one of these properties that is tolerable for Sigma's lenses but not acceptable for a telescope objective (or more to the point, not good enough to economically justify using FLD).

#10 DanaJ

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Posted 23 February 2012 - 07:55 PM

1) isn't FPL-53 "equivalent" to fluorite?

Fluorite has nd = 1.4526, Vd of 95 (says Ray, referencing K├Âtitz). The FPL-53 specs from Ohara show both 439950 (nd = 1.439, Vd = 94.9) and 440946 (nd = 1.440, Vd = 94.6) available. It's basically equivalent in Abbe number to fluorite. But there are other wavelengths than those used for Vd, and many other optical, thermal, mechanical, manufacturing, etc. properties that aren't the same. My reading of the linked press announcement (thanks Paul!) is that they have something similar to FPL-53.

If I'm cynical I could even stretch to read that they put FPL-53 in those lenses and are calling it "Sigma's new "F" Low Dispersion glass" because that makes nice press. The Feb 2010 press release on dpreview says "[...] FLD ("F" Low Dispersion) glass, which has a performance equal to fluorite glass, has been adopted and used in our new high performance lenses." Note that that's Sigma's release, while the earlier one is from a consumer reseller. Sigma's text makes it sounds much more like they started using FPL-53 and are calling it FLD (vs. SLD, UD, ED, etc.), rather than that a camera company suddenly started making their own glass that just happens to be similar to FPL-53.

#11 orlyandico

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Posted 24 February 2012 - 12:30 AM

i think in spot size at the center of field, even the sub-$100 doublet will beat a Canon L.

but once you get even 5mm off the center, and even more so as you approach the edges, the lens will start looking better and better...

it's no accident that Pentax A* 300mm f/4 medium format lens was also sold as SDUF 75mm APO refractor. And the A* 400mm f/4 was sold as SDUF 100mm f/4 APO.

#12 DanaJ

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Posted 24 February 2012 - 02:12 AM

Telescope objects and modern camera lenses have differences that will mean different tradeoffs and optimizations. That TEC, A-P, Tak, etc. have very high standards for their products, especially in the important areas we care about, is undeniable. They're not making water-resistant 6" f/4 refractors with 17 elements including gyros and motorized 3-d control of various elements, and optimize things like bokeh which is a critical factor in quality camera lenses.

If it's just FPL-53 that Sigma is calling FLD, similar to how Canon calls their ED glass "UD (ultra-low dispersion) glass", then it's all moot anyway since it's the same thing. Canon uses fluorite elements (sometimes more than one) with Vd 95, nd ~1.44, "UD" elements with Vd ~82, nd ~1.50, and plenty of other optical glass. The funny thing is, Canon's graph in their book "EF Lens Work II" even has a label "51" next to the UD dot, and the Abbe number and refractive index for their "UD glass" correspond to FPL-51. So .. FPL-51, FPL-53, and Fluorite. Sounds like quality camera lenses and quality telescopes are using the same glass types. However ... there are a lot of quality variations within the types (as the Schott booklet indicates), and there's a lot of work from the designer and manufacturer that is just as important.

#13 Jon Isaacs

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Posted 24 February 2012 - 08:17 AM

1) isn't FPL-53 "equivalent" to fluorite?

2) some camera lenses can give telescopes a serious run for their money - http://www.samirkhar...evue_canon.html

]

The Canon lens has a much more even performance, apart from some slight vignetting, easily treated by flat-fielding, but it is also weaker in terms of clarity and bite, likely a reflection of the fact that it has 17 lens elements rather than just 4.



Telescopes are designed to operate at the theoretical limit, camera lenses are not. Even a relatively simple telescope is capable of resolving, in the center of the field, right to the limit.

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#14 peleuba

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Posted 24 February 2012 - 10:41 AM

In some areas, but I find the idea that Canon, Nikon, Leica, Zeiss, etc. use less "precision" for the elements in a $10k+ lens than Synta does for a a sub-$100 doublet to be hard to swallow. Or perhaps I'm being too literal with your quote. All camera lenses have lower requirements in all areas than every telescope objective? You can't possibly mean that -- is Hasselblad going to tell Zeiss they're switching to Sears/Jason lenses?



I don't believe I said that, but your argument is an interesting one. I've not ever owned a Synta manufactured telescope so cannot comment on the relative quality from lots of hours in the field looking up. But I've seen one on a bench - an ED80 - and came away with the impression that it was a terrific performer (Strehl in mid .90's) for the price. I doubt that you would find the Strehl ratio of a $7k camera lens with 26 elements as good.

In any event much of my information is perhaps anectdotal, but has been gleaned from speaking, emailing and reading information from Yuri Petrunin and Roland Christen. I am a customer of theirs and trust what they tell me. A lot of what they say on this subject is also contained within the archives of the APUG and TEC Yahoo groups. I believe there was a recent dicussion on this about the AP130GT - search the archives if your motivated to do so. They contend that most camera lenses pale, from a pure performance standpoint - contrast and resolution, when compared to an APO of similar aperture.

Anyway, a great discussion.

#15 Jon Isaacs

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Posted 24 February 2012 - 11:45 AM

For the things a telescope is designed to do best, pure center contrast and resolution, it does the best...

Camera lens are designed for a different task and never case the challenge of operating in a high contrast environment at magnifications that require diffraction limited optics.

Cameras are forgiving in the center but demand a flat field with zero
vignetting.
Jon

#16 peleuba

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Posted 24 February 2012 - 01:53 PM

For the things a telescope is designed to do best, pure center contrast and resolution, it does the best...

Camera lens are designed for a different task and never case the challenge of operating in a high contrast environment at magnifications that require diffraction limited optics.

Cameras are forgiving in the center but demand a flat field with zero
vignetting.
Jon




So this begs the question earlier in the thread: are telescope objective lenses made to a higher standard then camera lenses? By higher standard I am speaking of optical figure. And I think they are. To wit, a fairly large amount of sphercial abberation and chromatic/spherochromatic abberition can be tolerated in a camera lens before it affects the image. As you say, the field needs to be flat. The optical figure of an obejctive lens has a much, much tighter tolerance then that of a camera.

#17 DanaJ

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Posted 24 February 2012 - 03:42 PM

Some of this, I believe, is explained by my thinking in terms of a lens element: does the glass need to be made precisely? Does it need high purity and homogeneity? I don't think an MTF of fully put together systems necessarily tells you that. After all, you can give me the most perfect glass piece and I'll make you an awful telescope. :)

If a camera lens in its entirety has 17 elements (e.g. the Canon 600 f/4), then we would hope each element was made well, otherwise the combined result would be awful. If the end result after 17 elements, many of which are designed to move independently or in combination with other groups, does not match a well made ED doublet in the telescope's chosen important specification, does not imply that each element uses poorer glass made with less care. Also, portrait lenses from the likes of Zeiss or Leica can have larger budgets than even these big telephotos, higher standards, and often much smaller elements -- there's simply no reason for them to spec poor glass.

As Jon said and I've tried to point out, they optimize different things. That doesn't imply they don't care about the optical qualities and "do not require the precision, purity and homogeneousness of glass." Sure, a bunch of that budget goes into hiring master opticians and software engineers, making aspherical lenses (precision?), optimizing other issues such as a flat field across a medium format area, etc. But there is no reason for them to just spec *BLEEP* glass for the hell of it. We're not talking about point and shoot or mall-store zoom lenses here.

On the subject of the f/1.0 lenses, the MTF chart for Canon's 50mm f/1.0 (and Castleman's measurements) are awful. The sub-$50 mostly-plastic 50mm f/1.8 is better, for goodness sake. But it doesn't go to 11 (er, f/1.0). I'm sure there are nice things it does, and with two large aspherical elements and the desire for maximum light gathering this would be really hard to make, but it seems like the dancing bear. The end result would make an atrocious telescope, but that doesn't mean they shoddily assemble cruddy glass and slap a $2500 price tag on it.

In any event much of my information is perhaps anectdotal, but has been gleaned from speaking, emailing and reading information from Yuri Petrunin and Roland Christen. I am a customer of theirs and trust what they tell me. A lot of what they say on this subject is also contained within the archives of the APUG and TEC Yahoo groups. I believe there was a recent dicussion on this about the AP130GT - search the archives if your motivated to do so. They contend that most camera lenses pale, from a pure performance standpoint - contrast and resolution, when compared to an APO of similar aperture.

Yuri and Roland know more than I'll ever know about this subject, and I've seen RC be very careful about exactly what he's reporting. That a fully assembled lens does not perform as well on these tests as an A-P or TEC scope doesn't really surprise me. I can also note that Sports Illustrated isn't using either of them at football games (which just means there's a lot of other important features that matter for them, e.g. the ability to change the focus).

I'm still not convinced that in any of these cases, even the lofty standards of A-P, TEC, and Tak are higher than that Zeiss or Leica would apply to a small portrait lens (e.g. Leica 90mm Summicron. That the telescope turns out to be a better telescope is irrelevant in this. A-P and TEC do have the advantages of (1) hand-making them or close to it, and (2) being run by quality-obsessed people who are very good at what they do. Their standards for glass quality may be higher than Sigmas, and their polishing standards probably are a lot better (they also have only 6 surfaces instead of 14-30+). RC has a post on Astromart indicating some of the issues he has getting good FPL53 and fluorite glass in the size, quantity, and quality he needs. Now take one of the Chinese refractor companies. They don't have Roland custom ordering glass batches. Someone has quoted RC (but I don't have his post so this is third hand) as saying some manufacturers were suing a cheap CDGM glass he considered unacceptable in homogeneity. Yet it's being used for telescopes.... Here is a quote from RC in 2009:

You can see why a 6" achromat can be made ultra-cheap. 6" blanks of normal crown and flint - approx $10. Diamond grind each surface - direct labor time about 1 minute each. Pell lap fine grind - direct labor time about 1 minute. High speed polish of each surface - direct labor time about 1 minute. Kiss with pitch for final surface polish (if you want better than 1/4 wave performance), coat, assemble into cell. Done! No need for testing because color and spherical correction has a huge leeway within the allowable radius error. Internal curves are so mild that collimation and spacing can be done by a blind person. Like driving a tiny Hot Wheels car on an abandoned 6 lane interstate.

He goes on to explain why making a 6" ED triplet for A-P standards is far, far harder and costly. Given that quote, I think I'm justified in saying there do exist camera lens manufacturers that have higher standards for both precision and glass quality than that of some mass produced telescopes.


By the way, I am enjoying learning and discussing this, and hope I don't come off as too argumentative. It's been interesting reading. I also think it's amusing to see Jon posting in this thread as well as in most of the historical threads I've found on these topics :).

#18 DanaJ

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Posted 24 February 2012 - 03:58 PM

So this begs the question earlier in the thread: are telescope objective lenses made to a higher standard then camera lenses? By higher standard I am speaking of optical figure.

But the original statement was "[...] camera lenses which do not require the precision, purity and homogeneousness of glass needed for a telescope." Specifically the composition of the glass elements and the precision to which they are made. That's my point -- looking at the MTF chart of the full system doesn't tell you that. I will shamefully use a tenuous car analogy: that a Toyota Prius or Lexus LS does not out-accelerate a Corvette does not indicate it did not require as much attention to engine design or quality of engine components. That the Corvette is a better track car isn't surprising.

All that said, I admit it's possible that the statement as written is completely true. Especially if it's reworked as something like "typical consumer-grade camera lenses and even many professional lenses do not require the [p, p,and h] of glass that I expect from A-P or TEC." I'll buy that.

#19 Yu Gu

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Posted 24 February 2012 - 05:29 PM

Keep in mind that those expensive Leica and Zeiss glasses are tested comparably (sometimes better, sometimes worse) to their Canon and Nikon counterparts.
www.photozone.de
So we can not just assume the quality only base on price.

#20 DanaJ

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Posted 24 February 2012 - 06:25 PM

Indeed, and there is a lot of sample variation in many of the Canon lenses. Again though, these are entire lenses. Let's pretend the transmittance is 99% for the glass used for a doublet objective and for a 17 element camera lens (the transmittance is unlikely to be this low in the visual range and thicknesses considered according to Schott's document, so this is an exaggeration on this measure, and I'm totally ignoring thickness, coatings, and goodness knows what else). The total transmittance for the doublet is (0.99)^2 = 98%. For the 17-element design it would be (0.99)^17 = 84%. So the camera lens, with the same quality of glass in this measure, comes out far behind. No wonder a premium telescope using very fine materials (e.g. A-P 130GT) can beat even a premium telephoto (e.g. Canon 800mm f/5.6 with nice though not best-of-Canon-telephoto MTF chart and 18 elements). This is part of my thinking: just to break even, the camera lenses have to use much better quality glass, not worse! When the telescopes being compared are using nearly the top commercial grade glass, there's not much that can make that up. Even using lousy 93% transmittance glass would still make the telescope come out ahead in this measure. Yet it used worse glass.

#21 DanaJ

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Posted 24 February 2012 - 07:00 PM

On that same A-P group thread, there is a comment:

"I actually use my FSQ106N for indoor sports, gymnastics. College level. Astounding clarity when compared to my 200 1.8L lens." which I found very interesting. I shot college gymnastics for a few years, and my first thought is that even an f/5 scope would be horribly slow. I found f/2.8 to be frustratingly slow and usually used f/1.8 and f/2. But newer sensors with good quality high ISO almost certainly make that moot (other than DoF issues). There are also a lot of events you can do just fine with manual focus (beam and vault, and you could probably do uneven bars), which makes it an interesting case where a telescope would work just as well as a fancy autofocus lens (except for floor).

The kicker is the comparison to the 200/1.8. When I was doing gymnastics photography, that was the holy grail of gymnastics lenses (at least for us Canon people). Pretty amazing that he'd say that about the scope. Now some of what people wanted was the 200mm f/1.8 and beautifully out of focus backgrounds, which the scope isn't going to give, but some of it was the sharpness, and may I say, clarity. Very nice. I hear the Tak works well on stars too. :)


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