To answer Ron, the color fringing that I see is reddish purple on one side, yellow green on the other. Only kind I see.
I like Henry's suggestion that the uptick in CA reports in modern non-ED bins is due to the change in internal focus compound objectives.
If it weren't for the fact that I've also compared porros of the same design where the newer versions had more CA (though still less than the roofs), and if I hadn't also read similar reports about other "udpated" porros, I'd gladly jump aboard the compound objective = > CA bandwagon, because in general, I see more CA in modern roofs than I do in modern porros of the same configuration.
Not that also seeing more CA in newer porros negates the compound objective = > CA idea. It's just that it's beginning to appear from the other points that have been discussed earlier such as more awareness of CA, more birders, older birder population, etc. that rather than there being a "Lone Gunman," there could be multiple "shooters" conspiring together to create that CA uptick.
None-the-less, to beat the dead horse one more time...
I found another Ohara report that sheds some (color fringed) light on the lead vs. unleaded gas, er... I mean glass differences.
One interesting comment that amused me was this:
'In flint glasses, lead oxide plays a vital role in producing a negative anomalous partial dispersion. The B2O3-PbO system such as encountered in Ohara’s glass types BPH8, BPH5, BPM51 shows significant negative anomalous partial dispersion. However, since it is thought that the characteristic of showing negative anomalous partial dispersion with high dispersion is a unique property of B2O3-PbO system,1 it has been difficult up until now to eliminate lead oxide."
Translating into the vernacular, it means that their own presuppositions about what was possible delayed the development of lead free glass. Though I would bet that government regulations to "get the lead out" caused them to open their eyes and take another look.
What they found at first confirmed their suspicions...
"The lead-free flint glasses, for example the glass system SiO2-TiO2, showed the opposite behavior - positive anomalous partial dispersion (> CA). As environmentally friendly optical flint glasses having negative anomalous partial dispersion, the glass system SiO2-GeO2-Ta2O5 was
introduced."
The power of three was the charm except for its economic and physical limits....
"This [lead free] flint glass however is economically disadvantageous because, in order to achieve the desired negative anomalous partial dispersion, it required a large amount of GeO2 (germanium dioxide) and Ta2O5 (tantalum pentoxide), materials which are very expensive. In addition, this glass is difficult to melt and
therefore, it is hard to obtain material homogeneity."
Nobium to the Rescue
"In niobium containing glass systems, Ohara found that the SiO2-Nb2O5-ZrO2 system [Quartz+Niobium pentoxide+Zirconium dioxide] for Ohara’s glass types S-NBH8, S-NBH5, S-NBM51 has negative anomalous partial dispersion.
"These glasses attained
almost the same values for θg,F as conventional lead containing flint glasses as shown in Table I.
Niobium oxide is effective for increasing the negative anomalous partial dispersion property of the flint glasses if the amount of niobium oxide is above 20%. As
a result, niobium oxide plays a vital role in environmental friendly glass having negative anomalous partial dispersion."
Hallelujah! Are we there yet? Not quite...
Problem of Niobium Oxide Containing Environmentally Friendly Optical Glasses
"Whether a melt of specific composition vitrifies or crystallizes depends on the composition and
the cooling rate.... As Figure 6 shows, lead oxide has an unusual characteristic such that the glass forming
tendency of even binary PbO-SiO2 system leads to lead oxide content of more than 60 wt%. So it is possible for lead containing glasses to achieve a very high refractive index."
The plot (and coating) thickens...
Give me Lead or Give me Jaundice
"However, niobium oxide has a narrower glass-forming region than lead oxide does. It shows that the
glasses tend to crystallize stronger when the content of niobium oxide is more than 40 wt%. In order to prevent the crystallization for niobium containing glasses with high refractive index, it is necessary to adjust the refractive index by using additional elements like titanium oxide....
(and another "however")....
However, since titanium oxide has the fundamental absorption in the range of the near ultraviolet, the absorption limit moves toward longer wavelength compared with lead containing glass as shown in Figure 7. This makes the glass slightly yellowish. Some extra
attention in material selection is required just in case the optical system is designed for use at the short wavelength part of the visible range."
and so on, the reports shows the "Long and Winding Road" (da da) that leads to the door of finding the right combination punches that KO'd lead glass (or "almost").
I've attached the entire document, complete with equations, abbe numbers, and all sorts of delightful technical information for those who like that sort of stuff.
But for those who don't, let's skip to the conclusion....
"Niobium oxide plays an important role as one of the components that are substituted for lead
oxide. Niobium oxide containing glasses are effective in attaining
almost [there's that word again] the same relative partial dispersion values as conventional lead containing glasses....
"In the optical industry, the development of lead free products has become such an important
consideration that it is also part of the marketing strategy.[We are "cool" because we make Ecobins].
As a result, the demand for niobium oxide has doubled over the past ten years."
[The paper isn't dated but since lead free glass wasn't introduced until the late 1990s, I'm guessing this was written sometime in the past few years.]
"Future development of optical materials with special characteristics is desirable to accompany cost reductions including those in manufacturing methods and higher performance optical system design."
Given that Schott and other glass makers very likely went through this same trial and error process (assuming no corporate espionage was going on), I have to wonder if that R & D initially added some cost to the alphas and Nikon HGLs even though lead free glass has now trickled down to entry level optics?
Or if the cost of having to properly dispose of the lead and the safety concerns related to workers making lead glass offset the costs?
Regardless of the cause(s), if you see more CA in lead free glass bins of either porro or roof design, and it bothers you, there are ED/HD/FL/L glass bins available for the sensitive and discriminate birder at various price points. So no worries.
And there are always old porros and pre-internal focus roofs for sale....
http://cgi.ebay.com/Zeiss-Classic-8x56-B-GA-binoculars-Mint-case-/160573316986?pt=Binocular&hash=item2562ea5f7a
To be safe, carry a freezer baggie with you in case there's a downpour. But if the baggie's from China, make sure it doesn't have any lead.
Brock