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Eleven fast 85 mm Lenses compared
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PostPosted: Tue Feb 09, 2021 8:13 pm    Post subject: Reply with quote
stevemark wrote:
But that would be something for a "pre-war" portrait test, along with the Thambar 2.2/9cm and the Sonnar 2/85mm. And the 4/9cm Elmar amd the 5.6/8.5 cm Staeble Telon, just for fun!


9cm Elmar vs. 8.5cm Triotar would be an interesting comparison...


PostPosted: Tue Feb 09, 2021 9:07 pm    Post subject: Reply with quote
stevemark wrote:
Yes, I was really surprised as well - I had laid away the MC-X 1.7/85mm since I'd bought it "by mistake" a few years ago (it had fungus and a stuck aperture). Only a few weeks ago I finally completely dismantled an cleaned it. As you may have noticed, the MC-X images (as well as the MC-II, not shown here) have a slight yellowish tint. The MD 1.7/85mm doesn't have this yellowish tint. The same can be observed with earlier (yellowish) and later (neutral) MC-X 1.2/58mm lenses. It seems that Minolta has changed some glass, probably slightly modifying the optical computation as well.

S
Yes, the first variant of both MC 58/1.2 and MC 85/1.7 used thoriated optical elements. Hence the yellow cast. However, your "rubberized" MC-X 85/1.7 should be the post-1973 model which is reportedly thorium free. So the change might actually occur later - with the MD series - or your copy might be a transition model...

I was also surprised that the MD 85/2 fared this bad - even the amount of CA is pretty nasty! For example, there is a Phillip Reeve's attempt to compare 85/1.7 against 85/2. He notes his 85/2 copy was not perfect but still managed to notably beat 85/1.7 in the center sharpness.

Another notable difference was the FD vs FDn Canons. Obviously, the FDn is smaller and lighter, but the optics were supposed to be identical. This doesn't seem to be the case. I'm starting to wonder about the sample variance here...

Too bad a CZJ 80/1.8 wasn't included Smile


PostPosted: Wed Feb 10, 2021 12:52 am    Post subject: Reply with quote
y wrote:
Yes, the first variant of both MC 58/1.2 and MC 85/1.7 used thoriated optical elements. Hence the yellow cast. However, your "rubberized" MC-X 85/1.7 should be the post-1973 model which is reportedly thorium free. So the change might actually occur later - with the MD series - or your copy might be a transition model...

I just have been discussing this issue with another Minolta collector. I'm pretty sure the very slight yellowing (and slight radioactivity) of both the early MC 1.2/58mm and MC 1.7/85mm are not caused by an intentional thorium content of the glass. Rare earth elements (incuding lanthanum) usually are being produced from the mineral monazite, which nearly always contains some thorium (sometimes up to 20%). Being a chemist myself, I know that separating thorium from rare earth elements originally was a challenge. I assume that lanthanum glass manufactured around 1970 still was containing considerable amounts of thorium, depending on the original source of the monazite (and therefore its thorium content) used for its production.


y wrote:

I was also surprised that the MD 85/2 fared this bad - even the amount of CA is pretty nasty! For example, there is a Phillip Reeve's attempt to compare 85/1.7 against 85/2. He notes his 85/2 copy was not perfect but still managed to notably beat 85/1.7 in the center sharpness.

If we look at Philips test, we clearly can see that he did not focus carefully: the center crop of his MC 1.7/85mm has greenish LoCAs, the center crop of his MD 2/85mm has reddish LoCAs. I usually focus "to best detail" which usually means that there are slight magenta LoCAs. If Philipp would have done the same with his MC 1.7/85mm, it would look sharper as well.


y wrote:

Another notable difference was the FD vs FDn Canons. Obviously, the FDn is smaller and lighter, but the optics were supposed to be identical. This doesn't seem to be the case. I'm starting to wonder about the sample variance here...

To me, this difference is negligeable (not relevant in practical photography). It may be sample variation, or it may be a slight variation in the optical design. Don't forget that these slight variations (without actually announcing them) were quite normal back then. Marco Cavina has made an excellent article about the variations of several Nikkors (http://www.marcocavina.com/articoli_fotografici/Nikkor_optical_variations/00_pag.htm), and similar variations obviously are found with other manufacturers.

Not shown in my test, for instance, is the early MC-II 1.7/85mm and the early (full metal grip) version of the Hexanon AR 1.8/85mm. The first version of the Hexanon AR 1.8/85mm has a slightly yellowish color cast as well, just like the early MC 1.7/85mm and MC 1.2/58mm lenses.

y wrote:

Too bad a CZJ 80/1.8 wasn't included Smile

Yeah, too bad indeed Wink ...

S


PostPosted: Wed Feb 10, 2021 6:40 am    Post subject: Reply with quote
stevemark wrote:
I just have been discussing this issue with another Minolta collector. I'm pretty sure the very slight yellowing (and slight radioactivity) of both the early MC 1.2/58mm and MC 1.7/85mm are not caused by an intentional thorium content of the glass. Rare earth elements (incuding lanthanum) usually are being produced from the mineral monazite, which nearly always contains some thorium (sometimes up to 20%). Being a chemist myself, I know that separating thorium from rare earth elements originally was a challenge. I assume that lanthanum glass manufactured around 1970 still was containing considerable amounts of thorium, depending on the original source of the monazite (and therefore its thorium content) used for its production.
Thanks a lot for this background information.

It always surprises me that the quite common lanthanum has been considered to be radioactive, but compared to thorium lanthanum's emitting is low it can't be even measured by the usual tools. Lanthanum elements are employed in the old M42 Soviet lenses like Zenitar 50/1.7 or Industar L/Z 50/2.8 and also in the present-day Chinese Zhongyi 50/0.95 lenses.

Marco Kröger wrote an excellent article about CZJ Prakticar 50/1.4 switching from "thoriated" to "lanthanum" glass. The thorium Schwerstkron SSK11 was replaced by Lanthan-Kron LaK75 along with a required design recalculation. The first version is known to be yellowed and radioactive, the second one is not.

So I'm still a bit confused. Is the amount of contained lanthanum relatively low and therefore that 20% content of thorium makes a very small part of the total volume? Might be, right?


PostPosted: Wed Feb 10, 2021 2:56 pm    Post subject: Reply with quote
y wrote:

It always surprises me that the quite common lanthanum has been considered to be radioactive, but compared to thorium lanthanum's emitting is low it can't be even measured by the usual tools.

I've roughly calculated the number of atoms decaying in 1 g pure Lanthanum per second: I'ts one atom per two seconds. That's extremely low radioactivity (compare this to the roughly 5000 decays per second resulting from natural potassium-40 decay in our body).

However, neither the lanthanum used now nor the lanthanum used in the 1960s was pure, of course. I have consulted Holleman-Wibergs "Lehrbuch der Anorganischen Chemie" (1960) which gives some clear insights about the correlation of thorium and lanthanum production around 1960. At that time, most of the monazite ore was used for thorium production (which in turn was used as "gas mantles" for gas lamps). Rare earth elements (mostly lanthanum an cerium) were a by-product of thorium production. Separation of the different rare earth elements was difficult in the 1950s (sometimes requiring several thousand cycles of re-crystallisation). Therefore usually the crude mixture of lanthanides (mostly lanthanum, but also other lanthanides and thorium) was used for optical applications. Only later (I guess around 1975) new separation techniques such as ion exchange were available on a (semi-) industrial scale, thus resulting in relatively pure lanthanum (and other lanthanide elements).


y wrote:
Lanthanum elements are employed in the old M42 Soviet lenses like Zenitar 50/1.7 or Industar L/Z 50/2.8 and also in the present-day Chinese Zhongyi 50/0.95 lenses.

Probably more or less any fast (f2, f1.8 ...) or super fast (f1.4, f1.2, ...) lens made after 1960 contains lanthanum glass. Even lenses such as the Macro Rokkor 3.5/50mm do have rare earth element glass.

y wrote:

Marco Kröger wrote an excellent article about CZJ Prakticar 50/1.4 switching from "thoriated" to "lanthanum" glass. The thorium Schwerstkron SSK11 was replaced by Lanthan-Kron LaK75 along with a required design recalculation. The first version is known to be yellowed and radioactive, the second one is not.

That's a typical replacement of a real thorium glass (about 20% thorium(IV) oxide) with a "thorium free" (well, nearly thorium free, as we have seen above) lanthanum glass. Other examples for such a replacement are the Canon FD 2/35mm, the Takumar / Pentax 1.4/50mm, and the Minolta MC 2.5/25mm which was replaced by thorium-free MC 2/28mm and MC 2.8/28mm lenses.

y wrote:

So I'm still a bit confused. Is the amount of contained lanthanum relatively low and therefore that 20% content of thorium makes a very small part of the total volume? Might be, right?

No. I never said that the lanthanum used for optical purposes would have 20% thorium impurities!!

What I said was: Monazite (the mineral / ore from which both thorium and lanthanum is extracted) may contain up to 20% thorium (usually it's less, maybe around 10%). Since no chemical separation on a (semi) industrial level is perfect, the lanthanum does contain some thorium. How much thorium depends on the separation process and on the ore used: In 1960 it may have been in the range of 1%, theses days certainly less. In other words: a better purification of the lanthanum results in less thorium traces in the "thorium-free" lanthanum glass.

S


PostPosted: Wed Feb 10, 2021 5:28 pm    Post subject: Reply with quote
stevemark wrote:
No. I never said that the lanthanum used for optical purposes would have 20% thorium impurities!!

What I said was: Monazite (the mineral / ore from which both thorium and lanthanum is extracted) may contain up to 20% thorium (usually it's less, maybe around 10%). Since no chemical separation on a (semi) industrial level is perfect, the lanthanum does contain some thorium. How much thorium depends on the separation process and on the ore used: In 1960 it may have been in the range of 1%, theses days certainly less. In other words: a better purification of the lanthanum results in less thorium traces in the "thorium-free" lanthanum glass.
Now it all makes sense. Thank you very much!
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