Voigtlander B serial transition to Portrait Objectiv serial engraving & Nomenclature

[lungovw]

Hello, I just acquired a Voigtlander lens. Serial number is 23494. It is a Petzval type but the one redesigned by Sommer. The separated doublet at the rear end follows the scheme on page 275 on Moritz Von Rohr book from 1899 (attached). I did not measure the radius on glass surfaces, but the sequence flat/convex followed by an external meniscus resembles totally that Sommer version. It has a number 6 engraved on barrel. No letters, just 6. I measured the focal length and it is 335 mm. Entrance pupil is about 95mm and that gives me f/3.5. I am confused about what lens is that. I read that between the traditional Petzval design and the portrait lens with a cemented rear doublet Sommer had introduced a lens f/2.7. This one I have matches the design, but it is not a all a f/2.7. I still need to go through this thread and maybe the answer is already here. Anyway, here is another Voigtlander lens for the database from the transition period.

[mhayashi]

Dear Wagner,
Thank you for your detailed analysis and regression calculation.
Yours was made in 1878, the important year when Voigtlander changed designs, relabeled lenses and introducing new optical layouts and when PWF Voigtlander died.

Your sample is NO.6 335mm f3.6 d92mm in the catalog (not NO.6B) and belongs to the 6th column from the title column and the 7th row counting from from the No.0 row. The definition of f-number in my table is just FL/front lens opening diameter for the sake of identification, not the entrance pupil technically. Let me know the front element opening diameter, which is said as “Oeffnung” in the German catalogs.
In this period, this series was the normal speed petzval series or called first series.

the sequence flat/convex

I think if you put a piece of paper on the surface of what it looks plano(flat), it should be slightly concave.

I am confused about what lens is that. I read that between the traditional Petzval design and the portrait lens with a cemented rear doublet Sommer had introduced a lens f/2.7. This one I have matches the design, but it is not a all a f/2.7.

As I said above, this lens is the next generation answer to the normal petzval layout and speed by 1878.
The first series called in this period and also said normal speed, in pararell to the faster (second series) speed Schnellarbeiter XB series.
In English catalogs, normal speed series was called "quick worker" and the faster(second) series was called "rapid worker" in this period.
Later when the normal speed series was extinct, the second XB series became the first series and was called "quick worker" and dropped the label B but only numbers like yours but the different series.
In German catalog, these confusing nicknames were always consistent unlike English catalogs, so the first normal speed series was never called Schnellarbeiter in German. It’s understandable that many people are confused with the first series and the second B series by these inconsistencies and say both are quick workers.

Voigtlander never released f2.7 design petzval you mention until Hans Hartings revised and tweeked the Sommer's original design in 1900, then labeled the new design as Portrait-Objectiv Ia, which is not f2.7 but f2.2-f2.4 in the last column in my summary table.

The close f2.7 is No.4 petzval with the traditional petzval layout before 1878 but this design was there around 1845 before Hans Sommer started to work for Voigtlander. So it has nothing to do with Sommer's design.

Your No.6 is not the version of the cemented rear doublet Sommer had introduced in 1879.
The cemented doublet is called “improved compound”, which was released as a supplement to exchange the rear group.
This improved compound seems not released as a standalone series as a whole lens set and ceased in production by around 1882.

There is also No.B
167mm f3.2 d52:54mm
becomes
136mm f2.7 d52:54mm
by swapping the rear group from the traditional petzval to the improved compound.

Refer to #20 self reply about the improved compound.

Or probably you mean f2.37 written in the page 275 of Rohr’s book, not f2.7.
If so, the direct descendant in terms of speed is the Portrait-Objectiv Ia. But notice the rear group of Ia is much smaller than the front but the diagram of Rohr’s shows the rear group is larger than the front group as in the traditional petzval.

But I hypothesize the normal speed No.X and the faster No.XB are both based on the Sommer’s design of the Rohr’s diagram,
Which I mention in the lower right in the flowchart pdf as well as the rear glued compound.
There is at least one difference between the Rohr’s optical layout diagram and XB’s and X’s series.
The front achromat rear most surface curvature r3 of Rohr’s is convex but concave for the latters.
On the other hand, r3 of Ia is convex.
These optical layout is denoted by the legends like C, V, P and A in both my table and flowchart.

Also note the XB series optical layout has changed from the tranditional petzval layout for No.3B, 4B, and 5B to the Dallmeyer layout sometime in 1879 to 1880. No.6B and 7B were the Dallmeyer layout from the beginning.
These changes are described in the generation 5 and post-5 in the summary table.

[mhayashi]

Dear Wagner,
I updated the sample listing based on your sample analysis.
Also I made a minor correction in the transition table summary.
pdfs are available in the #6 reply.

Btw, your Orthoskop in your homepage is No.2, 377mm d52:35mm.