Dallmeyer Group lens f4 info? The faster version of the triple achromatic series.

[mhayashi]

I have been offered one of the rare Dallmeyer Group lens f4 series, which I didn’t know until yesterday.
The series was made for 8x10, 10x12 and 12x15 during the short period in 1866 only, based on the Dallmeyer archive index.

http://thedallmeyerarchive.com/Records/Volumes/stockbook2/Indexes,%20lens%20lists/lens%20list%205-2.pdf

Vade Mecum states the series was made since 1861, but the Dallmeyer archive negates it.
In VM, it says the middle achromat group of the triple achromat series was replaced to the larger one to form the group lens.
I don’t have both series with the same front and rear lens diameter group combination in hand so not quite sure about the truth.
It is certain that this group lens series was not successful as the number of productions was only 220 for the whole series.

These are all I have got about the group lens.
If you know about this series, let me know.

[Steven Tribe]

I assume that the “F4” is really a modern F6.3?

[mhayashi]

Good morning, Steven.
Maybe yes or no.
I loooked up VM again and it says f4.

Google search found some examples but the exact measurements are unknown, as the effective focal lengths may be measured wrong, the front apparent lens opening diameter or simply the front lens opening diameter is not stated….

eg
https://www.worthpoint.com/worthopedia/dallmeyer-group-lens-10-8-circa-1867-466069826
https://www.liveauctioneers.com/item/76469782_a-j-h-dallmeyer-12x10-group-waterhouse-stop-lens

Certainly this series has nothing to do with the petzval “group lens” D series as a reminder….

The lens is engraved as one of these.
10 x 8 GROUP LENS
12 x 10 GROUP LENS
15 x 12 GROUP LENS
other than J.H.Dallmeyer LONDON and the serial number.

Two types exist for each,
rack and pinion focus
sliding focus.

The newest third book from Corrado about 19th century GB and Ireland lens book may have some more descriptions about this series but not in my hands at a moment.

[mhayashi]

My friend returned the book. This series is described from p224 to p226.
On p225, it says No.4, 5 and 6 of the slower triple achromatic series are modified to make the group series.
The rear groups are the same, but the middle and front groups were increased in diameter.

No.4 of the triple achromatic has back focus 15”. The corresponding group lens has the effctive focal length 15.5”. So by modifying the triple achromatic lens, the effective focal length is shortened based on the descriptions in the book.

The group lens f number is f7.5.
The triple achromatic f number is f10.