New arrived HERMAGIS casket lens

[seanwang]

It seems that the conversion table was made for a larger set, so it may not be correct for the one you have. That table also indicates that the focal lengths of the cells were NOT constant, since my No.7 cell is 360mm and the one in the table is 250mm.

As you might be able to see from the picture, my Hermagis Trousse Aplanatique has a kind of bayonet fitting - you only have to rotate the cell a fraction of a turn to screw it tight, the threads are removed in three sectors.

Mine also has three cells marked 7, 8 and 9, while the fourth cell is marked Somethingorother (I'm at work now) 210mm f:6.8.

Yes, I mount it on my 810 camera, 4 elements can combine 6 combinations, from 210mm to 310mm. For single cell, how about quality when shoot photo? And is the No.8 and No.9 hard to find?

[Ole Tjugen]

All casket sets are hard to find and/or expensive. Finding single cells and incomplete sets is easier, but in my experience these never fit the barrel you have - and I have lots.

Hermagis seem to have produced casket sets over a period of more than 30 years, so both optical and mechanical changes are very likely - meaning that the chance of an orphan cell fitting another set is quite unlikely.

There also seems to have been at least two very different optical designs used, which means that the quality of a single cell from one set says very little about the same from another set.

[Sven Schroder]

Hi
Attached is the table from the Hermagis 1925 Catalogue so hope it helps.
Hollis, I don't think I have a table for your set, but with a berthiot set I found a
cheque stub with the focal lengths, and it showed the maths involved.
The focal lengths are added together Say 390+305=695 this is then divided by 4 which gives 173.75 it seem close enough.
Regards
Sven

[Ole Tjugen]

The correct maths is Ft= 1/( 1/F1 + 1/F2 + d/F1*F2), where the F1 and F2 are the focal lengths of the cells, Ft is total focal length, and d is a spacing correction factor. The last term has been written in very many different ways through the ages, depending on how and by whom the spacing d was measured.

Ft=(F1+F2)/4 is wrong, but will give an approximately correct answer for two very similar focal lengths with no spacing.

[Sven Schroder]

Hi Ole

Thanks for the correction, maths is not a strong point with me, I am sure I have number blindness as well as word blindness.
Thanks and Regards
Sven

[seanwang]

Sven Schroder, Ole, Thanks!
Already printed

[Ole Tjugen]

Mine also has three cells marked 7, 8 and 9, while the fourth cell is marked Somethingorother (I'm at work now) 210mm f:6.8.

for "somethingorother", read "Hermagis Paris Aplanastigmatit 1:6.8 F=210 m/m"

[Ole Tjugen]

And by the way: They are 3 elements to each cell, looks like a Dagor-type design at a casual glance.