question about dallmeyer 3b water house stops

[alex from holland]

hello,

i have a dallmeyer r3b with it's original set of water house stops.
the numbers on these stops are :

X, X2, 2,3,4,5 and 6

can somebody help me how to calculate which stop is, lets say F 5.6 ?
thanks

Alex

[Steven Tribe]

This may be a question that only Sean can answer. Or someone who has done the measurements on an original stop set.
I do remember a posting here when he explained that sets of stops matched quite a few different objectives (based on diameters, of course) - even including some of the large RRs. So there was probably a code that translated the letters/numbers to F values to the specific objective sold.
This is similar to the system used by Zeiss and Busch (?) with mm opening in the diaphragm.
Last year Sean wrote:

"The same size stops for different lenses have the appropriate stop sizes engraved for the lens with which they are sold, either Dallmeyer's own aperture notation system early on or later in towards the end of the 19th. century the normal f stop system.

But the inscriptions don't really matter as it is easy enough to calculate values for the lens with which one is using that set of stops.



So someone with a later 3B can read off the F values for you!

[eddie]

measure the largest stop you have....the one that is f3. then divide that number by the square root of 2. this will give you f 4ish (2.8 then 4 then 5.6....close enough for me). divide that number by the square root of 2 once again. measure your stops. pick the one that is closest to the #. that is f5.6.

or take the 2nd from the largest opening stop....(provided the largest stop is f3)

[Steven Tribe]

I have just received a PDF file from Seàn about the early codes on Dallmeyer stops.
I will have a look and see if I can post it here. Otherwise, I'll forward directly to Alex (and others if interested).

[Steven Tribe]

The pdf file is too big to go into the system.
The answer is that there were 3 systems. The XXX2 etc. is the earliest system and there was another before they went over to F3 etc.
His example is the 3B models (there were two!) and I reckon you are missing some stops at the large end, Alex. Will send the PDF to you and copy some of the more important contents in an hour or so here.

[Steven Tribe]

This is the sequence (in descending hole size) for Seán's 3B - but which was made for an earlier "Quick Acting" 3B (no. 2562).

Stamped --- mm --- Modern F

2562 (i.e serial no.) --- 82 --- 3
X --- 72 --- 3.6
2 --- 59 --- 4.2
4 --- 41 --- 6
X --- 28 --- 8.4
5 --- 20 --- 12
X2 --- 18 --- 14
6 --- 15 --- 17


I cannot explain why you have a 3 - unless it is an later F3 stop from another 3B?
The X system seems to act as 1/2 stops.

[alex from holland]

This is the sequence (in descending hole size) for Seán's 3B - but which was made for an earlier "Quick Acting" 3B (no. 2562).
Stamped mm modern F

2562 (i.e serial no.) --- 82 --- 3
X --- 72 --- 3.6
2 --- 59 --- 4.2
4 --- 41 --- 6
X --- 28 --- 8.4
5 --- 20 --- 12
X2 --- 18 --- 14
6 --- 15 --- 17


I cannot explain why you have a 3 - unless it is an later F3 stop from another 3B?
The X system seems to act as 1/2 stops.

Steven,

thanks.
I added my measurements and hey seem to be a little different.
This is the original set with same serialnumber as the lens has.
The lens is from 17-01-1885 accordig to the info i got from Seán.

Stamped -----------mm ---------Modern F------Mine

2562 (i.e serial no.) 82 ----------- 3 ----- 81 ---36530
X ---------------- 72 ----------- 3.6 ---- 68 ---X
2 ---------------- 59 ----------- 4.2 ----- 56 ---2
4 ---------------- 41 ----------- 6 ------ 47 --- x2
X ---------------- 28 ----------- 8.4 ----- 39 ---3
5 ---------------- 20 ----------- 12 ----- 28 ---4
X2 --------------- 18 ----------- 14 ----- 19 ---5
6 ---------------- 15 ----------- 17 ----- 8,5 ---6



alex

[Steven Tribe]

Some logical matches, here - but also a lot of new problems arise!

Too late in the day to think of explanations. Except, that this very late to be using this system - so perhaps only the numbered stop is of that date (1885) and the others are from an very early (1860's) non-3B (but with similar diameter) objective?
Doing the usual calculation F/focal/diameter doesnt work when trying to work out your focal length.

[alex from holland]

Some logical matches, here - but also a lot of new problems arise!

Too late in the day to think of explanations. Except, that this very late to be using this system - so perhaps only the numbered stop is of that date (1885) and the others are from an very early (1860's) non-3B (but with similar diameter) objective?
Doing the usual calculation F/focal/diameter doesnt work when trying to work out your focal length.

Hi steven,

it could be but not likely.
This set is in it's original case and they all look identical ( exept for the hole ofcourse)

alex

[Randy]

Perhaps I did it incorrectly but I just measure the focal length of the lens focused at infinity (if it is not stamped on the lens).
Then I measure in mm's the opening of the lens (without any stop inserted) with a ruler across the rear element.

Example: 300mm focal length at infinity, 40mm diameter (across rear element) without any stops inserted (I do not measure the diameter of the rear element, but the maximum opening as I can see it through the rear element).

300 divided by 45 = f7.5 (round to f/8).

Then insert each stop and measure across with ruler and do the devision again to determine what each waterhouse stop is.

I have tested this with my Rodenstock Apo-Gerogon 360mm with modern built in aperture and this technique works perfectly - 360(mm) divided by 11(mm) = f/32.7mm, and I had the diaphragm set at f/32.