Rodenstock center filter numbers

[Leonard Evens]

I just got a Rodenstock center filter from B&H, but the various numbers on the cover of the case and on the filter housing are a bit confusing. It came without any instructions, and so far I haven't been able to locate any specifications for the filter anywhere on the web.

One label on the case it came in says "Centerfilter E 67/86 1.5 x", and it has what looks like what might be some sort of serial number---four groups of numbers separated by periods. The filter mount itself also says "E 67/86 centerfilter" but instead of "1.5 x" says "ND 0.45." There is nothing else on the mount except the Rodenstock name and "Made in Germany". Finally, there are two other labels on the case both of which say "Rodenstock 67 mm center filter 3 X F/Grandagon-N" and have some other different numbers, along with UPC codes.

I am trying to make sense of all these numbers. A density of 0.45 corresponds to 1 /1/2 stops, so if the 1.5 refers to number of stops, that might make sense. I checked the filter with my spotmeter and it does appear to reduce the light intensity at the center by about that amount or perhaps slightly more. If I interpret the 3 X to mean that the exposure has to be increased by that factor, that would correspond to a change of about 1.6 stops, which would be more or less consistent with the other interpretations.

I plan to use the center filter with a Rodenstock f/6.8 Grandagon-N and a Rodenstock f/4.5 Grandagon-N, each of which has a 67 mm filter mount. (The filter was clearly designed for those lenses.) Some calculations---or Kerry Thalmann's tables in the latest issue of View Camera Magazine---show that the fall off for the 90 mm lens with movements and the 75 mm lens without movements probably is just about 1.5 stops, and that is how I plan to use the filter.

Do I have this right, or is something else going on here?

The other interesting thing is that the filter itself appears to be about 86 mm in diameter, but the opening in the mount is 67 mm. I presume that means that in principle, the filter could be remounted in an 86 mm mount, and used that way. Is that right?

[Bob Salomon]

OK, Leonard let's take it from the top.

Centerfilter is obvious although there should be a space between the two words.
E is the thread pitch. 67 is the rear size of the filter thread and 86 is the front thread.
1.5X is the suggeted increse in exposure you should use.

There are two different Rodenstock CF for 67/86. One for the Apo Grandagon series and one for the Grandagon 75mm 4.5 and the 90mm 6.8 lenses with 67mm thread which has a 2.5x increase in exposure.

1094.2403.139.020 or whatever numbers you have is the factory part number.

Made in Germany is obvious as is the Linos address.

Your filter should be wrapped in white paper and be in a folded plastic bag if it is new and have a lens cap on it.

ND 0.45 is the density in the center. Old Rodenstock CF had a different center density and were not as neutral in color as the current ones.

Please call us if you need further help. 800 735 4373

[Bob Salomon]

""Rodenstock 67 mm center filter 3 X F/Grandagon-N" and have some other different numbers, along with UPC codes."

That is the store's label. It is not a Rodenstock label as Rodenstock does not show UPC codes on their items. As they are German any code they would show would be the standard European EAN 13 code which is probably not on your label.

"I presume that means that in principle, the filter could be remounted in an 86 mm mount, and used that way. Is that right?"

No. This is a wide angle filter for wide angle lenses so to eliminate vignetting the front diameter is larger then the rear thread size.

[Leonard Evens]

Thanks Bob. But I'm still a bit confused.

The part number is 1094.2403.139.020. Is this indeed the one designed for the f/4.5 75 mm and f/6.8 90 mm Grandagon-N lenses? Mostly I want to be sure I got what I thought I had ordered, a current Rodenstock center filter designed for my lenses.

And you've given me yet another number, namely 2.5. To make it even more confusing, I looked at the Calumet website, and in the picture for the same filter, they do appear to show a +2.5 on the housing. On the other hand, I finally found a Linos website, which lists the technical data for these filters, and for the filter designed for the f/4.5 75 mm and f/6.8 90 mm Grandagon-N lenses, it specifies a 1.5 stop exposure correction. A 2.5 stop correction is specified for some of the other filters. Is it possible you meant 1.5 rather than 2.5?

[Bob Salomon]

" 1094.2403.139.020. Is this indeed the one designed for the f/4.5 75 mm and f/6.8 90 mm Grandagon-N lenses?"

Yes.

As for your other question I answered that in my first response. 2.5x is for the Apo grandagon. Call us rather then get more confused by net answers.

[Leonard Evens]

Thanks Bob again. You have provided me with all the information I need.

I'm posting here for reference purposes. After all, someone else may have the same question and look here for an answer on the web.

I think I have it right now. The difference between exposure multiplier (in time) and exposure increase in number of stops scan be confusing when listed in specifications since it is not always clear which is meant. As best I can tell, my original estimates were correct. ND 0.45, +1.5 stops, and 3 X increase in exposure all mean approximately the same thing. One stop is equivalent to a number very close to ND 0.3, so 1.5 stops is equivalent to ND 0.45. Taking the logarithm of the multiplicative factor and dividing by the logarithm of 2 yields the change in stops. Applying this to 3 X yields 1.58 stops, which is close enough, since who is going to worry about .08 stops. (Inversely, 1.5 stops is equivalent to about 2.83 X.)

For a 90 mm lens following cos^3 falloff, the difference between center and corner without movements is 1.18 stops, so using this center filter would over correct by about a third of a stop. But if one used a rise of 20 mm, that would increase the difference to 1.5 stops. For a 75 mm lens following cos^3 falloff, the difference is about 1.5 stops, and movements would lead to an undercorrection. However, the f/4.5 75 mm Grandagon-N has a relatively small image circle and would not ordinarily be used with extensive movements.

The Linos web site containing the technical specifications is www.linos-photonics.com/en/prod/zubehoer_centerfilter.php.

[Bob Salomon]

"Thanks Bob again. You have provided me with all the information I need."

Glad it helped.