Experience with unusual f/stop scales?

I recently picked up a Plaubel Roll-Op from a seller here and am enjoying it. But its aperture scale is...

2.8 - 3.5 - 4.5 - 6 - 9 - 12 - 18 - 25

Two questions- in practical use, for B&W negative film, can I just use the marking as the more traditional 2.8/4/5.6/8/11/16/22 (and ignore 3.5) in my thinking? Or is it best to guesstimate and put the indicator mark at, e.g. a touch short of 18 for f/16?

Second question- any explanations for some of these scales? One like this is a standard doubling scale just using a different starting point. Would these kind of scales be related to the 1 - 1/2 - 1/5 - 1/10 shutter speed scale, for example?

Thanks.

I'd just set the aperture halfway between the 9-12-18-25 marks for 11-16-22, probably would be close enough. I've mostly seen similar f-stops on lenses that have some oddball maximum aperture (iirc Plaubel also did it with older versions of the 73mm f/6.8 Orthar)-- in those cases the peculiar 1/f numbers were successive full stops from the widest aperture available. No idea why they would do this on an f/2.8 lens...

wierd...6-9-12-18-25 is almost quarter-f/stop progression, except that has f/11-f/13 but not f/12!

I'd be more concerned with the shutter. For example it indicates 1/400. I'll bet it doesn't come close unless it has just been serviced.

I treat 1/100 and f:9 the same as 1/125 and f:8 (and so on) and have never had any problems. Serviced shutters and shutters that seem correct to my ear but might be a little off.

I would approximate the f/stops you are familiar with.

That's the old German (Continental) system I think... pretty sure it is based on the numbers you have listed.

Since they are both based on the ratio of the aperture to the focal length, they effectively mean the same thing (f9 in one system is the same as f9 in the other), but the difference is that the Germans baselined it off f100 and the English seemingly baselined everything off f1.

Look at the Continental system, it has 100, 50, 25, 12.5, etc... and the English system has 128, 64, 32, 16, 8, etc. So, it basically means they just chose different values to show on the lens they deemed "important ratios". I'd just translate them up a touch to get the aperture into the proper area so you aren't underexposing all the time.

It's rare, but in this case, I think the English did it better.

The aperture range of my 4,5/105 Rietzschel Linear: 4,5-5,5-6,3-7,7-11-15-22-31. It looks odd, but works.

Thanks for the replies. It's a bit disconcerting to have to play with these numbers after decades of the 'normal' scale. I'm not too precise in most cases shooting B&W negative film, expecting shutter speeds to be off, etc. I've been inside enough shutters to know that even aperture scales are not set in stone and that there can be slop in the levers and indicators, etc. I usually end up getting a feel for a particular camera's exposure slop and compensate. I'll go with the estimated position for what I am used to, e.g. a slight nudge short of 9 for f/8, and see how the negatives come out.

That camera is in my bucket list! The 6x6 version.
And yes, my Plaubel Makina 6x9 has those aperture markings, never had a problem, just go the nearest down

Those values are about a 1/3 stop from the usual ones. So rounding should be fine.

I’m in agreement with post #7.

In the Wikipedia article, History of photographic lens design, you’ll see the discussion of the Continental (German) sequence of: 1.1, 1.6, 2.2, 3.2, 4.5, 6.3, 9, 12.5, 18, 25, 36, 50, 71, 100 ratios under the heading Aperture Stops.

This sequence can be generated by starting on any value and successively multiplying or dividing by the square root of 2. To get smaller numbers, divide. To get larger numbers, multiply. For example, if you start with the “seed” 100 and successively divide by the square root of 2, you’ll generate a sequence, that when rounded (or truncated in some cases), will give you the given sequence.

The differences in the following pairs from the sequence: (1.1, 2.2), (1.6, 3.2), (4.5, 9), (9, 18), (12.5, 25), (18, 36), (25, 50), (50, 100) are 2 stops apart.

The adjacent numbers in the old German aperture sequence are all one-stop apart. The exact values can be calculated, but are truncated for simplicity per convention. The actual light values transmitted are one stop apart.

https://en.wikipedia.org/wiki/History_of_photographic_lens_design

For the aperture sequence given for the Plaubel camera in post #1, the numbers 2.8, 3.5, 6, 12 are “odd ducks” relative to the German system as stated in the article. I think these are simply truncated differently on the Plaubel camera, although 2.8 doesn’t seem to fit.

Note that the modern aperture sequence is generated by raising the square root of 2 to various positive integer powers.

For even powers, we get exact values: 1, 2, 4, 8, 16, 32, 64, 128, . . .

The odd powers are truncated per convention to: 1.4, 2.8, 5.6, 11, 22, 45, 90, . . . These are not exact numerical values (but the amount light transmitted is correct).

Both systems give the aperture ratios defined as (focal length)/(aperture diameter).

If you use a conventional light meter with the Plaubel camera, you can interpret the numbers as requiring the following light meter readings for correct exposure at the given film speed and chosen shutter speed.

2.8

3.5 (f.2.8 + 0.6 stop)

4.5 (f/4 + 0.3 stop)

6 (f.5.6 + 0.3 stop)

9 (f/8 + 0.3 stop)

12 (f/11 + 0.3 stop)

18 (f/16 + 0.3 stop)

25 (f/22 + 0.3 stop)

Uncoated lenses transmit less light, and the shutter speeds are most often slower than stated, so it evens out. With negative film I rather overexpose a little than risking loss of detail.