Computing f/stops ancient lens

juan

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I have a frankenlens combo - a really old 23” B&L Protar front and 18” Protar rear lens ( there are fractions to these lenses I’m not going to worry about) to make a combined fl of just under 12”. They are mounted in a non-working shutter that has f/stop scales for the 18” and a 27”. Can I compute the f/stops this way? Assume f/16 as an example.
16/27= x/23
27x = 368
X= 13.63 or about f/14 or about a half stop off the indicated scale. I’m looking for just good enough.

I’m an artiste, not a mathematician.
 

Don_ih

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The aperture scale is based on the aperture diameter itself, so it as a scale of ratios will remain accurate, you just need to know what the new ratios are. So, measure the focal length and divide by diameter of the wide-open aperture to get the max aperture number. Then every mark down on the scale will be a stop down from that.
 

Nicholas Lindan

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The definition of f-stop is

f = focal length / diameter of the entrance pupil

Measure the diameter of the entrance with a ruler - stand about as far away as you can. Then focus at something very far away and measure the distance from the diaphragm to the film plane. These measurements will do a 'good enough' measurement of focal length.

Once you have the wide open f-stop then you can scale subsequent f-stops. If the diaphragm mechanism was for an f.5.6 lens and you measure and it is now f5.0 then the progression of f-stops is 5.0, 7.0, 10, 14 .... just make up a new scale and attach it to the lens.
 
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juan

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Thanks, I always try to make things too complicated.
 

Don_ih

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I have an unmarked lens in a shutter on a wooden folding bed 5x7 that's marked f7.7 and calculates to f13 or something. When dealing with older lenses and shutters, it's almost always good to just calculate because the lens may have nothing to do with the numbers written on the shutter.
 

jimgalli

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Bausch and Lomb (actually Zeiss) purposely designed the spans between individual cells in 3rd stops from each other. 19 - 23 - 27 - 30 etc etc as you go up and down the list. Thus if you simply use your 27" scale you're a 3rd stop off. If you want f22, go to f22 1/3 on the scale. Zeiss when they made the barrels included a scale in millimeters. Thus if you were using a lens not on the calculated scales you could use the mm openings. If you had a 16 1/8 element you know it's 410mm and if you open to 41mm you know you're at f10. A 6" mini slide rule was a good companion then, but of course we're all cripples now with our telephones.
 
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juan

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Thanks, Jim. That’s even simpler. I never remember to take my phone with me. But then my slide rule is in a briefcase in the closet.
The 23” is marked Baush & Lomb - Zeiss, so think that means it’s very early. Thanks
 

jimgalli

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Yes, early - ish. Not sure what year the "Zeiss" went away. I have a brass barrel No. 7 Series VIIa in front of me that has the Z word @ 8635xx serial no. and a #6 VIIa, all black lacquer, 31795xx serial no. that does not have it. The earliest US versions from BnL license round off the focal's different from Europe also. 11 1/2" 14" while Europe was always 11 3/8" 13 3/4". BnL fell in line with Europe pretty quickly. Those early oddball ones are 1895 ish. Find an original Bausch & Lomb catalog complete on ebay and buy it. I've worn out my 1912 catalog looking up the series VII pages so often. Yes, it's on line, but like the slide rule, it's very nice to have the answer at your own finger tips.
 
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