US stops to F;stops

[Dave Henry]

Hello, I'm wondering if some of the really knowledgeable folks such as Ernest, Kerry etc. have a reasonably close date that we, in North America, converted from the US Number stop to the present English f:stop system?

Also, when was f:6.3 dropped from the English system?

I am not having much success researching this information.

It would be of great help. Thanks so much

[Peter K]

Sorry Dave,

I'm not Ernest or Kerry but possible I'm etc.

The f-number of a lens is the relation between the focal lenght and the diameter of the pupillary entrance.

f-number = f / pe

The f-stop is a mechanical device for easier handling of a camera. Most LF-cameras don't have f-stops but a scale with f-numbers. E.g. at 6,3 the diaphragm is a little bit smaller than at 5,6. When your lens is equiped with an old scale you have to mark or engrave the international row for easier handling.

The international f-number row is

f/0.7; f/1; f/1.4; f/2; f/2.8; f/4; f/5.6; f/8; f/11; f/16; f/22; f/32; f/45; f/64; f/90.

Peter K

[Ole Tjugen]

Since I'm another "etc.", I'll add a little bit more.

The Uniform System had the apertures marked so that each doubling of the number corresponded to a doubling of the light. This matches up with the more familiar f-numbers at 16, so US 16 = f/16. Since it doubles for each step, US 32 is f/22 and so on.

The f/6,3 is a relic of a popular numbering in Germany which is 17 stop offset from the more "modern" one given by Peter K - 3.16, 4.5, 6.3, 9, 13, 18, 25, 36, 50 and so on.

The modern row is based on 1, the old German scale was based on the square root of 10. When introduced it was a transition between the old French system which started with 1 = f/sqrt(10) and doubled for each step like the US did.

There were quite a few other numbering systems in use, although in my experience most English lenses used AN (Arbitrary Numbers).

[Ralph Barker]

The Wikipedia entry at: http://en.wikipedia.org/wiki/F-number has a chart showing various aperture systems in use around 1899. The switch to the modern series of f-numbers appears to have taken place around the turn of the last century.

[Dave Henry]

I had seen the Wikipedia listing from my Google search and the 1899 and 1920s dates from Kodak led me to seek out all your input.

I thought that this scale might be used as a clue to dating various lenses in my collection. I've accumulated several drawers full of lenses and now have the time to begin research on their origins etc.

It appears that this conversion may have taken place over many years and that manufacturers may not have converted all at once.

Let's see what more information comes up.

[Diane Maher]

Why not post a thread just listing the lenses and maybe part of their serial numbers? You would be surprised at who knows what just by that.

[Dave Henry]

Thanks Diane for the reply but there are way too many lenses to list here. I found some information in the Cameraeccentric catalogs that was helpful as well. That gave some dates by catalog publication and some interesting lens construction notes.

[Paul Fitzgerald]

Hi Dave,

A while ago I had Wollensak series IV in an Optimo shutter, patent appl. for, with the US system. Also had another Wollensak in an Optimo, patent June 14, 1910, with the modern aperture scale.

Have a B&L tessar 6.3 in a Volute shutter, patent appl. for, marked with the modern scale.

Kodak Anastigmats still carried 6.3 on the scale up to 1940, Wollensak carried 6.3 at least into the 1930s.

I think 6.3 was considered a 'magic' number, referring to the Zeiss IIb 6.3 Tessar, "the Eye of the Devil", the first truely modern lens and the most copied lens in history.

Have fun with the hunt.

[Dave Henry]

Hi Paul,

That's interesting. I have a Zeiss Triple convertible, Ross Double convertible and a B&L Protar Series V (8x10) in Volute shutters (all work smoothly) and all in the f:stop scale. None in the US scale.

[Tin Can]

Found this thread first, great to find US 16 stop = modern f16. Now need to check speed on a clean Zeiss 11A 5X8 with stable shutter and clean Wide angle, Telephoto And yellow filter.. Now all I need are those KS glass plates for these holders...

I'll shoot modern holders with X-Ray.

Thanks Ole Tjugen and Ralph Barker!