Rapid Angulaire Lens

[Ole Tjugen]

As a rough guide, insert thirds between f:8 and f:11. 8 +1/3 stop = f:9, 8 + 2/3 stop = f:10.

4.5, 6.3, 9, 13, 18, 25, 36, 50, 72, 100 and so on should be familiar to all who use old German lenses - especially as max aperture!

5, 7, 10, 14, 20, 28 and so on is less familiar, but possible on old French lenses.

[Emmanuel BIGLER]

Hello !

The question of determinig the f-number of an old lens has been raised on this forum before.

If you consider that you know the value of the focal length, the diameter to be measured is not the actual diameter of the waterhouse stop itself, but the diameter of the entrance pupil i.e. the diameter of the image of the f-stop as seen from the entrance of the lens through the first lens groups.

f-number = (focal length)/(diameter or the entrance pupil)

It is likely that at the time when this lens was fabricated, people had no idea of the role played by the entrance pupil.
For this unknown old lens, I have no idea whether this can induce a significant difference in the actual f-number with respect to simply taking the measurement of the iris itself.

I had explained in this post how photographers did this measurement one century ago and how you can do it simply today.
http://www.largeformatphotography.in...pil#post850369

The quickest way to verifiy the actual value of the focal length of the unknown old lens is to compare on the ground glass the distances between two objects located far away, with a reference lens e.g. a 150 mm : 6" , and the unknown lens. If the lens is a 4" - 100 mm, the distance will be reduced by a factor 1.5. The measurement with a simple ruler can be precise enough to determine both the diameter of the entrance pupil and the focal length.
In fact you only need to know the value with an accuracy of, say, 5% i.e. an error of 5 mm on 100mm, something easily achievable manually with any ruler.

[seawolf66]

Thank you again Ole Tjugen very much

As a rough guide, insert thirds between f:8 and f:11. 8 +1/3 stop = f:9, 8 + 2/3 stop = f:10.

4.5, 6.3, 9, 13, 18, 25, 36, 50, 72, 100 and so on should be familiar to all who use old German lenses - especially as max aperture!

5, 7, 10, 14, 20, 28 and so on is less familiar, but possible on old French lenses.

[seawolf66]

will read over this thread again and the other thread for that information on old lens : Thank you very much

Hello !

The question of determinig the f-number of an old lens has been raised on this forum before.

If you consider that you know the value of the focal length, the diameter to be measured is not the actual diameter of the waterhouse stop itself, but the diameter of the entrance pupil i.e. the diameter of the image of the f-stop as seen from the entrance of the lens through the first lens groups.

f-number = (focal length)/(diameter or the entrance pupil)

It is likely that at the time when this lens was fabricated, people had no idea of the role played by the entrance pupil.
For this unknown old lens, I have no idea whether this can induce a significant difference in the actual f-number with respect to simply taking the measurement of the iris itself.

I had explained in this post how photographers did this measurement one century ago and how you can do it simply today.
http://www.largeformatphotography.in...pil#post850369

The quickest way to verifiy the actual value of the focal length of the unknown old lens is to compare on the ground glass the distances between two objects located far away, with a reference lens e.g. a 150 mm : 6" , and the unknown lens. If the lens is a 4" - 100 mm, the distance will be reduced by a factor 1.5. The measurement with a simple ruler can be precise enough to determine both the diameter of the entrance pupil and the focal length.
In fact you only need to know the value with an accuracy of, say, 5% i.e. an error of 5 mm on 100mm, something easily achievable manually with any ruler.

[seawolf66]

And to all others that have commented here I thank you very much

[Emmanuel BIGLER]

I add two references to earlier discussions regarding old engravings & old f-stop systems

one discussion here where Ole gave us the explanations about the old French f-number system
http://www.largeformatphotography.in...-Hermagis-lens

and an interesting wikipedia page obout the origins of f-numbers
http://en.wikipedia.org/wiki/F-numbe...ative_aperture
where you'll find a scan of an old textbook with a comparative chart of various old F-number systems.

I have merged this info into a single chart; sorry, the (tiny bits of) text are in French, but the chart is self-explanatory. It is part of an article I have written in French about pupils and related formulae.
the old F-number chart in pdf (one page)

And for those who can read French:
The article on pupils in html: http://www.galerie-photo.com/pupille...tographie.html

Plus a technical annex in pdf : http://www.galerie-photo.com/annexe-pupilles.pdf