Wollensak 6"1/4 W.A. and Extreme W.A. aperture scale not equal to focal/iris

[Pierre 2]

I am the proud new owner of two Wollensak 159mm Wide Angle lenses for 8x10 :

6"1/4 8x10 Serie IIIa EX.W.A. f12.5 in barrel (serial 313657 - uncoated - no dots)

6"1/4 8x10 Series III f9.5 Wide Angle (serial 249391 - uncoated - no dots) mounted on Alphax shutter

After servicing the shutter of the f9.5, I noticed that the aperture scale on the shutter goes from f3.5 to f22 hereby leading me to think that maybe this lens was simply mounted on a shutter that was just available without due concern for the scale, even though the shutter is fitted with a plate that reduces greatly the maximum aperture that one could get from this shutter.

Since I have a close relative of that lens (the f12.5), I figured I could make myself an appropriate shutter scale for the f9.5 by comparing aperture diameter on both lenses, checking myself along the way with the f stop = focal length/apparent aperture diameter rule. Well, I was surprised to find out that my f12.5 barrel lens with original diaphragm does NOT conform to that rule, whether I measure the diaphragm diameter with or without the elements. Also, if ever the scale on my shutter is really a proper scale, the corresponding apertures would be quite different from the aperture observed on the barrel lens.

My uneducated guess is that the f stop = focal length / aperture diameter rule simply cannot be applied for such Wide Angle lenses. Anyone would care to comment / provide a simple explanation ?

Also, anyone with an Alphax mounted W.A. f9.5 would care to share the extent of the aperture displayed on his shutter and confirm that this f3.5 to f22 scale is indeed the appropriate scale for that lens (even though reading below 9.5 are meaningless) ?

Thanks,
Puzzled Pierre

[Steven Tribe]

The F3.5 scale is obviously a US F scale (probably a F6.8 Dagor!) so calculations aperture/fl/"F" will be different than when using real F.

[Jim Jones]

Steven may well be right. In the US system, the numbers double at each stop. Thus, US 2 = f.5.6, US 4 = f/8, US 8 = f/11, US 16 = f/16. US 32 = f/22, US 64 = f/45, etc. With this in mind one can measure the iris opening to see if the aperture markings follow the obsolete US system. The rule of the f stop = focal length / aperture diameter does apply to all lenses when the aperture diameter is measured through the front elements of the lens with any needed correction for parallax.

[Pierre 2]

Thanks Steven and Jim. Never thought I would have to check aperture rings as well as shutter speed (on a barrel lens anyway).

Solution to the f12.5 puzzle is as follows :

On the f12.5 Extreme W.A., the lens opening is indeed f12.5 if one brings the ring to the stop (well past the f12.5 mark) which means that the aperture ring is improperly set in relation to the actual aperture of the blades. Just checked that with what the constant mechanical offset I am trying to hold through the scale, the other indications are then correct.

Anyway, I remember reading somewhere here that someone was under the impression that his f12.5 could be opened up quite a bit wider than f12.5 marking - Guess that was just his aperture ring and that the f9.5 indeed has a clear advantage for focusing. Anybody with an f12.5 in barrel should probably check his ring for proper indication.

Thanks again !
Pierre, who will sleep better tonight.

Now, it so happens that there is what appears to be a fairly tight set screw on that ring. However, as I untighten this set screw, the aperture ring locks !!! update : have been able to loosen that screw which is not a "set screw" but a ring locking screw. Screw locking hole may be in improper location.

[Darren Kruger]

After servicing the shutter of the f9.5, I noticed that the aperture scale on the shutter goes from f3.5 to f22 hereby leading me to think that maybe this lens was simply mounted on a shutter that was just available without due concern for the scale, even though the shutter is fitted with a plate that reduces greatly the maximum aperture that one could get from this shutter.

I have seen a lot of Alphax #3 shutters that had f/3.5 apertures (from oscilloscope?) I want to say those were for 75mm lenses so the scale is probably way off. I have a Series III f/9.5 in an Alphax #3 where the engraved scale starts at f/1.9 but someone has put on a scale with tape that looks closer to being accurate

-Darren

[Dan Fromm]

The F3.5 scale is obviously a US F scale (probably a F6.8 Dagor!) so calculations aperture/fl/"F" will be different than when using real F.

Interesting if true. Why do you think this is obvious?

I ask because Alphax (so marked) shutters weren't made when the US aperture system was in use.

[Steven Tribe]

Interesting if true. Why do you think this is obvious?

I ask because Alphax (so marked) shutters weren't made when the US aperture system was in use.

Which is something I was clear about within hours of posting!
Ex-Oscilloscope shutter is obviously the correct explanation!