Wollensak 13" Velostigmat Series I Aperture Scale

[Jeremy]

Can anyone give me some pointers on using this triple convertible. I know that using both gives me 13" and an fstop of f/6.8, but what does the fstop become when using it in the 19" (I'm guessing) and 25" (also a guess from literature around the web) set-ups. This is also even more fun because I have a US scale to work from, but I am going to paste the f-stop scale above it so that will no longer be an obstacle.

 

[fparnold]

To a first approximation, treat it like a telescope objective. Measure the diameter of the element, and divide the focal length by that number, to get wide open.

In your example, 13" f6.8 implies a front element of 1.91" diameter. This seems about right, as that's close to the front diameter of my 12" f6.8 Wolly (non-convertible). So, 19"/1.91" gives you f9.9, wide open. Ballparkish, this would indicate that f8 on the current scale becomes f11, etc.

 

[jimgalli]

Hey Jeremy, congrats on a GREAT lens. It's the same as this one that I bragged about (there was a url link here which no longer exists). If it's the old Regno shutter, I can make you a scan but not until I get back to work on Monday. Hope you like yours as much as I like mine.

 

[Jeremy]

Hey Jeremy, congrats on a GREAT lens. It's the same as this one that I bragged about (there was a url link here which no longer exists). If it's the old Regno shutter, I can make you a scan but not until I get back to work on Monday. Hope you like yours as much as I like mine.

It's actually in a wolly autex shutter. Haven't shot with it yet as one of the elements had become decemented, but that's what epoxy is for


Boy am I going to get it for this one....

 

[HPorter]

Jeremy,

On my Wollensak 1A the apeture range is:

13" is f6.8 - f64
20" is f12.5 - f64
25 1/2" is f16 - f64.

Hope this is what you were looking for.

Harold

 

[Ole]

A lens with 13" focal length and f:6.8 will have an aperture opening of 13/6.8 or about 2". Since the focal length of the front group is as long as 25", you can safely ignore the "loupe effect" of this - the error in the calculations will be less than the error in even a freshly CLA'd shutter!

So 25"/2" gives f:12.5 when set at 6.8, and the 19" gives f:9.5. Remember that the single element should be used behind the aperture, and it's easy to calculate - for that matter, you could simply measute the opening of the aperture at different settings!

The US scale is fairly simple - US 16 = f:16, and the US doubles for each aperture step. So US 8 = f:11, US 32 = f:22 and so on.

 

[Jeremy]

Thanks guys! Now I just need to get the shutter working (or go with the ol' hat method). It didn't come with a flange, but the entire unit does screw into my Ilex #5... sadly it protrudes too far back into the shutter to allow me to actually use the Ilex #5, but this does require one less lensboard and I don't have to have a flange.