wollensak lens info

[anthony marsh]

I have a Wollensak lens that came on a 5x7 SEROCO ca. 1900.It has a piston on either side ,speeds 1 sec 1/2 1/5 25 50 and 100 B T on a round dial on top,lens speed f4.The barrel reads 5x7 extra rapid convertible.What I would like to know is, what is the focal length?Since it says convertible does this mean that there are different front elements for it?

[Chauncey Walden]

I'm guessing that those are US stops (4, 8, 16, 32, 64, 128, 256) which makes it an f/8 lens and a rapid rectilinear, probably of around 210mm. Removing the front component will increase the focal length (and soften it up).

[anthony marsh]

Chauncey,the stops read 4,8,16 and 32.Perhaps I wasn't very clear in my description.There is only one component,no rear component. Would a lens of this vintage(1900 patent date) have only one element?

[Ernest Purdum]

Your description suggests that someone removed a cell to "convert" it and then didn't put it back. The name of the lens means that it had two cells when made.

It is customary to remove the front cell, not the rear, when converting.

[Chauncey Walden]

And, since the component you have has all the info marked on it, someone has removed and lost the rear component. It may still work out fine. Try it!

[Steven Tribe]

But, of course, the F scale will not be correct for the single cell - unless there is a second row of figures!

[Chauncey Walden]

Anthony, in a darkened room with a window, hold the lens on a yardstick pointed out the window and move it away from a white sheet of paper until an image forms on the paper of something in the distance. Check the distance of the lens from the paper - your effective focal length. Now measure the apparent opening of the aperture through the front of the lens and divide this into the focal length to get your real f stop. Mark it on the lens barrel with a Sharpie. Do it for all the original US stops. That's half the battle. Determining the real shutter speeds is more complicated but you can almost be guaranteed that they won't be as marked on the old shutter - except B and T, of course. Put it on a camera and play with it.

[eddie]

it is 7 inches combined (front and rear element) and about 14 converted (using the rear element only). it should have two scales....i can not remember for sure. probably about f12-14 using only the rear.

that is the old system. f4 is f8 in today's system.
eddie

[anthony marsh]

Eddie,the lens has only the front element.It seems the rear element got lost sometime during the past 100 years,Tony

[eddie]

put the rear front on the rear....it is symmetrical....