Want to make a Lens Extension for a 4x5 Double Polar Lens

Been using a Double Polar Carl Zeiss protarlinse vll with two f 350mm lenses on my 1950 Pacemaker 4x5, and Love it. But can only use two of the three aspects of the lens. With both lenses on, front and back, we get examples: (see below) l1, ll1, lll1 ... and l2, ll2, lll2 with only the front lens on.
Would like to try #3, with just the back lens on, but the 4x5 speed bellows will not extend that far of course, as the lens was really made for a 5x7 camera.

(Reference for both images)
https://web.archive.org/web/20170202074125/http://www.cameraeccentric.com/html/info/zeiss_4.html
This page #36


So need to make a lens extension. Never having done such a thing before, need some help.

To make the lens extension I'm going to need to know first how long it needs to be!
Now my Double Polar has two 350mm lenses.. which is not shown in the table below, but basically as explained to me back in 2012 on Graflex.org, that combined with both lenses its 180m, and will need apx. 7"+ to focus. Which is proven in use with this lens on the 4x5, and is close to what is shown in the table for a 350/285.
With just the front 350mm on, focusing on the 4x5 is 11+".
Now the table below shows with just the back 350mm on its needs at least 13.75" to focus.
When using just the one lens on the front, my bellows is just about fully extended when focusing on a close object, and is extended 13+". In the table below there is no reference for a 350mm lens on the front, but it shows for a 350mm on the back to be 13.75", about equal to what we have on the 4x5, but for the lens on the front!!!
With the 350mm on the back of the 4x5, and with the bellows fully extended, were not even close to focusing on anything.
So what gives???

Page #39


Several questions ...
A) Is there something I'm misunderstanding in the table?
B) If not, how does one figure out how long the lens extension needs to be, in order to focus with just the one lens on the back?

With lens mounted on a lens board, cell focal distance to be measured mounted on shutter in desired orientation, shutter open on T or locked open on B, aperture wide open, darken the room, point the lens at an infinity target through a window, place a light colored rigid material behind the lens and move the material until the infinity target is tack sharp. Measure from the projected on material to the lens, subtract the camera bellows length, the result is the necessary extra extension to just reach infinity, add an additional inch to inch and a half for closer focusing.

WOW... makes sense, had to read it several times.
Ok.. the lens is going to have to be steady and anchored. So if I take the Pacemaker, set the lens on the rear of the shutter, with the bellows totally in, as when the camera is set up to be shut, mount on tripod, with the back and ground glass off, and do the do.
That's so simple! Neat!
Thanks, will have to wait for daylight to give it a try.

Why? The Zeiss table you posted contains single Protarlenses and pairs with unequal cells. Your lens has two identical 350 mm cells. What will you gain by using one behind the board?

1901 Zeiss London catalog: http://books.google.com/books?id=cnkWAAAAYAAJ&dq=zeiss+unar&source=gbs_navlinks_s Click on "The Protarlens," scroll to p. 70 for the table.

Dan Fromm said:

What will you gain by using one behind the board?

Click to expand...

My understanding from the original source above, that I used in post 1, page 36, is that there would be a gain, as there was with just one cell on the front!
Your table is more in depth, but please exclaim to a newbie on all this, what are you referring to on that page p70, that shows any reference to a single cell? Front or back.
So could you please give a little more explanation of what I'm missing in your post, that is so obvious to you.
It's all part of a learning experience and a little experimenting helps in understanding and I thank you for clarifying what were missing in our understanding.

Well getting educated: Got it.
Did a quick setup as described in post #2, and there seems to be no difference between the two images.
So your right. The only difference is that being on the back, it needs 15+" to focus, which is only the difference really in the lens body and position of the lens being further back.
I'll be darned. Makes sense.. as you stated, there both 350.. so to get the result shown on page 36, post #1, these lenses were purchased with two different focal lengths, when new. In fact, on page 37, in explanation of the images given on p36, they state this, with the ratio between the two lenses given.
Ha.. what a gas.. so simple, yet perplexing.. when not understood right off, and misinterprets what is obvious. Life

Thanks again to all... so this project comes to an end.. no lens extension needed.