Making Another Expansion Back - 8x20 for 11x14

[Fragomeni]

Hi all,

A little over a year ago I made an 11x14 expansion back for my 8x10 Deardorff V11. I am now interested in making an 8x20 expansion back for my 11x14 Deardorff V11. My original plan was to make a 14x17 back which would be rather easy because of the proportions but if I can do it with an 8x20 back then I think I'd like to do that. The last project ended up being an expansion back with its own set of bellows and its own rear frame that was mounted to a separate tripod. This time around I would like it to be a rigid unit that fastens directly to the rear standard without the need for an additional tripod. This would be easy to fabricate using a 14x17 spring back but will probably prove a little more challenging with an 8x20 back. I'm thinking about using an 8x20 back traditionally for a 12x20 camera to make the attachment. My concerns are making sure the weight of the expansion back would be light enough to ensure that it doesn't stress the rear standard of the camera and cause damage. The dimensions of 8x20 would be a little more difficult to pull off because of the more extreme dimensional difference between the longest sides. I'm wondering how deep the adapter would need to be in order to ensure no vignetting caused by the dimensions of the V11 rear frame. There is effectively 3 extra inches on each side to go from 14" to 20" but I'm wondering if there is a relatively simple way to figure out the necessary depth without having everything in front of me. I don't want to buy the stuff before I know if it is feasible.

Many people here were extraordinarily helpful with the last expansion back project so i wanted to bounce this off of the group before I begin. So any ideas on how to pull this off? Ideas of how to figure out the depth? Think weight will be too much? Thoughts? Thank you!

[John Jarosz]

The shortest focal length lens will determine your internal clearances. For 8x20 the shortest you are likely to use is 300mm lens. I'd extend the short side of the bellows (along it's internal angle while you simulate a 300mm lens to film plane distance) to intersect with your film plane. That will give you the back distance space you must accommodate. You then have to confirm that your bellows will compress enough to allow infinity focus at 300mm focal length. I'd make an 8x20 cardboard rectangle. Then I would support it at the center from a dowel extending from the lens mounting hole. Using the extended plane from the inside of the bellows to the film plane will tell you how much space you may or may not have for movements. That's how I'd do it.

John

[Fragomeni]

Thanks John. I wish I had some drawing to go along with your explanation to help solidify it in my mind but I think I got the gist of it. I was able to snag an 8x20 back for a great price yesterday so now its just a matter of waiting for it to arrive. I'm confident this will be much more successful hen the last one.

My lenses for 11x14 are a Nikkor-M 450mm and Fujinon-W 600mm and those are what I plan to use with 8x20 as well.

[Oren Grad]

My lenses for 11x14 are a Nikkor-M 450mm and Fujinon-W 600mm and those are what I plan to use with 8x20 as well.

This may not be ideal. Think about the geometry: the further the front standard is extended to attain focus, the further the film plane of your expansion back will need to be pushed back from the rear standard of your camera to avoid cutoff. By the same token, be sure to simulate actual focusing distances that you intend, not just infinity focus.

To turn it around, the shorter the lens, and thus the closer it sits to the back of the camera when you are in focus, the easier it is to avoid cutoff.

EDIT, to elaborate: In general, with expansion backs, you're stuck between two problems: image cutoff from having the lens too far away, and bellows compression preventing you from bringing the lens closer. That's why products like the inexpensive 6x17 rollholders sold for 4x5 cameras have a narrow range of usable focal lengths.

[John Jarosz]

Yes, Oren has a point. You need to simulate both the closest position of the front standard and the furthest.

My other comment would be is that while I use my 420mm lens for 8x20 a lot, the 300mm lens gives very interesting images. I would try to preserve that capability (300mm) if at all possible. For the 'grand vistas' that are possible out West in the US I can see a need for lenses longer than 600mm (but those longer lenses are $$ and the logistics are tough).

[Fragomeni]

Great advice guys, thank you. You're making me think of things I might have overlooked. I'm not overly worried about the lens focal length issue. I plan to use both the 450mm and the 600mm as those are the lenses my 11x14 kit is built around and I don't have a 300mm that'll cover 11x14 or 8x20 and I'm not interested in picking one up at the moment. I've seen 8x20 work done with both of these lenses and they match my eye. The trick will be to get the geometry to work for me.

I've got a few drawings sketched out and I've looked at things on a smaller scale using my Deardorff V8 and a roughly to scale mock up of the back. I've attached one of the drawing here. Sorry about the poor image quality, all I have access to is my iphone at the moment. Essentially, the main elements of this are; 1) the longest lens's focal length and the subsequent extension of the camera at that focus distance, the extension of the bellows at the closest focus distance I intend to use, and the lenses angle of coverage at infinity and at the closest intended focus distance. Those factors will determine the necessary depth of the expansion back to prevent any clipping. I need the camera and back in front of me to get any real idea of numbers so I'll be waiting eagerly for the mail.

If you guys or anyone else sees any flaws in my thinking or has other ideas or insight please do share. Things like this are always learning experiences

[Oren Grad]

You've got the right idea re the geometry.

Hmmm - here's a back-of-the-envelope calculation, thinking about similar triangles defined by what's needed to just clear the rear standard. I'm completely brain-dead right now, so I may mangle this - somebody check me. For a 450 at infinity: Figuring a width of 500mm for the expansion back and a rear standard opening of about 350mm, the back will have to hang out 450*[1-(350/500)] mm, or 135mm. For a 600 at infinity, it would be 600*[1-(350/500)] mm, or 180mm.

[John Jarosz]

Here's the way I look at this problem. Top view is the unmodified 11x14 showing the 600mm and 450mm lens in correct position and to scale. In both views you are looking down on the camera (in section) along the long film axis. The bottom view show the two lenses correctly positioned imaging onto a 20 inch wide film. The bottom sketch shows the add-on rigid extension with the bellows compressed. (The bellows representation is a cartoon - I did not attempt to show the actual number of folds in the bellows properly, it is only a representation). Oren's numbers are approximately correct, the actual values will depend on the hardware design details.

[Fragomeni]

Oren and John, let me just say thanks so much for contributing to this! You guys have been exceptionally helpful and I appreciate it.

Oren's math makes sense to me so I'll take actual measurements off the actual equipment to get the exact numbers when everything arrives.

John, your sketch looks a lot better then mine! Thank you! I'm putting a copy of this in my notebook. Do you mind if I use it when I make a post about this on my website later during the project?

I used my 8x10 with the back removed and a yard stick inside the camera along the approximate projection plane of the lens and then racked the front and rear standard in and out to get a visual of exactly whats happening with the angle of coverage as the focal length shortens and lengthens. This was a useful exercise to really get it in my mind. I hadn't thought about it like this before but now seeing exactly where the angles are going in three dimensions I'm much more comfortable with how I'm going to approach this

[John Jarosz]

Glad to help in a small way, Francesco. You can make use of that sketch in any way that will help your project. Good Luck with it.

John