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 (http://www.largeformatphotography.info/forum/showthread.php?74053-Enlarging-Back-(as-opposed-to-reducing-back)-Project). 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 :)

71325

[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! :p 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

[Oren Grad]

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 :)

Excellent! Looking forward to seeing where you go with this...

[Monty McCutchen]

Even though I have nothing to add to your project, this thread exemplifies why this community is so great. Sharing at its best. Bravo!

Hope to see some wonderful work from your camera's Fragomeni

best,

Monty

[Fragomeni]

Even though I have nothing to add to your project, this thread exemplifies why this community is so great. Sharing at its best. Bravo!
Exactly!

[Michael Roberts]

I would make a 14x20 expansion back capable of 12x20, 8x20, and 14x17. At least that's what I've been planning for some time.

[Fragomeni]

My original intention was a 14x17 expansion back because of the obvious dimensional reasons but then I saw some extraordinary 8x20 images and I couldn't get the format out of my head. I believe in keeping the number of things to think about at an efficient minimum so I have no interest in playing with a slew of format sizes and then the cost of all the various holders and film sizes needed. The V11 does 11x14 and 8x10 which realistically is enough for me but if I can produce a back that'll do 8x20 as well then I'll be even happier. Weight is also an issue as I'm making a single piece back which attaches and needs to be supported by the hardware supporting the rear standard. If I were making a separate back like I did last time the a larger back might not be a problem.

[Michael Roberts]

Keeping it simple is always a good idea. OTOH, there's something to be said for planning ahead and keeping options open. Seems to me you are already talking about 14x20 for max height and width for film. Having an option to add a 12x20 spring back in the future has zero additional cost at this point. I just happen to prefer the 60-70% aspect ratio to 40% or 80%, but to each his own--and there's certainly a time and place for each format.

In any event, what I have in mind is a rigid 16x22 rectangular expansion back (or 15.5 x 23.5 inches depending on the outside dimension of the 11x14 camera this back will attach to). My plan is for one rectangular section that is 9 inches deep. This will allow lenses as long as 24 inches w/o vignetting. Then a second, four-inch deep add-on expansion spacer for lenses of 30 and 36 inches when needed (what I have in my kit).

I've already made the 8x20 and 12x20 spring backs. The expansion back itself will be a breeze. Here's a similar expansion back I made to go from 8x10 to 7x11: http://www.largeformatphotography.info/forum/showthread.php?62789-adding-a-7x11-expansion-back-to-an-8x10/page2&highlight=7x11+expansion
See posts 11-13 for the expansion-spacer.

The only difference will be a need to fill in the sides on the front end that attaches to the 11x14 camera to fit a 15.5 inch square frame.

Like you, my first expansion back (11x14 to 12x20) included an additional bellows: http://www.largeformatphotography.info/forum/showthread.php?47057-Ultra-light-DIY-ULF-Camera/page2&highlight=ultra-light+diy
See posts 15 & 16.

Having passed on my 11x14 DIY camera and acquired a King 11x14 (http://www.largeformatphotography.info/forum/showthread.php?36782-Show-off-your-camera!&p=745226&viewfull=1#post745226), like you, I am now planning a new expansion back that will be a rigid box.

I will again be using balsa wood, either completely covered in a thin 1/32 mahagony veneer or with the mahagony veneer just to reinforce stress points (i.e., joints). Balsa wood weighs practically nothing. It can be quite strong depending on the thickness and the angle at which it is stressed (it's used in making surfboards among other items). You just need to use a harder wood wherever there will be a pin or a bracket attached.

If you are interested in more info, let me know. If not, best wishes on your project.
Michael

[Michael Roberts]

focal length diagram, 1/4th scale

[Michael Roberts]

Francesco,
I just reread your OP and realized that you are no longer interested in 14x17; sorry for any confusion. Nevertheless, you might want to consider keeping the height of the entire expansion box at the same height as the V11 back. If you attach a 9.5x21.5 or 10x22 box to a frame that "sticks up" to 15.5 or 16 inches to match the V11 height, this could be a weak point in the design. Seems like you might wind up trading off increased weight of a bigger box for increased fragility (likelihood of being damaged when stored or handled) of a narrower box on an outsized frame. Balsa would allow you to use the bigger box with little weight added, but if you use a heavier, harder wood you would have to limit the weight/size as much as possible.
Michael

[Fragomeni]

Hi Michael,

Thank you for chiming in with all of this information. There is a lot of really good information in your posts. I think I'll take your advice and use Balsa as weight is obviously a factor and I'll probably end up making this entirely by hand and wont have the tools to work with a harder wood. Balsa with a mahogany veneer should be plenty strong for this. The expansion back will be of an appropriate size to accommodate a 12x20 spring back. The back that I got is actually a 8x20 reducing back for a 12x20 camera so I'll build the expansion back around the original 12x20 dimensions rather then cutting the back down to save a small bit of weight. Aside from that, I think you're right about a bigger box vs a smaller box that attaches to a larger frame and the subsequent weak points that would result. Attached is a picture of the 8x20 back that is on its way.

Would you mind sending me the focal length diagram via email? I can't see if very well in the forum image viewer and I'd like to take a closer look. Also, can you go over how you worked out your numbers in the diagram?

71377

[Michael Roberts]

email sent. Here is the procedure I used:
1. draw a line on the right side to represent 20 inches--min coverage needed for 12x20--the line is five inches long, so this is 1/4 scale
2. draw a centered, perpendicular line to represent distance to lenses
3. mark a point on the horizontal line 1/4 of the length of each lens f.l. you plan to use on the expanded back, e.g., 450mm = 18in; 18/4 = 4.5 in
4. draw lines from the lens point to the ends of the film plane on the right side, i.e., to the top and bottom points; these lines will look like rays
5. for this diagram, the 14 inch opening of the 11x14 camera was the vignetting limitation. 14/4 = 3.5 inches. So, use a ruler, parallel to the film plane and move the ruler to a point on each "ray" from a given lens to locate a 3.5 inch line that just reaches from the top to the bottom of each "ray." Draw this line; then--on the center line--measure the distance from the intersection of this line with the center line back to the film plane on the right. Multiply by 4 to determine how far your film needs to be from the back of the 11x14 to avoid vignetting.

I drew this out some time ago. Now that I am looking at it again, it looks like a 7.2-inch deep box would be sufficient for a 24-inch lens to focus at infinity; my plan was to use nine inches to allow focus at closer distances, though I would check this out on my 8x10 or 11x14 to see if an additional 1.8 inches is enough before starting construction.

[Fragomeni]

Wow Michael, thanks for this explanation! I really like this approach and I understood you instructions perfectly. This will be hugely helpful!! Definitely adding this to my notebook!! :D

[Fragomeni]

So I'm going over everything thats been discussed so far and I'm looking at Michael's diagram and the instructions he posted. I should be able to make one diagram that will give me all of the information needed to determine the depth of the expansion box needed to prevent vignetting at any focus distance that I'll use with any of my lenses. Essentially, I just take the longest lens I intend to use, 600mm, and extend the bellows to the closest focus distance I intend to use with this lens and use that as the focal length measurement for the diagrams and measurements rather then the focal length at infinity. If I work out the math using the focal length at the closest focus distance for the longest lens then everything else for the same lens at shorter focus distances to infinity and all focus distances for the shorter lens should be covered. Does this sound like a correct assumption to make?

[Michael Roberts]

Sounds right to me.

[Oren Grad]

Does this sound like a correct assumption to make?

Yes. The only trouble you might conceivably run into is that if calculating for a 600 at closer-than-infinity focus results in having a box that extends well off the back of the Deardorff's rear standard, you might run into a bellows compression constraint if you decide you want to use a much shorter lens like a 300.

Go ahead and do the calculation - you'll know if you have a potential problem before you start to cut anything.

[Fragomeni]

Just a quick update. I've got the camera and the 8x20 back on hand now. I visited with a friend who is going to help me build this. We'll probably use Birch for its light weight and a veneer of some sort for extra strength. Anyway, I made all the measurements and drew up the diagram which is attached (hopefully you can see it as I had to photograph it with my iphone again). In order for my longest lens that I intend to use (600mm Fujinon-C) to focus at infinity the expansion back needs to be 7.5 inches deep. For the lens to focus at the closest focus distance (max bellows draw is 42 inches), the expansion back needs to be 12.6 inches deep. Now we just need to figure out how to make it light weight as to not stress the rear standard. Birch should be a good option I think. Balsa would obviously be the lightest but its so soft that I worry about it.

72308

[Fragomeni]

So after looking at the dimensions and assessing them while looking at the camera I think I'm going to nix the project. The problem is that I like longer lenses and longer lenses necessitate a deeper back which is bigger and weighs more. This concern was brought up earlier in the conversation by a few I believe and it has indeed become the deciding factor. I've begun the last minute search to see if I could find an 8x20 camera anywhere but funds are running low at this point. I'll keep the 8x20 back and my friend who was going to help me with the build has agreed to help me build a whole 8x20 camera around it later on. He built his own ULF camera so he has the necessary skills.

Huge thanks to all who contributed to this discussion! Obviously I wish making the back was more feasible but this was a great learning experience! Thank you!!