Making Another Expansion Back - 8x20 for 11x14

[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.in...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.in...ltra-light+diy
See posts 15 & 16.

Having passed on my 11x14 DIY camera and acquired a King 11x14 (http://www.largeformatphotography.in...l=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?

[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.