Extension back design questions...

Hi, all -

I'm wanting to build a 5x12 extension back for my Seneca Competitor 5x7. I'm trying to reason through the design a bit: I pulled the back on the Seneca. Measured out to where 8.25" is straight back from the lensboard (for infinity focus with the 210mm lens I ultimately want to get). Measured 6" from the centerline at infinity focus to establish width of the ground glass. Racked the standard in a ways. Laid a ruler against the centerline of the lensboard and against the inside back of the rear standard, and moved the standard back until the ruler intersected the 12" mark at infinity. That *should* give me the angle and depth I need to make the back, correct? The top and bottom is flat, since the 5" dimension is the same. This back is just longer side-to-side.

Am I missing something in the process? (I'm guessing I am.) Anyone have some sage wisdom for me a'fore I start in to resawing mahogany?

Thanks for any imput.
Scott

I've no idea what you've done :confused: I think it gave me a head ache

Take the opening you've got currently at the back of the camera. That's the front of the extension since the two bits need to match up. The back of the back is big enough for the 5x12. So lets say an opening of 7x14?

You've got a triangle. One side is the 8.25". A second side is half the 14" so 7". You've got a 90degree angle between those sides so you can figure out the third side.

Now go back to the opening on your back. That opening needs to fit the triangle some place. The depth of the extension must be enough to move that opening into the triangle.

I think I've got the geometry right but we need a math major or a high school student to check

In for a penny.

Once you've got the three angles. Take the current back opening use that for the back of a new triangle with the figured out angles. Then figure out how far it is from the len. The difference between that number and the 8.25 is the depth you need.

Now for sure somebody needs to check my math.

You rock, Nick!

Ok, here's what I got:


Assuming a 14" opening, I get an overall hypotenuse length, x, of 10.82". Using tangent, I get an angle, Ө, of 40.3°. Given an existing opening, minus 1/4" all around for the new back to sit in, of 7-3/4" (half of which is 3-7/8"), using sine I get a short hypotenuse, y, of 5.99", meaning the angled extension, z, is going to be 4.83" wide, and the overall depth is going to be 3.68".

Now, I've gotta do a reality check and make sure the assumed dimensions are good, but that looks about right, doesn't it?

Thanks nick!

Seem right. The only thing I'd add is draw a couple of more triangles for different lenses. Longer and shorter. You'll want to make sure the size you end up making works for more then the 210mm lens.

Ok, FWIW, I ran through s'more calculations today. Looks like a back sized for 210mm will give me some flexibility upwards. Too long, and the back becomes unwieldy. Too short, and the lenses become too expensive. 210 looks just right.

Need to figure out joinery, decide if I'm planing down mahogany or walnut, and get to building!