4x5 to 8x10 adapter video [Archive] - Large Format Photography Forum

Michael Roberts

26-Dec-2015, 09:46

Really nice video, Tony. Thanks for sharing!

I realize you have now upgraded to an 8x10 camera, but for those viewing your video and thinking of building their own expansion back, I have two suggestions:
1. put adhesive felt on the runners (frame) where the film holder will rest to help block light leaks from imperfections in the wood/build.
2. measure out the distance you will need with various lenses ahead of time to make sure your smaller camera back does not vignette. See the link below for how to do this.

Here is a link to a thread on expansion backs: http://www.largeformatphotography.info/forum/showthread.php?120318-fabricating-wooden-camera-backs-reducing-backs-expansion-and-enlarging-backs&highlight=11x14+expansion+back

And another thread: http://www.largeformatphotography.info/forum/showthread.php?89071-Making-Another-Expansion-Back-8x20-for-11x14&highlight=11x14+expansion+back

Here is the procedure I used for an 11x14 to 12x20 expansion back:
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.