I was about to order some bag bellows, but remembered that I had some BK-5 blackout cloth (ThorLabs) left over from the last bag bellows experience. I made a few quick bellows frames from Baltic birch ply, laid out a few patterns and was done almost before I realized what was happening.
Template out of WC paper. I fold it in half and lay out and cut from one side only to be sure the pattern is symmetrical. I flared the middle of the template a bit to allow the lens board stage to nest inside the bag a little easier. The 1/2″ margins on the sides are for the seams between the panels; front and back they provide the excess needed to attach the bellows to the frames. For a 5×7, 9″ overall length is about right, including the margins. I located the flare transition a little past the half-way point, 5″ from the back.
I’ve made this style of bellows before, so didn’t bother making a mock-up out of paper, but this would probably be a good idea for a first attempt to get a handle on how it will come together.
Sandwich of 2 layers of BK-5 cloth, with a layer of black ripstop in between. One layer of BK-5 isn’t completely opaque, so it needs to be doubled up. I added the ripstop in between so the glue surfaces weren’t rubber-on-rubber; BK-5 is a little picky about which adhesives it will stick to. BK-5 is also very thin, so I taped it to the tabletop to get it as wrinkle-free as possible. Positioned the ripstop on top, then flipped half of the ripstop over a screed bar, like an open book, then sprayed the mating faces with adhesive (RCD #52). Hard to see the aluminum screed bar in the photo above, but using the bar I drew the cloth together in one smooth squeegee motion. After rolling it out with a rubber brayer, the process is repeated for the other half. Then I added the final layer of BK-5 in the same way. The results will only be as wrinkle-free as the fabric itself, so running a cool clothes iron over the materials first is helpful- but don’t iron the rubber side of the BK-5, even a ‘cool’ setting may melt or distort it.
With all three layers, the material is a little over ten thousandths of an inch thick.
Template is used to cut the 4 panels. Working inside out, I joined the seams using double stick tape. This is really simple to do, but tricky to describe. I did the 4 lensboard-side seams first, then the 4 back seams last. With the good side of the panel facing up, I applied the tape to the right-side seam only, then positioned the next panel directly on top, outsides of the panels facing each other, and aligned the edges. Turning the bellows clockwise I repeated the process for all the front seams, then repeated for the back. Once done, the bellows are inside out.
Before inverting, I cut 3/4″ strips from the scraps of the fabric laminate, and applied double stick tape to each side of each seam, and wrapped each seam with the laminate strips, stopping 1/2″ front and back to leave flexibility in the corners for attaching the frames. This step is important- the seam cover provides an extra layer of light-proof cloth and extra strength as well. It also gives the bellows enough rigidity to keep them from collapsing and blocking the lens during an exposure.
Flipped right-side out. Before attaching the frames I pressed each of the seams in a vise with plywood soft jaws to get a better crease. This worked a lot better than sheet metal vise grips.
Joining them to the frames was a little tricky at the corners. I rigged a small pedestal with a board the size of the front bellows frame screwed to the top, then taped the bellows opening squarely to this backer board with just enough masking tape to cover 1/16″ of the bellows opening. Then I applied the double-stick tape to all edges, peeled back only the corners, and adjusted the position before removing the rest of the tape backing.
Obviously this won’t work for the back because you’d never get the backer board out, so for the rear frame, I covered the margins of the frame with double stick tape, then only exposed the bottom taped edge. I seated the bottom edge of the bellows fully, then slowly peeled by the paper backing at the corners, and then the rest of the way up as I worked up each side in tandem towards the top.
It would have been much easier to attach to the frames if I’d pre-creased the 1/2″ margins at the openings with a bone folder, or maybe even put a little card stock backer in those margins before joining the panels together.
Silly amount of rise, but good to know there will be no stress at all during normal amounts of wide angle movement. Also get a fair amount of extension out of this, might be usable up to a 180mm lens.
Fits in a ziplock bag when folded up. I have a lot of these bags.