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Bag Bellows

The last bag bellows I made was an ongoing disaster. Unable to find a replacement for the blackout cloth that Porter’s Camera used to carry, I pulled apart some old bellows screw-ups with lacquer thinner and cut them into pieces to be stitched together. Trying to remove the glued-on paper stiffeners from the pleats pulled off some of the vulcanized rubber coating from the material as well, and the resulting bag bellows leaked like a sieve. I tried to patch the interior by laminating more scraps of blackout cloth to the interior and when that didn’t work very well I painted the whole inside with liquid electrical tape. The result was a  humid, out-gassing and shapeless mass with a semi-gloss interior that, while light tight, still flared unmercifully even in low light situations from lens light scattering around inside.
Some camera-builders have been experimenting with a Thor Labs material called BK-5 for bellows. It’s a nylon material with a vulcanized rubber coating on one side that is very similar to the Porter’s cloth. The rubber coating is thinner, so it’s not as light-proof as the Porter’s cloth, but the reports are that when the fabric is doubled up it is light-tight. There also have been issues with finding the proper glue; typical contact-cement and spray adhesives like Spray 77 do not work very well. Also, the stuff is a bit shiny, so a matte non-reflective liner would be needed for the inside to keep light scatter down. That this stuff needs 3 layers of cloth, that glue doesn’t stick very well, and that it’s fairly expensive are a few of the reasons why it’s taken me over 3 years to even try this stuff after first reading about it.

I used to make a type of bag bellows that is essentially a closed pillow case with frames attached to the front and back. It’s very simple to make, but it needs to be very big on order to have any extension at all. I’ve experimented with making bellows that have hexagonal panels in the past- that is, panels that get bigger in the middle, but the problem is the many seams and potential for light leaks. A pillow-case style only has three seams, if you fold over the top from a large piece of cloth. A hexagonal style bellows can need up to 8 seams, which can complicate light-proofing. With a big enough piece of cloth however, many of the seams can be replaced with simple folds.

First pattern in card stock. I’m glad I made a mock-up because the first pattern was too small at the rear frame, only 8″. For my 5×7, the actual dimensions are 8 1/2″ for the rear frame, 5″ for the front, 11″ overall length including the folds to attach each end to the frames. The widest part flares to 12″, 5″ from the rear frame. I lay out the full pattern, but fold it in half before cutting out to make sure it is symmetrical. A slight oddity can make bellows twist when assembled.

A paper mock up pattern was made first to check the geometry and practice assembly.  Note that at every seam, 3/4 is added to the panels to give plenty of glue surface for the joints. Also helpful to crease the folds and seams beforehand.

Once it all works out, another full size pattern is created to simplify laying-out on fabric. You could probably just disassemble the mockup if the paper is durable and no changes need to be made to dimensions.  How to trim the corners to attach to the frames can be bewildering after it assembled, so I tried to account for that in the patterns.  The small squares in the corners of the bottom of the pattern are cut out; this will allow the pattern to be folded 90 degrees inward to attach to the frame. The front frame folds are a weak point of this pattern- because of the intersecting angles they have much less overlap and glue surface than the rear, but more about this later.

Adhesives to practice with.

The most stressful part of the day was laminating the BK-5 cloth. It’s thin, tends to buckle when adhesive hits it, so I stretched  it fairly taught and taped it down to stabilize it. I used 2 pieces instead of folding a single larger piece in half. The pieces are laid rubber-to-rubber and aligned as they will be once glued. The top piece is folded back several inches to expose a full edge along each piece. Th edges are sprayed with glue. After this edge is laminated, the top piece is folded back open like a book to expose the rubber side. Both pieces of fabric are then sprayed. I used the RCD 52 for the fabric laminations. It comes out of the nozzle like spider webs, in long strands instead of a mist, and doesn’t soak and buckle the material as much as Spray 77.  An aluminum bar (1/8″ x 2″x 36″) is slid under the top piece as a sort of screed bar, then I pull it evenly towards me making sure the fabric doesn’t bunch or wrinkle along the leading edge of the bar. It’s sort of a leap of faith since you can’t really see what’s happening, but the results were good. Might try another method next time, but for the ease and speed of laminating the fabric it works well enough. Maybe a better way would be to stretch the material on some frames, like canvas, then align the frames and bond  together. Anyway, after the BK-5 is bonded, the linen liner is bonded to the BK-5 in the same way.

Pattern is taped down to the fabric, then cut out. I didn’t bother tracing, just cut directly with the pattern overlay. After cutting, I did mark the fold lines on the linen side for reference. Before assembly, I ironed creases in the frame attachment fold lines. I also masked of the nylon side where the seams would be joined. Next time I will probably iron ‘hems’ in there as well so the seams are better defined. One issue with this method is it’s not practical to put stiffeners in to reinforce the frame attachment folds. I tried to peel back the linen liner to fit some stiffeners in after it was cut out, but the glue held too well. Stiffeners would make attaching to the frames much easier. Or at least iron clear creases so the folds are defined.

Seriously? Carpet tape? Nothing else I had on hand  could match the strength of this fiber-reinforced tape. It’s not really tape though. It’s much thinner- it’s like a membrane of glue in tape form, and a little difficult to work with. I only used this for the seams. I would have used it to attach the frames as well, but it was just too tricky to handle. The backed-tape is applied to the seams, trimmed, the backing pulled off, then bonded. Works great. For attaching  the fabric to the frames I used pliobond, but any contact cement would probably work as well with nylon outer-side of the BK-5.

Front detail- due to small corner overlap at the front frame, I cut a 6″x6″ square of the fabric lamination ( left over after cutting out the pattern) and applied it with carpet tape over the front frame, then trimmed out the circle. I use bookbinder’s tape for reinforcement around this panel, and also around the rear frame. Not only for extra strength, but to guard against the fabrics de-laminating over time.

 

It’s compact when folded, but balloons out nicely. Regular 5×7 bellows for scale.

With 3 plies total, the fabric is bulky. I probably wont try BK-5 for pleated bellows. The extra rigidity is nice for bag bellows though, it won’t sag and block the lens. With the 60″ x 3 yard minimum order, there is still plenty left, even after doubling it up. If I make another one I might make the flare in the middle a little narrower, and the overall length at little longer to make it easier to fold it up in the camera and use a wider variety of lenses. I’ll probably go back to individual panels too, since the carpet tape worked so well for the seams. That way I could put stiffeners in is as needed, and leave extra material in the pattern for glue surface in the front corners.