This instructable is about building your own adapter to mount your mirrorless camera in place of film.
This isn't the first digital sensor mount I've made for a large-format camera; in http://aggregate.org/DIT/4X5/ I describe how I packaged the sensor from a cheap webcam so it could be used in place of a 4x5 film holder. It was way back in the late 1970s that I built a back to mount my Minolta SRT101 film SLR on the very same 4x5 camera. In fact, on eBay for between $150 and $200, there are now various sellers offering similar backs with the additional feature that you can slide your DSLR horizontally in order to create stitched panoramas. They don't seem to come with mounts for mirrorless cameras, but you can always just stick an adapter on the DSLR mount to convert it to your mirrorless mount. Sounds pretty good, right? So why build your own?
Well, first off, the unit described in this instructable costs more like $10 to build. No, the digital camera can't slide for panoramas -- but that's ok, because most large format cameras allow the lens and/or the entire back to do that, giving exactly the same functionality! In fact, the lens can shift in both the horizontal and vertical directions, giving better functionality. However, the primary motivation is more subtle and more important: the mounts of necessity force a DSLR to be quite far behind the original film plane (unless you can push your DSLR partly inside the open back of the camera, as in http://www.thingiverse.com/thing:18989), making it impossible to reach infinity focus with some lenses. The shorter flange distance on mirrorless cameras, combined with the lack of front-facing protrusions, allows them to come much closer to aligning with the intended film plane of the large format camera... so infinity focus is much more likely to be feasible and lens tilt and shift features are less likely to be impeded by mechanical obstructions.
Step 1: Stuff You'll Need
- A mirrorless camera (which should not be harmed by any of this)
- A large-format camera (which should not be harmed by any of this)
- A cheap extension tube set -- the kind with front, back, and three screw-threaded in-between segments
- Any tools required for removing the large-format back (typically a screwdriver)
- Material for the board; probably either scant board or plywood
- A saw and sandpaper to cut and finish the board to size
- An adjustable hole saw or other device that can make a hole in the board
- Paint and brush or other finishing materials
- Glue for setting the extension tube in the board hole
- Electrical tape or other light-sealing material (e.g., black paint)
- Thin metal/plastic and craft foam to make flat springs to hold the back in place
There is a good chance you have pretty much all the above -- except the extension tube set. You can get that on eBay for less than $7 shipped. They're made in China, but you can get them shipped from within the US for about $0.50 more.
Step 2: Back Where It Belongs
I think my 4x5 B&J is fairly typical. The spring clips -- and everything -- is held on by just two screws: one above and one below. Removing those screws exposes the metal bed upon which a cut film holder would be held. That bed defines the size and shape of the board we'll have to cut for our new back.
Measure the bed and cut a rectangular piece of material to fit. The easiest materials are either plywood or scant board. I used scant board because I like how it looks and I had some scrap that had been sitting for years. Beware using "new" scant board; plywood is more more dimensionally stable unless the scant board is well dried, and it is easier to find plywood in various thicknesses.
Step 3: There's a Hole
Take the board and hold it in place on the back. Open the front of your camera and remove the lensboard. Reach in and mark the corners of the film opening on your board.
Take the board out and flip it over to the side with the corner marks. We're going to use those marks to find the center of the lens -- which is also what you want to be the center of your extension tube. Use a straight edge to draw lines connecting opposite corners. Where the lines intersect is the center of the lens. Note that it probably isn't the center of your board because one side of the bed is open, and your board probably extends further past the imaging area on that side.
Now you're ready to drill that hole... but read the next step for that.
Step 4: Holey Precision, Batman!
The problem is that hole needs to be very precise in order to hold the extension tube, and your mirrorless camera body, in place. Drilling a hole with a very precise diameter is easy if you have a drill bit that size, but trust me -- you don't. The good folks in China who make those extension tubes don't want to waste any material, so the extension tubes are made precisely the diameter that's cheapest to fit to that particular lens mount, which is generally some random diameter in mm.
If you've got a lathe or a CNC milling machine, feel free to use it now. For the rest of us, it's time to find that adjustable hole cutter you bought when you had to replace the lock set on some door. The cheap hole cutters aren't easy to set very precisely, so I used a few iterations of trial and error to adjust mine testing with a piece of scrap wood. If you have a scroll saw, or even a hand jig saw, you could probably cut the hole with that. You want it to be a firm fit around the portion of the extension tube that will go inside it, but a little play is ok.
I used a drill press to drill the hole centered in the position marked in the previous step, but you can do this with any drill. Just be careful to drill straight down into the wood, Also, don't drill all the way through from one side; drill halfway and then flip the board and drill the rest from the other side, using the center hole as a guide. This will prevent chipping at the edge.
Step 5: Paint
I like natural wood finishes, so I gave the outside and edges two coats of clear, semi-gloss, polyurethane.
After that dried, I gave the inside two coats of flat black latex enamel. Shiny light wood isn't good in the light path.
Step 6: Which Way Is Up?
For most lenses, it really doesn't matter if the lens is sitting a bit rotated on the camera. It just makes it harder to read the distance scale on the lens. However, if the thread doesn't stop in the "up" position for your mount here, the mirrorless camera will be tilted when the large-format camera it is riding on is level on a tripod. That's not ok.
Put the tubes you're going to use together and place a bit of masking tape inside to mark where the top should be when the tubes are mounted on your mirrorless camera.
Step 7: Glue, Baby, Glue!
Take a piece of aluminum foil and place the board on top of it on a flat table.
Take whatever glue you're going to use and line the inside of the hole with it. I used a hot glue gun, which works very well, but requires that you work very quickly before the glue sets. Gorilla glue would be a potentially better choice, but you would need to clamp the board and the tube firmly for the complete drying time.
Orient the tube so that the alignment mark you made in the previous step is at the top. While pressing the board flat black side down, push the extension tube into the hole and flat against the aluminum foil on the table. This will ensure that the mount is perpendicular to the film plane as the glue sets.
After the glue has dried, simply peel-off the aluminum foil that stuck to the excess glue.
Step 8: Yet Another Inappropriate Use of Electrical Tape
Little pieces of black electrical tape over the seam ensure a good light seal no matter how sloppy the hole fit was. Actually, after using electrical tape, I realized that the precision of my cut was high enough that I could just fill the minor gaps with a little flat black paint. Just do anything that will work; I could even see packing a little wood filler into the gaps if they were really big... or perhaps just lay a bit more glue over the seam if your glue isn't translucent like standard hot glue.
Step 9: Spring Clamps?
Not having any flat springs, I faked it. I used two pieces of plastic (separators for a little parts cabinet). These do not flex like metal springs, but were easy to drill holes in. Screwed-into the holes on the back, they overhang just above the board we just made. I used a little piece of craft foam glued to these to serve as the spring. I'm not sure it will hold-up well, but it appears to work very well.
Ah, but shortly after posting this Instructable I found a better answer than the clear plastic parts: use a sample piece of countertop laminate! Free samples are available in many colors and textures, and this material is stiffer than the clear plastic and machines very nicely -- provided you put masking tape over it to prevent chipping when cutting. The laminate is still backed by a little piece of craft foam to serve as the spring. The extra photos added to this step should make it obvious....
Step 10: Conclusions (and Future Work?)
In case you've been wondering how you use the shutters built-into these large-format cameras and lenses, the answer is you don't. There is generally a "T" exposure setting which locks the shutter open until the shutter is triggered a second time. Simply set the shutter on "T", fire it, and use the shutter in your mirrorless camera for the actual exposures. There is generally no harm in leaving the shutter open in "T" for extended periods of time -- that's how you would have set it while composing images on the ground glass.
One more operating note: without electronics in the lens, mirrorless cameras will not even know you have a lens attached. Many will not allow you to fire the shutter without a lens attached unless you set some option buried deep inside the menus on your camera. You've been warned. Obviously, you don't get autofocus or electronic control of the aperture of the lens mounted on the large-format camera, so you'll generally be stuck with M or A mode (manual or aperture-priority auto exposure).
Take a look at the images here. The first two were shot with the 127mm, the third with a 19in, and the fourth is a crop from the third. I know Instructables doesn't allow very high image quality, but they look good, right? It may surprise you that the resolution per mm of lenses for large format cameras is virtually identical to that of lenses for 35mm cameras: usually about 50 line pairs per mm. That's partly because these lenses span a relatively narrow focal length range in which very simple designs perform quite well. In any case, resolution is unlikely to be a problem. Contrast is another issue; even coated large-format optics tend to be rather low in contrast, although that is easily corrected in digital post processing.
It is also worth noting that you can use the other end of the extension tube set to make a lensboard that has the mirrorless mount on it (e.g., E-mount) so you can use any of those lenses on the large-format camera. They will not focus to infinity, but they will focus very close and still be able to use the tilt and shift features of your large format camera -- these lenses might even cover 4x5 film or larger in the macro focus range. You'd build a lensboard for mounting smaller-format lenses pretty much the same way we built this back! Simply use the other end of the extension tube set with the #1 section. The lensboard usually needs to be thinner, and typically is held by something other than spring clamps, but it is otherwise the same construction. Not clear? Well, I've done an Instructable on that now too: https://www.instructables.com/id/Custom-Lensboards-For-A-Large-Format-Camera/