Part three in the journey of building a camera, to be exact a 4×5 full frame field camera.
Having abandoned the idea of an aluminium sandwich between wood as the base I decided to start on the camera back. Perhaps the most complicated part of the whole build. Complicated because there are several parts that must go together and also accommodate a film holder. Moreover there is a part of this build that must be exactly 4.8mm but more on that later.
Having chosen Jarrah as the wood, not something I would choose again as it tends to chip and the grain can be difficult to work when doing touch up planning, I started the build.
I built the main back frame using finger joints and then inserted a mitred frame inside. The description I was following suggested plywood for this. However buying a large sheet just for a small square of plywood seemed to be somewhat of a waste. Next it was build the film holder back that goes into this. It is made separately so that one can change from landscape to portrait mode. Finally it was make the glass plane holder. Again all three parts do need to be reasonably accurate including a small groove in the film holder so that the film tray can lock in and also provide a light trap. The last part of this build was to make some springs to hold the glass back in place but still allow for the film tray to be inserted. I thought that when I had nearly finished it that I had made it too small and the part would just fall off. However the spring mechanism, simple as it is works well and holds the glass frame in place as long as I am gentle.
Now why the 4.8mm measurement that I mentioned earlier. Well the focus plane, ie the glass back has to be perfectly aligned to the film plane other wise all of the shots will be out of focus. The film tray has a nominal depth of .197 inches which converts to 5mm, or very close to that. After the film thickness if taken into consideration the depth becomes .192inches or 4.8mm. The plan suggested getting some 4mm plywood and adding a .8mm brass shim. However when I measured a sample of the plywood it was not 4mm it was 3.2. All in all this made it too difficult. So looking at the brass I had bought for making the hinges etc I realised that the brass had a nominal depth of 4.76mm with a tolerance. As luck would have it the brass when measured on the vernier callipers was exactly 4.8mm. So it then became a simple solution of mounting a thin wooden frame on two strips of brass. Problem solved.
The final step was to shellac the wood a few times, rub it back with some very fine steel wool and then use beeswax to give it a soft lustre.
Here are two shots of the built back, minus a few brass pieces that I have to make to hold the parts together.
Is it perfect, no it is not. Does that worry me, yes a little however I am realistic about my woodworking skills. Some parts of woodworking require years of practise which I don’t have so I have to put up with little defects here and there. However in the end the camera will work, it will look ok and is something that will be a conversation piece when out using it.
The next steps are to make the lens board holder and then the base, followed by the brass bits to hold it all together.
Oh the glass panel is frosted. Learnt how to make that today. Had to but some very fine grit, #320 carborundum and #500 aluminium oxide. Add a small amount of the 320 with a drop or two of water to make a watery paste and the rub the glass against another thicker piece of glass until it is frosted. Wash it clean, turn the thicker glass piece over and add the aluminium oxide plus water to make a fine paste and rub some more. After a short time viola, there is the ground glass. Clean it up and draw a few lines with a pencil on the frosted side and insert into frame. So easy.