Panomicron Holmium - 6x7 Rangefinder

 
 
A close up of the 3D printed engravings

A close up of the 3D printed engravings

During July 2018 I started planning to build a rangefinder camera. I decided that 6x7 would be the first format I would try to make, essentially seeing how close I could get to an all mechanical Mamiya 7. I had had this idea for a few years, and the rangefinder, or any other accurate focusing system was the thing separating most homemade camera projects from real commercial cameras. This would also enable me to use different lenses and not be limited to hyperfocal focusing.

Thus I was off, madly sketching ideas for designs, researching old rangefinders, looking all over the internet to try to find bits of information that would help me get to a functional rangefinder. The information was very scarce ( who would have guessed right?) but I eventually figured it out and could make sure that the rangefinder would be accurate within the DoF for a 80mm f/2.8 lens on 6x7, focused down to around a meter.

Even if the rangefinder would technically be accurate enough for a 80mm lens, I decided to start with something that would give me some more leeway. the Nikkor 65mm f/4 large format lens seemed like a good fit, I found a used but not too beaten copy and went for it.


Besides a rangefinder I wanted the camera to have it’s viewfinder integrated, so I designed a viewfinder that would be around 30mm long, and give an angle of view wide enough for the 65mm frame to fill it entirely. I also decided to put a 1.5x magnifier in front of the rangefinder, doubting the accuracy of the materials I was using, I at least wanted to remove user error as best as I could.

Looking at other homemade medium format cameras, I thought the two knob system seemed sort of lazy, for big wide format (6x12, 6x14 etc…) camera it’s understandable, because film flatness is hard to keep over such a long area, but for a smaller aspect like 6x7 I thought I could do better, so I designed a ratchet system in the advance to keep the film in tension.

I then decided to try something fancy, make a counter that would stop the winding of the lever when the film has moved the correct amount. Sadly the method i decided to use put to much strain on the gears and would lock up. Medium format is tricky in that way, since you don’t have any sprockets, you have to rely entirely on film to roller friction, or make a complex mechanical system to non linearly count. I have a few more ideas to make the counter work, but in the mean time I might go back to a simple dial indicator that I’ll have to keep an eye on when I wind.

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Issues

Once everything was 3D printed and assembled a few issues were obvious.

The helicoid I had designed, was not smooth enough to operate the rangefinder. It worked, and was accurate, but caused the rangefinder to shake which threw it out of alignment, which made it pretty hard to calibrate ( I swear I spent 3 or 4 hours the first time trying to get the images to line up). The second issue was that for some reason the top plate was interfering with the rangefinder, it was touching one of the mirrors which was changing the vertical alignment, so every time I had to calibrate the image too high, as the top plate would angle the mirror forward and push the image down. It’s still a bit of an issue, although I’ve tried my best to fix it by sanding down the top plate. The next iteration will leave ample room to make sure nothing is touching at all, ever.

I also had issues with the actual film spool inserting and removal. The orignal concept had the bottom spool holders bending down through the flexibility of the plastic. This was enough to insert an empty spool into the camera, but the roll of film would prevent the spool from flexing enough to slip in. So at the moment I remove the bottom holder and screw it back in every time I load/ unload the camera. In the next version the holders simply twist out from the bottom, sort of like the Pentax 67.

 

A bit later, a new idea came to mind, something even a bit more special, I found that my 50mm f/2.8 Bronica lens just about covered 6x7. So I decided to redesign the rangefinder to change its base length to 50mm and mounted the 50mm on the camera. After a few hours of calibrating (again…) It was up and running, I took a few test shots and I’m seriously impressed by its performance. It has character for sure, and it’s a little soft in the edges, but there seems to be very minimal distortion, and since it goes all the way to f/2.8 it’s probably one of the most flexible wide angles for 6x7.

Sorry for the test shots not being the most fascinating, had to make do with limited time.

Special Thanks to Alex Nelson and Nico Llasera for dealing with my excitedly incoherent rambling when I need to bounce ideas off of someone. And of course a special thank you to Bellamy for featuring the camera on his site.

Holmium with 50mm f/2.8 mounted

Holmium with 50mm f/2.8 mounted