Maybe you remember I converted my Bolex H16M camera to Ultra-16. Well, for a long time I’ve been using the normal Octameter side viewfinder and frankly guessing the U16 frame ! So it’s about time I pulled up my socks and finish the modification. I really just need to know the correct frame for a 25mm and a 50mm lens and maybe a 75mm too. My Octameter does have the adaptor for 10mm, but I decided that it wouldn’t work well with this U16 mod, so I removed it.
Here goes… I removed the 4 screws that hold the viewfinder together. Two of them are long. Also removed the two screws holding the parallax knob and withdrew it along with the circular scale. Once prised apart you’ll see a carriage with one fixed lens and another concave lens in front that moves along, controlled by a rack and gear wheel. The wheel is marked with the various focal lengths and at each position there’s a click-stop formed by a ball-bearing dropping into a small hole. Bolex engineering !
With the focal length set at 150mm it’s easy to unscrew the 4 screws that hold the front frame. It’s prised off carefully without losing the two tiny leaf springs ! I then filed out the frame to 29mm and painted it matte black before reassembling.
Back to the carriage. Now, it’s relatively easy to adjust the finder to get a wider image than the normal 25mm frame, by moving the front concave lens forward. But then it won’t have the desired click-stop. So to get this it’s necessary to do some work on the carriage. Unscrew the rear fixed lens, also the 3 screws and one nut holding the carriage to the chassis. Now the carriage can be easily removed. It needs to be brought forward by a few mm. So the two slotted holes at the front are filed out further, and about 3mm is sawn off. When replaced, the rear screw and nut still hold the carriage, but only just. The carriage is pushed forward to maximum. Also the front (moving) concave lensmount needs to be removed and the two slots made longer rearwards by about 2mm. Screw it back firmly.
Before I put the carriage back, the toothed wheel with the focal length numbers had to be set properly. This is easily done when the carriage is loose but the ball bearing has clicked on… simply adjust the cogs until the 25 comes into the window on top. Then screw back the carriage on to the chassis. Photo shows the correct position for a 25mm lens. The 35, 50, 75 and 100mm positions also click properly. But 16 and 150mm do not work on my set-up.
Now I checked to see if everything was OK when attached to the camera. I used a Bolex Gate Focuser to first examine the image, though tracing paper works fine as well. The shutter obviously has to be opened using a backwind key. For now I was just concentrating on the horizontal field of view, making sure the two images were equal. Bear in mind though the camera gate usually takes in slightly more than a viewfinder, as projector gates are smaller. I tripod-mounted the camera and observed distant objects. So there was no need to attach the viewfinder. The gate image is obviously upside down etc.
When satisfied all OK I reassembled the viewfinder casing with the 4 screws, making sure that the parallax rod connects and turns properly. Replaced the parallax scale and knob. When attached to the door, and parallax adjustment closed, the mark should show infinity.
Now time to do the top and bottom masking. I used two thin strips of aluminium: they go into the slot on the front of viewfinder. First I blackened their visible rear parts. The aspect ratio of Ultra-16 is 1.85:1. My H16M gate is wider than that, but I wanted the correct ratio so that the finder could be used on different cameras. As the front frame width is 29mm its height needs to be 15.7mm. Camera on tripod again, I aimed it at a horizontal line on the wall, using the gate focuser accessory. When the line was exactly central I withdrew the focuser and attached the door/viewfinder and again compared the image. Then inserted the two masks and carefully adjusted them until they were 15.7mm apart, whilst observing the horizontal line midway between. I marked the positions with a scriber, then eased them out slightly and applied Araldite. Pushed them back into position. Later finished with matte black paint.
So now I feel much more confident using this Bolex H16M camera for Ultra-16. It’s worth mentioning a Super-16 mod might be broadly (excuse the pun !) similar. Now I can be sure about the framing for three of my prime lenses. But not 15mm. For this I have another idea…. check back in a week or two !