Manual Focus Lenses Forum

marius.zaech · Joined: 27 Jan 2021 Posts: 62 Location: Switzerland

Coming soon...

I've had this project in my mind since quite some time. Now I've finally started with the CAD for the prototype. I'm designing it for a 90mm Schneider Kreuznach large format lens that i got with my Sinar kit. Luckily it's rear diameter is just slightly thinner than the thread diameter of the focus helicoid i got off ebay, greatly reducing the complexity of the front end design (It can thus be just one piece). I think for my first prototype i will not bother constructing a swinging back door but instead just one to screw on with small m6 thumbscrews.

The biggest question now is how to handle the pressure plate. My current solution would be a 2mm Aluminium plate mounted on a thin piece of compressible foam (which acts as a spring). Using a leaf spring system would certainly be better but I'm not quite sure how to do that apart from ripping one out of an old folder camera.

Of course this is still just a prototype. I will also have to see how well it will print as there are some places with considerable overhang no matter how you orient it.

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RokkorDoctor · Expire: 2025-05-01

How are you going to focus the images?

Will there be a swappable film back / focus screen or are you going to rely on yourself calibrating an accurate focus scale on the helicoid and work with small apertures and hyperfocal distances?

I'm also interested to see what solution you will use for winding the roll film whilst avoiding overlapping frames or wasted film (revive the old system of the small window in the back revealing the frame numbers printed on the backing paper maybe?).

marius.zaech · Joined: 27 Jan 2021 Posts: 62 Location: Switzerland

I bought a simple helicoid off ebay. It has a huge diameter but only 14mm extension, should be enough for the appliaction (near infinity).

I don't think I will bother with swappable backs and stuff like that since the whole idea is a portable, simple camera to take on hikes and stuff like that. So I'm just gonna determine infinity focus once with the ground glass from my mamiya 645 and then mark it on the lens, another possibilty would be to calibrate the distance with some 0.1mm spacers onto the hard stop of the helicoid but I will most likely not do that since it's much more tedious and I don't want to wear out the plastic threads that the helicoid is screwed in.
I'm then gonna mark some distances + hyperfocal range (easy to calculate).

The film winding is another open issue... my current idea was to mount a simple reduction gear on top with a dial fixed to it, so that winding the whole film length corresponds to less than one revolution. I could then mark on that dial (experimentally, with an old film) how much I need to wind on from the start mark + for each frame. In a further step i thought of lasercutting said dial with notches and a spring indent. But as mentioned that's just an idea by now. The small window is for sure the easier low-tech option but I do not really like it.