I’m developping a small 35mm scanner, using DS8 electronics and software from @Manuel_Angel. Here you can find a small video about the scanner called Lumi 35: https://youtu.be/xypYzaK6ssk?si=WI4-LQZuNFkQ4txw and here you can find the result https://youtu.be/AHLDQx7DdMg . It’s a prototype still under development.
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And my first trial to acquire the optical audio from scan https://youtu.be/TJQvOPVhgrY
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My latest setup is using the Rodenstock Rodagon 50mm f1:2,8 lens on the Rpi HQ camera. The distance between lens and film is about 350 mm. Is it possbile to find a lens system so it can be put closer to the film? Would putting the Rodenstock lens in reverse be of help here? https://youtu.be/w2RaOlH2nu8
I don’t have the scientific background most of you seem to have, so what I’m writing might be absolutely trivial, but for the first iteration of my 8 mm scanner I used a Canon f2/35mm Lens and was about 3 cm close to the film, as opposed to about 8 cm with the Schneider 50mm.
The lens has a close focusing limit of 25 cm, while the Schneider has 50 or 100cm, I think? So that would be the stat I’d look for. The Canon is great and fairly cheap, and at about f7.1 I didnt see any difference in sharpness compared to the Schneider, but you need a Canon camera to set the aperture, which ist not very convenient.
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Bedankt, thank you @verlakasalt. This is for me also unknown ground. Just managed to 3D design and print a 40,5 to 42 mm (male/male) adapter ring so I can start exploring the differences forward and reverse mount. My camera will continue to be the Rpi HQ camera, since I’m a fan of DS8. To be continued.
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For a 35mm lens the film/lens value 3cm seems curious to me? In the case of the HQ camera and the S8 film the optical calculation is especially simple, the magnification is approximately 1 and in this case the film/lens and lens/sensor distance are approximately equal to twice the focal length of the lens. The optical calculations are not very complicated and for an exact calculation I gave in a previous post an Excel calculation.
https://forums.kinograph.cc/t/c-mount-rpi-hq-camera-to-m39-componon-s-f2-8-extension-tube-for-3d-printing/2679/6?u=dgalland
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I used about 6-8 cm of extension tubes and I’m positive I had to get this close to the frame (it’s the reason why there’s a cutout in front of my film gate…).
Actually, I couldn’t follow the calculations in the Excel sheet (not because I know better, I just didn’t arrive at a result that made sense).
How do you know the lenses focal points? When I look at 35mm lenses, they all have a different close focusing limit. A 300mm lens can be a regular telephoto lens or a 1:1 macro. The latter I can use as-is, the former needs a couple of extensions to arrive at the same magnification. But there might be something essential that I’ve misunderstood…?
The closest point of focus with the Canon is at 25 cm. Magnification is 1.4 when used without extension tubes. But that’s for a 35 mm sensor, and the HQ camera only sees a fraction of it.
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Thank you @dgalland, will study this very interesting article and try to adapt the excel to 35mm.
Found two photos.
Admittedly it looks a bit more like 4 cm, but it’s still much closer than the Schneider 50mm, which is about 8-9 cm away. 
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Unfortunately, I do not have a great knowledge of optics. However, by analogy with lenses used in general photography, short focal length lenses produce smaller images of subjects than longer focal length lenses.
For example, when photographing a certain subject, a 35mm lens will produce a smaller image than a 50mm lens at the same capture distance.
To obtain a similar size image with the shorter focal length lens, we must logically get closer to the subject, which is precisely what we are looking for in this case, reducing the camera-subject distance in a scanner suitable for the 35mm format.
I would not be able to give mathematical calculations that allow a first theoretical approximation.
Simulators, in principle, give good approximations for simple lenses, but it is much more complicated in the case of complex photographic lenses. Let us bear in mind that we intend to focus on a subject at a very short distance, the image will be generated at a distance much greater than the focal length of the lens, the use of some extension device is absolutely mandatory.
On the other hand, the trick of reversing lenses is often used in macrophotography to increase the size of images of normally very small subjects. For example, photographing an insect of a few mm in size on a full-frame camera (24x36 mm). The problem with the 35 mm scanner is the opposite, we must reduce the 35 mm frame image to fit on the HQ camera sensor (6.287x4.712 mm).
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What I’m missing in @dgalland’s fornula is a representation of the lens-specific closest point of focus (still not sure about the correct English term for “Naheinstellgrenze”…). Because that would surely modify the distances between subject and sensor. But, again, I might’ve misunderstood something :).