hmmm. Didn’t realize that you are working with raw captures.
While a f-stop of 5.6 seems to be a sweet spot with the Schneider - have you tried slightly higher f-stops? I think I use a setting between f/5.6 and f/8.
Remember that the ten’s of thousands captures you are going to take when digitizing S8 footage will not treat your DSLR shutter nicely.
In any way an interesting comparison. The S8 image (whatever the capture method is) looks very good - do you know by any chance which S8 camera/lens was used?
Image quality also depends on the type of light source you are using. I assume the light source is the same in all of your captures. But generally, directed light enhances micro-contrast (= “sharper images”) compared to more diffuse light setups. I am using an integrating sphere, the most diffuse light source you can have.
The difference in image quality might not really matter once you are progressing toward a final product, which tends to have at max a resolution of 1440 x 1080 or so for S8 material. And even that resolution is hard to achieve with certain camera/film stock combinations. Of course, the better the source material, the better the end result, usually…