Vignetting with Componon-S

Hi everyone, I have bought the Componon-S (c-mount, 1:4 50mm).
I mounted it on a 2/3 sensor with 10 cm extenders, so that I can frame the printed area of the frame, but I’m noticing a bit of vignetting:

I used the photoshop levels on the right to better show the issue.
Does this happens to any of yours too?

What is most used is the 50mm f2.8 Componon-S, the specs for it here. With either the Componon-S or the Nikkor EL (also f2.8 50mm) I have not seen significant vignette. Used both with a D3200 DSRL and the Rpi HQ sensors.

I see. I knew it was f 4, but I got it for about 12$ on ebay so I wanted to try.
For the moment I’ll continue to build the scanner with with lens.
Thank you very much.