I’ve never used a camera (other than my cell phone) before in my life and I have zero knowledge about how to take pics.

However I’m a historyfag and I’ve gone down the rabbit hole of developing film (my end goal is to make and develop my own wet plates). I was given a voightlander Avus camera recently and realized that 9x12cm film is hard to find as are plain sheet holders for the camera. So I bought a 6x9 roll film adapter. The problem is it’s too small for my camera. So I rigged up something that I think is light tight. It definitely works in practice. However my film all came out with a really dark line along the whole edge of the film. Is this from a light leak or bad development?

>tl;dr what caused this?