macro reverse with two lenses: am I doing it wrong?

Hallo,
here is my setup: I am using a full frame Canon camera with the 24-105 as primary lens and a 50mm 1.8 as secondary (reversed) lens. For that I need a step down ring because the 24-105mm is a 77 while the 50mm is a 52. I make sure that the 50mm is wide open before reverse mounting it (by mounting it in the normal way, open it wide and then dismounting it).

The problem is that in the viewfinder I can only see a small circle, the rest is black. Like if I would look through a small hole.

What's wrong with my setup? Thanks

What you are experiencing is "vignetting", try reversing the setup, i.e. 50mm on the camera and the 24-105 reversed.

Set the zoom to 105mm. If that is vignetting, you have to use a longer lens.