I got serious light leaks using cheap adapter (K&F Concept) to attach Leica-R lenses to my Sony A7ii. I got less light leak using a more expensive adapter (Novoflex), and covering the junction between adapter and lens with a simple hairband solved the issue.. but really?
Here are pics to show the problem and the sollution.
The portrait of the man has visible unwanted light at bottom part of picture.
The rest of the pictures are from my test. I shoot at ISO 20000 with lens cap on. 6 seconds shutter time.