I had to set my thickness way more than your setting to block light leak completely. I don't know which factor made the difference between you and me. I'm using version 2.93
Hey jeewon ,
Increase the thickness just enough to block the light leak in the corners of the room.
Try holding SHIFT while clicking and dragging the value of the thickness for smaller increments. The strength of the light will also make a difference.
Remember though your hardware is different to what's used in the lesson. The brightness and contrast of your monitor maybe different for example. There may also be differences in Blender's default settings that may have changed for you to notice more light leak and have a need to increase the thickness.
Ultimately the goal is to understand how to fix issues like light leak, however the values used are a guide. Trying different values to suit your needs is a good thing.
Thank you for the kind reply! I might have been obsessed about following instruction too exactly.
The key is to understand what's happening, not so much the exact numbers, and that will apply across many of the courses you watch.