Optical illusion

I pass this stairwell window often, whenever I take the stairs to my 5th floor office. I am fascinated by the fact, that when I look at it in passing, I only see it as a rectangle that I know it is. I never actually notice the convergence of its lines (perspectival diminution!). Only in a photo do I see what it looks like to my eyes, without my brain translating.

Curiouser and curiouser.