Not all of the bokeh are the same shape because not all of them are along the optical axis of the lens. Professor Ren described it this way: imagine that the cylinder holding the lens of the camera is like the brown cardboard piece at the center of the toilet roll. If you look straight down its center (or equivalently, the optical axis of the camera lens), you will see a circle. However, if you shift your eye off the optical axis, you will see a shape that looks like one of the bokeh on the image of this slide. This is because the circle in the frontal plane of the lens (i.e., the one closest to your eye), will obstruct part of the circle in the plane farthest from your eye. In the image, you can see how the orientation of the bokeh shape depends on the direction that the light is shifted off optical axis of the lens.
Not all of the bokeh are the same shape because not all of them are along the optical axis of the lens. Professor Ren described it this way: imagine that the cylinder holding the lens of the camera is like the brown cardboard piece at the center of the toilet roll. If you look straight down its center (or equivalently, the optical axis of the camera lens), you will see a circle. However, if you shift your eye off the optical axis, you will see a shape that looks like one of the bokeh on the image of this slide. This is because the circle in the frontal plane of the lens (i.e., the one closest to your eye), will obstruct part of the circle in the plane farthest from your eye. In the image, you can see how the orientation of the bokeh shape depends on the direction that the light is shifted off optical axis of the lens.