What you get in the image (ignoring lens issues like spherical aberration, etc) is the convolution of your aperture (stop) with every point in the scene, with the size of the aperture-image scaled by how far from the plane of focus the point is. So stuff in focus varies only in brightness with the area of the aperture, stuff that's out of focus will vary in shape.

The usual cliche is to make heart- and star-shaped cardboard stops for a 50/1.4 (35mm-equivalent), put a loved-one in front of christmas-tree-lights and fire away. You get glowing coloured hearts etc in the background.