Look at these images of an Apple remote:

1/125th sec, 3200 ISO, f/1.8

1/125th sec, 3200 ISO, f/1.4

1/125th sec, 3200 ISO, f/1.2

They get a tiny bit brighter as you go down. Correct. And that is because I opened the aperture a little more each time: f/1.8, f/1.4, and f/1.2, respectively, on my 50mm f/1.2 lens.

Ah. But now look at the circle next to the remote. The actual light source in the distance was the “ON” light on my TV, a tiny blue light. But the more open my aperture, the larger the circle caused by that out-of-focus light.

And now observe the most important thing. Only wide open do we in fact see a circle. In other images, the “circle” is not a circle, but the shape of the lens diaphragm that creates the aperture (“opening”). In my case, an octagon. Not quite a circle.

So if you want out-of-focus hexagons or octagons in the distance, stop down your lens. If you want out-of-focus circles, open the lens as far as it can go.