It’s an optical illusion. Open your image in an image editor and look at the color values.
True dat. If you stare at a fixed point of the ring it disappears in few seconds. Guess it’s simultaneous contrast effect between the clipped part and the halo part.
Interesting. I never would have imagined that it was like that…
(see in full screen)
Without the black border it gives the feeling of each circle being as if they were almost transparent/less strong in the center. With the black border, the colors inside the circle is better distinguished.
Excellent demonstration. It would seem to be the simultaneous contrast phenomena, or as one of my colour peeps suggested, lateral brightness inhibition / Mach banding. If the matte choked a little more closely up to the edge of the beginning of the transition, it probably would be an even more compelling demonstrating.
Just one more image to support optical illusion theory
Here we can clearly see that the cores are not dimmer than halos.
if you want linear, select “None” or “Raw” under srgb.
Yeah, thank you.
Just to be a good blender community member i want to leave a note, that such an optical illusion is called Mach Bands.