I came across a problem in Photoshop which for some odd reason seems to exist since the birth of the program and has not been fixed to this day. I say fixed, I'm not sure if it's actually a problem. It was a problem for me though.
What I found was that when I used one of Photoshop's own default brushes (a round one), it seemed to leave banding. It also happens when blurring in image.
If you look carefully at the screenshot above you can see banding/rings. What I was trying to do was have a slightly lighter colour on top, I used a brush for this. When doing this you don't get a smooth gradient look. And when you have a keen eye like me you notice these little things.
How I Fixed It
1. You want to create a new file but in colour mode choose 16 bit. (It's usually 8 bit by default)
2. Add your brush in a separate layer on top of the base colour. (As you can see I chose a very dark blue as my background colour. Then I chose a very slightly lighter colour and just hit the brush once in a new layer.)
3. You'll then see the banding. Go to "Filter > Noise > Add Noise".
4. Then add the slightest little bit of noise. You'll need to adjust it to suit your image. Just make sure to look closely and make sure you don't see the banding anymore.
You should then be done!
There are some very complex tutorials on the web showing how to overcome this. But this procedure is the easiest and quickest I've found.