A gradient applied to a large surface may cause a visible color banding effect (steps, strips) if start & end point color components (red, green and blue) are enough similar. To fix this problem you can use diffuse and Gaussian blur effects.
Let's take a look at a common example of color banding effect. You can clearly see strips (steps) of the gradient. That effect caused by a way the gradient was generated. I used Gradient Tool which generates an array of ideal strips. In the real world, gradients don't exist in such a perfect form. To get a smoother look you may add a noise and some distortion. A typical photo may be completely destroyed, but not gradients.
A Gaussian blur or Gaussian smoothing is an image processing effect that mixes pixels using a Gaussian function.
If a gradient was generated in an image editor that step may not help. To apply it use menu Filter → Blur → Gaussian Blur....
Set Radius parameter to 10 pixels and tweak it a bit to get best results. Click on OK to apply the effect.
The Gaussian blur effect worked well. Colors are much smoother, but not perfect. You can still note small steps.
To reduce color steps apply Diffuse effect. Go to Filter → Blur → Gaussian Blur...
Set Mode to Normal and click on OK.
You will notice a small difference. Apply the effect 5-10 more times to get a final result. To quickly repeat last effect press Alt + Ctrl + F in Adobe Photoshop CC version 19 or newer. Or press Ctrl + F in older versions.
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