If you spend much time working in photoshop you will eventually run into an image with banding. Banding is an unwanted visible break in what should be a smooth area. Sometimes banding is caused by the editing done in photoshop and sometimes the banding is already part of the base image you have started from.
The first thing to check is to find the source of the banding. Some times it is a mask. Sometimes it comes from a layer (or effect) applied at a small opacity value. Often it is in the base image – jpeg compression artifacts are a common cause.
If it comes from a layer at a small opacity value you can usually fix it by adding some noise to the layer mask (menu item: Filter->Noise->Add Noise).
If the banding is caused by heavy color correction use, you can often fix the banding by adding some noise below your color correction layers.
On your gradient layers make sure that the dither option us turned on. Watch out if you have a gradient layer going from solid to transparent – the dither option does not help, it only dithers the color not the transparency.
If it comes from an image layer here is how you fix it:
- Create a selection of the portion of the layer that is banding.
- Duplicate that section into a new layer (cmd-j).
- Reselect the mask you used and add it as a layer mask to the new layer (cmd-click the new layer in the layers window, then use the “Add layer mask” button at the bottom of the layers window).
- Duplicate this layer to a new document.
- Change the mode to 16 bit (menu item: Image->Mode->16 Bits/Channel).
- Create a high contrast layer to better see the banding.
- Select your image layer and bring up the Gaussian Blur filter (menu item: Filter->Blur->Gaussian Blur).
- Find the smallest blur radius that totally eliminates the banding and does not introduce unwanted shapes. When evaluating the image make sure you are zoomed in to 100% because for zoom levels < 66% photoshop falls back to an 8-bit preview mode.
- After applying the blur you will almost always want to add back a bit of noise to match the original image. I recommend this method for adding noise.
- Use the image layer as a clipping mask for the noise layer. (option-click the line between the two layers in the layers panel)
- Merge your noise layer down into your image layer (Select the noise layer and type cmd-E).
- Convert the document back into 8-bit mode (menu item: Image->Mode->8 Bits/Channel)
- Copy your layer back to your original document.
Doing the blur in 16-bit mode is not always necessary, but I nearly always use 16-bit mode when fixing banding due to it’s increased quality and smoothness.