In a continuous tone image, the transition from one tone to the next is seamless. In an image with evident banding, it’s as if what should be a smooth line on a graph is actually stair-stepped, and we’re able to see each of those transitions from one tone to another right there in the picture.
Banding happens when there aren’t enough tones available to recreate a seamless gradation; that’s why they’re most common in 8-bit image files and with image files that have been heavily compressed. In each case, there are less colors available with which to work.
If you’re concerned about banding, keep an eye out in the corners of an image when the background is a studio wall, seamless backdrop, blue sky, or any other fairly solid tone that subtly transitions from light to dark. Any area of an image that consists of a fairly solid color, particularly if that color transitions from lighter to darker, you may see banding.
To avoid banding in the first place, work on your images in 16-bit mode, so you’ll have exponentially more colors to work with. In most cases, that means converting a RAW file as a 16-bit TIFF when opening it from Lightroom or Adobe Camera Raw.
A second step you can take to minimize banding is to avoid overly compressing your image files. For that reason, an out-of-the-camera JPEG is more likely to exhibit banding than a RAW file. Likewise, saving image files as a TIFF, or minimally compressed JPEG, will keep the maximum number of tones available, whereas a highly compressed JPEG will eliminate subtle differences between colors and introduce that terrible stair-stepped, chunky banding.
To remove banding in an image file, first try to upsize from an 8-bit file to a 16-bit file. In Photoshop, with your image file open, choose Image>Mode and then click on 16-bit color space, which is found right below the already-checked 8-bit color space.
Often, serious banding requires a little more attention than simply converting the color depth to 16-bits per channel. The age-old trick for eliminating banding is actually to hide it with a bit of noise. To do this, simply duplicate the image onto a new layer and choose Noise>Add Noise from Photoshop’s Filter menu.
The noise is designed to mask the harsh edges of the transitions from one tone to another, so a little goes a long way. Try setting the noise to 1% for starters and adjust up and down as needed. Keep the distribution set to Gaussian and check the box for Monochrome to see if that’s better than full-color noise. Reposition the preview window in order to see just how much change the noise is producing.
Use as little noise as is needed to hide the sharp edges of banding. If the banding is only in the background, so the noise is only needed in the background, you can consider using a layer mask to eliminate the noise from the subject if you find it distracting. Between a little noise and the added dynamic range of 16-bits-per-channel color, there’s practically no reason to worry about banding ever again.