Today’s Question: What is “dither”, and should I enable this option when creating a gradient in Photoshop?
Tim’s Quick Answer: Dithering in the context of a digital image refers to the introduction of “noise” to help prevent issues such as posterization. In the context of a gradient, dithering involves the blending of various color and tonal values to help maintain the appearance of a smooth transition.
More Detail: When you are working with a 16-bit per channel image, dithering is an unnecessary concept. However, for 8-bit per channel images dithering can certainly become a very real concern in certain contexts. The Gradient tool is a good example of when dithering can be helpful.
A gradient by definition is aimed at producing a smooth transition between one value and another. For example, let’s assume you are attempting to create a gradient that transitions from white to black over a range of 3,000 pixels, or around 10 inches assuming a 300 pixel-per-inch print resolution.
If the image containing this gradient is an 8-bit per channel image, there are only 256 shades of gray available to create a white-to-black gradient. That, in turn, means that each individual tonal value on the gradient will require just about twelve pixels. In other words, there will be a series of bands of individual tonal values defining this gradient, each being 1/25th of an inch in size.
By enabling the Dither option (available as a checkbox on the Options bar for the Gradient tool) you can help to create a smoother appearance for that gradient. Instead of having bands of various shades of gray that are 1/25th of an inch wide, you will have the appearance of a smoother gradation thanks to the dithering.
So, dithering is not necessary for 16-bit per channel images in the context of a normal photographic workflow. However, it can be very helpful to ensure that dithering is enabled for any image being adjusted in the 8-bit per channel mode.