The pooling of similar grey levels into a single value resulting from the limited availability of tonal resolution in a scanner. The result is that slightly different grey levels may be recorded as identical. The effect is most often seen in shadow areas of scanned images.
Quarter tones
Tones between mid-tones and highlight or shadow tones. Those falling between shadow and mid-tones are known as three-quarter tones, and those between highlight and mid-tones are known as quarter tones.
