The concept of resolution in AFM is different from radiation based microscopies because AFM imaging is a three dimensional imaging technique. The ability to distinguish two separate points on an image is the standard by which lateral resolution is usually defined. There is clearly an important distinction between images resolved by wave optics and scanning probe techniques. The former is limited by diffraction, and later primarily by apical probe geometry and sample geometry. Usually the width of a DNA molecule is loosely used as a measure of resolution, because it has a known diameter of 2.0 nm in the B form. Some of the best values for AFM imaging are 3.0 nm quoted form DNA in propanol. Unfortunately, this definition of resolution can be misleading because the sample height clearly effects this value.
Indeed, many authors have seen that it is the radius of curvature that significantly influences the resolving ability of the AFM. Images of DNA made by the sharper tip have shown dramatic improvements in resolution widths (see the two pictures below). Even greater improvements in resolution have been attained with Tappingmode but contact imaging still is capable of high resolution imaging. For brief discussion on resolution see the article by Keller in Physics Today (October, 1995).