Achromatic Lens - A positive simple lens cemented to a negative simple lens. Achromatic lenses are corrected for two colors and produce flatter fields of view at higher powers. Double Lens - Two simple lenses used as a system but not cemented together. This system produces a far superior image than a simple lens. The number of times a lens or lenses multiplies an image with respect to the original size.
Low power is recommended for scanning large surfaces.
Higher powers are generally used for inspecting small areas. The maximum area seen through a magnifier. The entire field of view may not be in focus. See Flatness of Field. Due to the physical laws of optics, the outer part of the image formed by a lens may be out of focus.