Achromat objective lenses are corrected for chromatic aberrations at two wavelengths, one in the red and one in the blue. They are also fully corrected for spherical aberration at only one wavelength in the yellow-green line. At other wavelengths in the visible spectrum the correction for spherical aberration is good, but not fully complete, as field curvature is present.
So what exactly does this mean? Correction for only two colors and the inherent field curvature of the lens may limit the use of these microscope objective lenses to visual work of a more routine nature. Photomicrography, and espeically color photomicrography, makes more rigid demands of color corrections and flatness of field. For non-critical work or when black and white photomicrography is being utilized, achromat objective lenses can produce acceptable images.
|Semi Apochromat Microscope Objective Lenses|
Apochromat objective lenses are made up of several fluorite lenses in combination with glass lenses, achieving correction for chromatic aberrations at three wavelengths in the red, green and blue, and for spherical aberration throughout the visible spectrum to a greater extent than with achromat lenses. The apochromat microscope objective lens is typically higher in numerical aperature (NA) than those of corresponding magnification in achromat form only. Curvature of field is also present in this type of objective (you would need to select a "plan" apochromat objective lens in order to reduce field curvature).
Since Apochromat objective lenses are corrected for three color wavelengths rather than two, they produce a more vivid image that is typically better for intricate laboratory work and advanced photomicrography.