Abramowitz, M., Spring, K. R., Keller, H. E. and Davidson, M. W.
Basic principles of microscope objectives.  BioTechniques 33: 772-781 (2002).

Microscope objectives are perhaps the most important components of an optical microscope because they are responsible for primary image formation and play a central role in determining the quality of images that the microscope is capable of producing. Objectives are also instrumental in determining the magnification of a particular specimen and the resolution under which fine specimen detail can be observed in the microscope. The objective is the most difficult component of an optical microscope to design and assemble and is the first component that light encounters as it proceeds from the specimen to the image plane. Objectives derive their name from the fact that they are, by proximity, the closest component to the object (specimen) being imaged.