There are two main properties to keep in mind when choosing microscope components: objective mounting thread and mechanical tube length. CVI Melles Griot microscope objectives all have an RMS threaded (0.8 in., 36 TPI) interface, except for the Plan Apochromat Long-Working-Distance Objectives. This standard thread is used on our microscope bodies as well as on many other commercial microscopes. Check the documentation that came with your microscope body or objective.

Our standard visible and infrared microscope objectives are designed for a mechanical tube length of 160 mm. The mechanical tube length is the distance between the end of the tube you insert the eyepiece into and the shoulder of the objective. Both the eyepiece and objective should be designed for this spacing to perform at the specified magnification.

Finite-conjugate objectives form an intermediate image within the microscope body. Infinite-conjugate objectives do not, therefore they require a tube lens between the objective and the eyepiece to create a viewable image. There is a brief application note about this in our catalog Microscope Objectives Technical Notes (PDF).

Most standard microscope objectives will have two sets of numbers in this format:

160/0.17
4/0.12

160 is the mechanical tube length (in mm) for which the objective is designed. For an explanation of mechanical tube length, please see the previous FAQs.

0.17 is the thickness (in mm) of a glass cover slip that the objective is corrected for when being used for examining a biological specimen.

4 is the magnification (in this example, 4×)

0.12 is the numerical aperture (NA) of the objective.