The large N.A. of a microscope objective restricts the focusing requirements of the objective. The depth of focus is shown in the table as the accuracy with which the focal plane must be located in a direction along the axis of the microscope optics in order that the highest possible resolution can be obtained.
|objective focal length (mm)||numerical aperture (N.A.)||maximum useful magnification in compound microscope||maximum resolution on object (mm)||objective depth of focus (mm)|
|3||1.38||1,500× (oil immersion)||0.00018||0.00010|