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) |
|---|---|---|---|---|
| 32 | 0.10 | 100× | 0.0025 | 0.025 |
| 16 | 0.25 | 250× | 0.001 | 0.0038 |
| 8 | 0.50 | 500× | 0.0005 | 0.00086 |
| 4 | 0.95 | 1,000× | 0.00026 | 0.00024 |
| 3 | 1.38 | 1,500× (oil immersion) | 0.00018 | 0.00010 |