depth of focus, focal depth, DOF
depth of focus, focal depth
The depth of focus (DOF) or the focal depth is the range of distances for which the specimen (object) is imaged with an acceptable sharpness on the image plane. DOF is larger as the opening (aperture) angle of the incident probe is smaller but is smaller as the magnification of the image is higher.
The opening angle of the incident electron probe of the SEM is about 100 times smaller than that of the optical microscope. Thus, the DOF of the SEM is about 100 times larger than that of the optical microscope. Fig. 1 schematically shows the change in DOF when the diameter of the objective lens aperture is changed. Fig. 2 shows a comparison of SEM images taken at the different DOFs determined by the different objective lens aperture sizes.
Fig.1 Difference in DOF depending on the diameter of the objective lens aperture.
Fig.2 Comparison of two SEM images taken at the two different DOFs determined by the objective lens aperture sizes of 100 μm diameter (Left) and 200 μm diameter (Right).
Specimen: Filament of a light bulb. Accelerating voltage: 10 kV.
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