focal depth (depth of focus)

The "focal depth (depth of focus)" is the range of distances for which the object is imaged with an acceptable sharpness on the image plane. The focal depth is proportional to the spatial resolution of a microscope and to the square of magnification, and inversely proportional to the aperture angle. In the case of a TEM, since the aperture angle is smaller than that of an optical microscope, the focal depth is large. For example, if the magnification is 10,000×, the aperture angle is 1×10^-3 rad, and the resolution is 1 nm, the focal depth reaches 100 m.