The distance from the center of the lens to the focus point (or called focal point) of electrons is called the focal length (f) of the electromagnetic lens. In general, stronger lens excitation generates a shorter focal length. Magnetic lenses with short focal lengths are obtained by concentrating the magnetic field by means of magnetic polepieces. Because the focal length of magnetic lenses in electron microscopes is of the order of
millimeters, the specimen is immersed in the magnetic field.
The values of the focal lengths and the positions of the foci do not depend on the direction of magnetic field (Bz) .
The focal length is given by,
f = K (E0/i2) ------------------------------- 
K -- A constant based on the number of turns of lens coil wire and the geometry of the lens,
E0 -- The accelerating voltage,
i -- The current, put through the coil.
The typical focal length (fo) of objective lenses is ~ 1.5 mm for both the pre-field and post-field parts.
In TEM systems, the first condenser and final projector lenses have short focal length and their designs are similar to that of the objective lens. However, the second condenser and the other projector lenses may be much weaker.