Practical Electron Microscopy and Database

If no magnetic lens is used, the camera length is defined as the distance from the specimen to the recording plane (e.g. film or CCD camera). Because some lenses are used in a TEM, the actual camera length should be the effective camera length. With a parallel electron beam, the camera length L₀ is equal to the focal length f₀ of the objective lens as shown in Figure 3245. Due to the effect of magnifying lens (magnified from r₀ to r), the camera length is modified. By considering that the magnifying lens system magnifies the pattern and projects it onto the recording plane at magnification M (M=b/a), the final, effective cameral length L should be given by,

         L = L₀ · M --------------------------------- [3245]

To have a camera length exactly as labeled in the instrument, some key points need to be satisfied as follows:
         i) The specimen should be placed exactly at the Eucentric height in the objective lens.
         ii) A parallel electron beam should be employed.
         iii) The focus of the objective lens should be exactly adjusted to the focus position.
         iv) The focus of the first intermediate (with “Diff focus” knob) should be adjusted at the back focal plane of the objective lens.

Note that the magnification and camera length projected on the fluorescent screen are slightly smaller than the ones on the film because the fluorescent screen is located at a higher position than the film.