untethered atom · TEM

How to focus TEM diffraction patterns

What the diffraction-focus knob moves, and how to set it so the spots are sharp.

Drag the diffraction-focus knob below the diagram and watch the plane the intermediate lens is imaging slide on and off the back focal plane.

parallel illumination specimen A B C objective lens back focal plane (BFP) one focal length f behind the lens −g 000 +g IN FOCUS ✓ selected-area aperture image plane rays from the same point reunite here plane the diffraction lens is imaging intermediate (diffraction) lens ← this is what the FOCUS knob drives in DIFF mode viewing screen / camera
1Bragg sorts by angle, not by place. Every point of the specimen (A, B, C) diffracts into the same set of angles 2θB: one angle per set of hkl planes.
2The objective lens converts angle → position. All rays leaving the specimen at the same angle, from wherever, meet at one point a focal length f behind the lens. That plane is the back focal plane, and the map it holds (x ≈ f·2θ) is the diffraction pattern.
3Farther down, rays from the same point reunite: the image plane. The SA aperture sits here (conjugate to the specimen), so only your chosen region feeds the pattern. Here it passes B and blocks A and C.
4Diffraction focus = choosing which plane goes to the screen. Sharp spots ⇔ the red dashed plane sits exactly on the BFP. Off it, every spot spreads into a disc.
 what you see on the screen
FOCUS knob
(= DIFF FOCUS in diffraction mode)
Jump to

Why the “focus” knob isn’t moving the objective

In imaging mode the FOCUS knob drives the objective lens. The moment you press DIFF / D, the same physical knob is remapped to the intermediate (diffraction) lens: on some columns the knob is labelled DIFF FOCUS in this mode.

Turning it changes the intermediate-lens current, which slides its object plane up and down the column. Your job is to park that plane exactly on the objective’s back focal plane, because that is where the diffraction pattern physically lives.

Misfocused, the lens hands the projectors a plane near the BFP instead: each Bragg spot becomes a small disc, and at large defocus each disc is actually a tiny shadow image of your selected area.

Bench procedure for sharp SAED

  1. Set eucentric height, then focus the image with the objective knob. This fixes the objective current, and therefore where the BFP actually sits.
  2. Insert the SA aperture and sharpen its edge. The aperture and image are now conjugate: the aperture truly selects your area.
  3. Switch to DIFF and spread the beam with intensity/brightness (C2) until illumination is parallel: a converged beam gives discs (CBED-like) even at perfect focus.
  4. Turn the (diffraction) focus knob until the central 000 spot is smallest and sharpest: the other spots follow.
Don’t touch the objective focus in DIFF mode (on scopes where it stays accessible): changing the objective current moves the BFP itself, changes the effective camera length, and undoes your image focus.

Sharp focus is not cosmetic: spot radii R₁, R₂ are what you measure for the ratio method (R₂/R₁ = d₁/d₂), so a defocused pattern degrades your indexing; see the SAED indexing guide in this project.

References

  1. D. B. Williams and C. B. Carter, Transmission Electron Microscopy: A Textbook for Materials Science, 2nd ed., Springer (2009): the chapters on lenses, apertures and resolution, on the instrument (imaging and diffraction modes, the intermediate lens) and on obtaining diffraction patterns.
  2. L. Reimer and H. Kohl, Transmission Electron Microscopy: Physics of Image Formation, 5th ed., Springer (2008): ray paths of the imaging system, the back focal plane and selected-area diffraction.
  3. R. F. Egerton, Physical Principles of Electron Microscopy: An Introduction to TEM, SEM, and AEM, 2nd ed., Springer (2016): the TEM imaging system and diffraction mode.
  4. J. W. Edington, The Operation and Calibration of the Electron Microscope (Practical Electron Microscopy in Materials Science, Monograph 1), Macmillan (1974): camera-length calibration and setting up selected-area diffraction.
  5. P. B. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley and M. J. Whelan, Electron Microscopy of Thin Crystals, 2nd ed., Krieger (1977): the geometry of selected-area diffraction and its errors.
Cite this page: Tripathy, Manisha. “The Back Focal Plane & Diffraction Focus in SAED.” untethered atom, 2026, https://untetheredatom.com/tem/bfp-diffraction-focus.
BibTeX 
@misc{tripathy2026thebackfocalplanediffrac,
  author = {Tripathy, Manisha},
  title  = {The Back Focal Plane & Diffraction Focus in SAED},
  year   = {2026},
  howpublished = {\url{https://untetheredatom.com/tem/bfp-diffraction-focus}},
  note   = {Interactive teaching resource}
}
Last updated 16 October 2026.