untetheredatom.com / TEM concepts & techniques

The back focal plane & the focus knob in SAED

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)

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 JEOL columns it is literally labelled DIFF FOCUS.

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.