[讨论]过焦还是在欠焦

Rodenburg said:
the system is usually designed in such a way that the illumination is parallel at the specimen when the intensity (C2) knob is high.
http://www.rodenburg.org/guide/t1100.html
Peter Goodhew sadi:
when overfocused, beam convergences substantially because electrons come from only cross-over only
http://www.matter.org.uk/tem/lenses/beam_convergence.htm

个人觉得Rodenburg的解释更合理，见他网页中的ray diagram

调整电镜状态和使用时C2都是处在过焦状态（右旋，强度变弱），过焦状态时的照明孔径焦比欠焦时的照明孔径角小，因此相干性比较好。如图，照明孔径角的确定。a是正焦，b是过焦，c是欠焦