[3DEM] microscope indicated defocus values

Paul Chipman paulrc at bilbo.bio.purdue.edu
Wed May 23 09:13:59 PDT 2007

Dear Colleagues,

We've known for years that the indicated amount of defocus on our 
CM200 and CM300 was not accurate but was reproducible.  Since we 
generally only used a couple of different magnifications, it was 
pretty straightforward to plot out the actually values (linear) 
versus those indicated on the microscope.  Absolute precision was not 
really a requirement since the user determined minimum focus point 
could also be slightly off the target and a range of different 
defocus values was needed for the reconstructions.  Close was good 
enough and an accurate value could be determined during the data 
processing phase.  Recently we've noticed a change in the slope of 
the plotted values and have been using some previously unused 
magnifications.  Some of the indicated values can be quite far off 
the actual (underestimated by up to a factor of 4).  Again we can 
compensate but now I'm curious if anyone knows of a way to alter the 
indicated defocus (similar to changing the indicated magnification 
with a LensEdit program) or if it is dependent upon any of the 
microscope calibrations.  Also, how do other labs handle this 

All comments appreciated.


Paul Chipman
Director, Electron Microscopy Facility
Project Leader, Structural Virology EM Studies
Dept. of Biology, Purdue University
Lilly Hall, Rm. B216
Phone: 765-494-1487
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