2 Standard Focus

On a Tecnai microscope, the defocus display value on TUI is a relative number, because it can be changed by ResetDefocus without the actual objective lens current being changed.

However, there is a thing called Standard Focus. Its value is directly related to objective lens current, regardless what the display defocus value is. Therefore, sometimes, it is also called Absolute Focus. The term Absolute is perhaps slightly misleading, because it depends on scope alignment. When Eucentric Focus button on the right pad is pressed, the Standard Focus or Absolute Focus has value 0. In another word, it depends on Focus Preset for that specific magnification during column alignment procedure.

In a perfect world, the value of Standard Focus at Eucentric Focus should be 0. But in reality, this value can be off by a small or large number. The alignment might have not been done accurately, and the it might not be updated, due to, for example, a CompuStage repair. However, for a fixed alignment, a Standard Focus value corresponds to an unique objective lens current. We can utilize it to set objective current to a specific level, conveniently.

As you can see, if specimen is at Eucentricity and precisely focused, the Standard Focus value under this condition is corresponding to Eucentric Focus.

Note: Standard Focus property is hidden from TUI interface. One can not see its value and use it. However, we can do all that via scripting interface which SerialEM uses to control the microscope.