From emsuite wiki

Considerations In Relation To Magnification Accuracy in the SEM

Magnification is generated by the instrument knowing the final lens focal length and adjusting the scan to that appropriate to the selected magnification.

The variables in the system that may vary the accuracy are:

1. High Voltage – stability over time, the general drift as HT tank warms to the point of heat gained is equal to heat lost.
2. Magnetic history in the final lens and resultant focal values, is this very great and is one able to stabilise it?
3. Spread of current under a specific WD value (often several turns of the focus control before a change occurs), it is ideal if you are able to measure the current or voltage passing through the final lens.

Tests that may need to be made:

1. A range of measurements over a period of one hour with HT on to determine the point of stability.
2. Experiments with magnetic history, how much variation do repeated out and in focus adjustments make
3. Experiments with WD factors and determination of a standard procedure to ensure that the specimen is in exactly the same position
4. Accuracy of the instrument's inbuilt standardisation if such an adjustment is available.