Using the "=" would add a layer of complexity and reduce response time,
because you would have to use either the KeyPress or KeyDown events which
will have to evaluate every keystroke.
Here is a way that will give you the same results regardless of how the text
box (not a field, fields are in tables) is exited:
In the after update event of the text box:
Me.MyTextBoxName = Eval(Me.MyTextBoxName)
You will need to ensure you have an error handler in the event procedure,
because if a user enters a value that cannot be evaluated, it will cause an
error.