Creating Trigger For A Single Column/Field?

Hi all,

My code below creates a trigger that fires whenever a change occurs to a 'myTable' row, but this is not what I want. I only want to log changes made to a single field called 'Charges', when that field is changed I want to log it, can anyone tell me how to modify my code to do this, thanks

Create Trigger dbo.myTrigger
ON dbo.[myTable]
FOR UPDATE
AS
Declare @now DATETIME
Set @now = getdate()

BEGIN TRY
Insert INTO dbo.myAuditTable
(RowImage,Charges,ChangeDate,ChangeUser)
SELECT 'BEFORE',Charges,@now, suser_sname()
FROM DELETED
Insert INTO dbo.myAuditTable
(RowImage,Charges,ChangeDate,ChangeUser)
SELECT 'AFTER',Charges,@now, suser_sname()
FROM INSERTED
END TRY

BEGIN CATCH
ROLLBACK TRANSACTION
END CATCH