Signed by field made read only

[pdelicado]

Hello
I have created costum fields "Signed" and "Signed by". "Signed by" is a readonly field wich is to be filled by the "modifiedby" user in a formula (signedby=modifyedby.name). However if there is anothe user modifiyng the record then this filed is also changed.to the new user wich modified the record. How can I prevent this form happening and keep this field intact without making the user ttake action?
Thanks

[emillod]

You could create hook which would run only if someone modify signed field. AfterSave hook which would take propper person then from modifiedBy.