For those critical measurements longer than 20" (500 mm), the optical scale extension adds a 20" artifact to increase your precision measurement length up to 40".

The optical scale extension uses a magnetic base to securely mount the invar extension bar and a scale holder from which the precision optical scale extends. Optical scales are sold separately in 5", 10" and 20" and 130 mm, 250 mm and 500 mm lengths.