In one aspect of the invention, a sensor assembly for incorporation into a downhole tool string comprises at least one sensor adapted to detect changes in magnetic flux proximate a first portion of the downhole tool string. A second portion of the downhole tool string comprises a characteristic that interferes with a magnetic reading of the at least one sensor. At least one magnetic focusing array is disposed on an outer diameter of the tool string, and the at least one magnetic focusing array reduces an effect on the sensor of the interfering characteristic of the second portion.